Review on the Research and Development of Ti-Based Bulk Metallic Glasses

Ti-based bulk metallic glasses (BMGs) are very attractive for applications because of their excellent properties such as high specific strength and high corrosion resistance. In this paper, we briefly review the current status of the research and development of Ti-based bulk metallic glasses. Emphasis is laid on glass-forming ability, mechanical properties, corrosion resistance, and biocompatibility.

Metals 2016, 6, 264 2 of 59 review, we summarize the details of the developments on the glass-forming ability, mechanical properties, corrosion resistance, and biocompatibility of Ti-based BMGs.

Alloy Development Studies
As most bulk metallic glasses are multicomponent alloys, the composition design of bulk metallic glasses has been acknowledged to be a difficult task. However, all the developed BMGs can be traced on the binary glassy alloys. According to the binary phase diagrams, Ti can form deep eutectics with Cu, Be, Pd, Si, Co, Ni, etc. By rapid cooling (e.g., sputtering or single roller spun-melt technique), amorphous thin films or ribbons were fabricated in these binary systems [32][33][34][111][112][113]. In order to improve the GFA of these binary alloys and obtain Ti-based BMGs, different alloying elements have been added to the binary alloys to explore multicomponent Ti-based bulk metallic glass-forming alloy systems. Except for the trial-and-error method, which is widely used but tedious and time-consuming, some empirical criteria have also been used for choosing the alloying elements. According to Inoue's classic three rules [114], the common characteristics of most bulk glass-forming alloy systems are (1) multicomponent systems consisting of more than three constituent elements; (2) significant differences in atomic sizes with a size ratio above 12% among the main constituent elements; and (3) negative heats of mixing among the main constituent elements. In this sense, the alloying elements should possess large atomic size mismatches and large values of negative heats of mixing against Ti. According to Table 1, we summarize the candidate constituent elements for developed Ti-based BMGs. Their atomic radii (R) and heats of mixing (∆H mix ) against Ti are listed in Table 2 [115]. Generally, the classic empirical rules work well. There are also some special cases such as Sc, Y (∆H Sc-Ti = 8 kJ/mol, ∆H Y-Ti = 15 kJ/mol), and Ag (R Ag is similar as R Ti ). However, the content of these elements in Ti-based BMGs is relatively low. It has also been found that the substitution of similar atoms can markedly improve the glass-forming ability of BMGs [116]. Ti exhibits similar properties such as Zr, Hf, etc., while Cu can be replaced by similar atoms-Ni, Pd, or Ag. Therefore, based on the Ti-Cu binary system, multicomponent Ti(Zr, Hf)-Cu(Ni, Pd, Ag) alloys with higher GFA have been developed [82,86,98,99], indicating that this substitution method is also effective for exploring novel Ti-based BMGs. As mentioned earlier, Ti-based BMGs can be classified into three groups: Be-and Pd-free, Pd-containing, and Be-containing alloys. In the rest of this section, typical alloy systems for each group are introduced in detail.

Be-and Pd-Free Ti-Based BMGs
This type of Ti-based BMG is mainly developed based on Ti-Cu binary systems [111]. The Ti-Cu-Ni alloy system, one of the typical representatives, is among the few Ti-based ternary systems with bulk glass-forming ability. In 2008, Wang et al. [41] investigated the GFA of the Ti-Cu-Ni ternary system systematically. Figure 3 shows the reported BMG-forming composition map of the Ti-Cu-Ni system. It can be seen that the BMG-forming composition region for t max = 1 mm is located in the triangular region enclosed by the three intermetallic compounds TiNi, TiCu, and Ti 2 Cu (50-57 atom % Ti, 34-44 atom % Cu, and 6-10 atom % Ni). The Ti 50 Cu 43 Ni 7 and Ti 53 Cu 39 Ni 8 alloys possess the best GFA, which enables the preparation of fully glassy rods with diameters up to 1.5 mm. It has also been found that the critical diameter increases to 2 mm by more fine-tuning of Ti 50 Cu 43 Ni 7 to Ti 50 Cu 42 Ni 8 [40]. Subsequently, it has been shown that the replacement of Ti by Zr effectively improves the GFA of Ti-Cu-Ni glassy alloys. By the "3D pinpoint approach," the BMG-forming maps for t max = 1 mm was found in the composition range of 51-53 atom % (Ti + Zr), 38-41 atom % Cu, and 8-10 atom % Ni (as shown in Figure 4) [41]. This incentive played a trigger effect on the subsequent development of new Ti-based BMGs with critical diameters ranging from 1 to 6 mm, such as Ti-Zr-Cu [82,83], Ti-Zr-Cu-Ni [82], Ti-Zr-Cu-Ni-Sn [45,95], Ti-Zr-Cu-Ni-Sn-Si [104], Ti-Zr-Hf-Cu-Sn-Si [99], and Ti-Zr-Hf-Cu-Ni-Si-Sn [54]. Recently, a novel strategy for designing BMG with high mixing entropy and large GFA has been proposed. Using this method, a series of Ti-Cu-Zr-Fe(Co)-Sn-Si-Ag(Sc) [56,57] BMGs has been discovered by multi-substitution of similar elements. After multi-substitution, the mixing entropy of the alloys is significantly enhanced, and the crystallization process exhibits a higher level of complexity, implying a better GFA. It is worth mentioning that the critical diameter for glass formation was reported to be 7 mm for Ti 47 Cu 38 Zr 7.5 Fe 2.5 Sn 2 Si 1 Ag 2 alloy [56], which is the largest among the Be-and Pd-free Ti-based BMGs.  15, for the as-cast rods of 2 and 3 mm in diameter, respectively. Reproduced with permission from [41]. Copyright 2008, Springer.

Pd-Containing Ti-Based BMGs
This type of Ti-based BMG was firstly developed in the Ti-Zr-Cu-Pd system in 2007 [86]. Since Pd possesses properties similar to Cu, Pd was introduced to partially substitute for Cu to improve the GFA. It was noticed that a series of 6 mm-diameter Ti-Zr-Cu-Pd alloys were produced, and the largest critical diameter of Ti-Zr-Cu-Pd quaternary alloy was up to 7 mm. In order to further improve the GFA, Zhu et al. investigated the addition of Sn on the GFA of Ti-Zr-Cu-Pd BMG [47]. With the addition of 2-4 atom % Sn, the critical diameter was markedly increased to 10 mm, which is larger than that of other Be-free Ti-based BMGs. Figure 5 shows the appearance of the as-cast 10 mm-diameter Ti40Zr10Cu34Pd14Sn2 and Ti40Zr10Cu32Pd14Sn4 rods. Except Sn, Si [49], Nb [50] and Co [117] have also been adopted as alloying elements for Ti-Zr-Cu-Pd quaternary alloys. The maximum critical diameters of Ti-Zr-Cu-Pd-Si, Ti-Zr-Cu-Pd-Nb, and Ti-Zr-Cu-Pd-Co were reported to be 5 mm, >2 mm, and 10 mm, respectively. Without biological toxic elements such as Be, Ni, and Al, these Ti-based BMGs are expected to be used as biomedical materials. The limitation is that, because of the noble element Pd, the cost of this type of Ti-based BMG is relatively high.   15, for the as-cast rods of 2 and 3 mm in diameter, respectively. Reproduced with permission from [41]. Copyright 2008, Springer.

Pd-Containing Ti-Based BMGs
This type of Ti-based BMG was firstly developed in the Ti-Zr-Cu-Pd system in 2007 [86]. Since Pd possesses properties similar to Cu, Pd was introduced to partially substitute for Cu to improve the GFA. It was noticed that a series of 6 mm-diameter Ti-Zr-Cu-Pd alloys were produced, and the largest critical diameter of Ti-Zr-Cu-Pd quaternary alloy was up to 7 mm. In order to further improve the GFA, Zhu et al. investigated the addition of Sn on the GFA of Ti-Zr-Cu-Pd BMG [47]. With the addition of 2-4 atom % Sn, the critical diameter was markedly increased to 10 mm, which is larger than that of other Be-free Ti-based BMGs. Figure 5 shows the appearance of the as-cast 10 mm-diameter Ti40Zr10Cu34Pd14Sn2 and Ti40Zr10Cu32Pd14Sn4 rods. Except Sn, Si [49], Nb [50] and Co [117] have also been adopted as alloying elements for Ti-Zr-Cu-Pd quaternary alloys. The maximum critical diameters of Ti-Zr-Cu-Pd-Si, Ti-Zr-Cu-Pd-Nb, and Ti-Zr-Cu-Pd-Co were reported to be 5 mm, >2 mm, and 10 mm, respectively. Without biological toxic elements such as Be, Ni, and Al, these Ti-based BMGs are expected to be used as biomedical materials. The limitation is that, because of the noble element Pd, the cost of this type of Ti-based BMG is relatively high. . Composition maps of BMG formation in the (Ti 1−x Zr x )-Cu-Ni system. The two panels, (a) and (b), show the BMG-forming composition ranges at two ratios of Ti 1−x Zr x , (a) x = 0.1 and (b) x = 0.15, for the as-cast rods of 2 and 3 mm in diameter, respectively. Reproduced with permission from [41]. Copyright 2008, Springer.

Pd-Containing Ti-Based BMGs
This type of Ti-based BMG was firstly developed in the Ti-Zr-Cu-Pd system in 2007 [86]. Since Pd possesses properties similar to Cu, Pd was introduced to partially substitute for Cu to improve the GFA. It was noticed that a series of 6 mm-diameter Ti-Zr-Cu-Pd alloys were produced, and the largest critical diameter of Ti-Zr-Cu-Pd quaternary alloy was up to 7 mm. In order to further improve the GFA, Zhu et al. investigated the addition of Sn on the GFA of Ti-Zr-Cu-Pd BMG [47]. With the addition of 2-4 atom % Sn, the critical diameter was markedly increased to 10 mm, which is larger than that of other Be-free Ti-based BMGs. Figure 5 shows the appearance of the as-cast 10 mm-diameter Ti 40 Zr 10 Cu 34 Pd 14 Sn 2 and Ti 40 Zr 10 Cu 32 Pd 14 Sn 4 rods. Except Sn, Si [49], Nb [50] and Co [117] have also been adopted as alloying elements for Ti-Zr-Cu-Pd quaternary alloys. The maximum critical diameters of Ti-Zr-Cu-Pd-Si, Ti-Zr-Cu-Pd-Nb, and Ti-Zr-Cu-Pd-Co were reported to be 5 mm, >2 mm, and 10 mm, respectively. Without biological toxic elements such as Be, Ni, and Al, these Ti-based BMGs are expected to be used as biomedical materials. The limitation is that, because of the noble element Pd, the cost of this type of Ti-based BMG is relatively high.

Be-Containing Ti-BMGs
This type of Ti-based BMG is mainly developed based on the Ti-Be binary system [34]. Be is a magic metallic element with unique properties. The density of Be is only 1.85 g/cm 3 , which is much lower than that of Ti (4.50 g/cm 3 ). Therefore, Be-bearing Ti-based BMGs exhibit relatively low density and high specific strength compared with Be-free Ti-based BMGs. Moreover, as the atomic radius of Be is much smaller than that of most metallic atoms (as shown in Table 1), it has been found that the addition of Be significantly improves the GFA of Zr- [118,119], Cu- [120], and Ti-based BMGs [79][80][81]. Except for the Ti-Cu-Ni system mentioned in Section 2.1.1, the Ti-Zr-Be system is another important Ti-based ternary system with bulk glass-forming ability. In 2008, Duan et al. firstly reported a series of lightweight Ti-Zr-Be BMGs with critical diameters larger than 6 mm [79]. Without containing heavy late transition metals (e.g., Cu, Ni, and Fe), the density of these alloys was lower than 5 g/cm 3 , resulting in a high specific strength larger than 400 J/g, which is more than twice that of the commercial Ti-6Al-4V (the most widely used crystalline titanium alloy). In 2010, Zhang et al. systematically studied the GFA of the Ti-Zr-Be system [80,81]. As shown in Figure 6, two amorphous regions were found in the Ti-Zr-Be system. The first amorphous region includes two intermetallic compounds (BeTi and Be2Zr) and one solid solution (α-Ti). The second amorphous region contains only one intermetallic compound (Be2Zr) and two solid solutions (α-Ti and β-Zr) and exhibits better GFA (tmax = 5 mm) compared with the first one (tmax = 3 mm). This may be because intermetallic compounds are easier to be suppressed as they are always ordered phases. Based on experimental results, the best glass former was located at Ti41Zr25Be34, which is close to the predicted composition by binary-eutectic rule (Ti41Zr32Be27). Then, a series of Ti-Zr-Be-(Fe, Al, Ag, Cu, Ni, V, Cr) BMGs [79][80][81][87][88][89][90][91][92][93][94] with improved GFA was reported, and some of the developed quaternary alloys were found to possess a larger critical size up to a centimeter scale. A pseudo Ti-Zr-Cu-Ni-Be system, which possesses a much higher GFA, was discovered based on the Ti-Zr-Be system. In 2005, Park et al. evaluated the GFA of Ti-Zr-Cu-Ni-Be alloys by injection copper mold casting and systematically outlined the alloy composition ranges with a tmax larger than 1, 6, and 10 mm, as shown in Figure 7 [121]. In 2010, a breakthrough was made in the GFA of Ti-Be-based BMGs by Tang et al [72]. They found that the microstructure of a Ti50Zr23Ni3Cu6Be18 as-cast pancake mainly consisted of a primary β-Ti phase and an amorphous phase. Based on the composition of the amorphous phase, which promises higher GFA than that of Ti50Zr23Ni3Cu6Be18, they prepared a high number of pancakes with compositions nearby and measured the compositions of the corresponding amorphous phases. Using this

Be-Containing Ti-BMGs
This type of Ti-based BMG is mainly developed based on the Ti-Be binary system [34]. Be is a magic metallic element with unique properties. The density of Be is only 1.85 g/cm 3 , which is much lower than that of Ti (4.50 g/cm 3 ). Therefore, Be-bearing Ti-based BMGs exhibit relatively low density and high specific strength compared with Be-free Ti-based BMGs. Moreover, as the atomic radius of Be is much smaller than that of most metallic atoms (as shown in Table 1), it has been found that the addition of Be significantly improves the GFA of Zr- [118,119], Cu- [120], and Ti-based BMGs [79][80][81]. Except for the Ti-Cu-Ni system mentioned in Section 2.1.1, the Ti-Zr-Be system is another important Ti-based ternary system with bulk glass-forming ability. In 2008, Duan et al. firstly reported a series of lightweight Ti-Zr-Be BMGs with critical diameters larger than 6 mm [79]. Without containing heavy late transition metals (e.g., Cu, Ni, and Fe), the density of these alloys was lower than 5 g/cm 3 , resulting in a high specific strength larger than 400 J/g, which is more than twice that of the commercial Ti-6Al-4V (the most widely used crystalline titanium alloy). In 2010, Zhang et al. systematically studied the GFA of the Ti-Zr-Be system [80,81]. As shown in Figure 6, two amorphous regions were found in the Ti-Zr-Be system. The first amorphous region includes two intermetallic compounds (BeTi and Be 2 Zr) and one solid solution (α-Ti). The second amorphous region contains only one intermetallic compound (Be 2 Zr) and two solid solutions (α-Ti and β-Zr) and exhibits better GFA (t max = 5 mm) compared with the first one (t max = 3 mm). This may be because intermetallic compounds are easier to be suppressed as they are always ordered phases. Based on experimental results, the best glass former was located at Ti 41 Zr 25 Be 34 , which is close to the predicted composition by binary-eutectic rule (Ti 41 Zr 32 Be 27 ). Then, a series of Ti-Zr-Be-(Fe, Al, Ag, Cu, Ni, V, Cr) BMGs [79][80][81][87][88][89][90][91][92][93][94] with improved GFA was reported, and some of the developed quaternary alloys were found to possess a larger critical size up to a centimeter scale. A pseudo Ti-Zr-Cu-Ni-Be system, which possesses a much higher GFA, was discovered based on the Ti-Zr-Be system. In 2005, Park et al. evaluated the GFA of Ti-Zr-Cu-Ni-Be alloys by injection copper mold casting and systematically outlined the alloy composition ranges with a t max larger than 1, 6, and 10 mm, as shown in Figure 7 [121]. In 2010, a breakthrough was made in the GFA of Ti-Be-based BMGs by Tang et al [72]. They found that the microstructure of a Ti 50 Zr 23 Ni 3 Cu 6 Be 18 as-cast pancake mainly consisted of a primary β-Ti phase and an amorphous phase. Based on the composition of the amorphous phase, which promises higher GFA than that of Ti 50 Zr 23 Ni 3 Cu 6 Be 18 , they prepared a high number of pancakes with compositions nearby and measured the compositions of the corresponding amorphous phases. Using this technique, a series of Ti-Zr-Ni-Be-Cu BMGs with high GFA have been developed. It was reported that (Ti 36.1 Zr 33.2 Ni 5.8 Be 24.9 ) 91 Cu 9 is formable as a BMG alloy with a larger diameter of 50 mm via water quenching, which is a new record in the GFA of Ti-based BMGs [72]. By replacing Ni with similar atom Fe, Ti-Zr-Cu-Fe-Be BMGs with critical diameters up to 50 mm have also been developed [73]. By direct solidification, a 150 g Ti 36.2 Zr 30.3 Cu 8.3 Fe 4 Be 21.2 amorphous ingot was successfully prepared, indicating a superior GFA. Ti-Be-based BMGs possess low density and high strength together with high GFA, which favors their application as high performance structural materials. However, their application is also constrained to an extent because of the toxicity of Be. together with high GFA, which favors their application as high performance structural materials. However, their application is also constrained to an extent because of the toxicity of Be.

Role of Alloying Elements on GFA
Alloying additions have been widely applied to develop new metallic crystalline materials and optimize the properties of these alloys. Recently, this technique was proved to be a simple but effective way to improve the GFA of BMGs [122][123][124]. In this section, we will focus on the effect of alloying on the GFA of Ti-based BMGs. Table 3 summarizes the effects of different alloying elements on the thermal stability (reflected by ∆Tx) and GFA of Ti-based BMGs [39,49,52,54,57,79,87,[90][91][92]99,[105][106][107][108]110,117,[125][126][127][128][129][130]. together with high GFA, which favors their application as high performance structural materials. However, their application is also constrained to an extent because of the toxicity of Be.

The Additions of Metalloid Elements
Metalloid elements (such as O, N, B, and Si) have a strong affinity with Ti and can be absorbed during the preparation process of Ti-based BMGs. Therefore, it is necessary to investigate the effect of metalloid elements on the GFA of Ti-based BMGs. It was reported that a high oxygen concentration has a detrimental effect on glass formation for Zr-based BMGs [131,132]. As Ti possesses similar properties as Zr and easily reacts with oxygen, it can be inferred that oxygen introduced from the raw materials or the low vacuum may have a negative effect on GFA of Ti-based BMGs. Experimental results indicate that the GFA of Ti 42.5 Cu 40 Zr 10 Ni 5 Sn 2.5 is sensitive to the nitrogen doping level [125]; with the addition of less than 0.1 atom % nitrogen, the GFA is improved because of the suppression of the formation of a competing eutectic structure. However, if the nitrogen concentration exceeds 0.1 atom %, the formation of quasicrystals can be promoted, and the GFA is deteriorated. A minor Si addition also shows a great effect on the thermal stability and GFA of Ti-based BMGs. For example, a 2 atom % Si addition to Ti-Zr-Cu-Pd BMG can extend the ∆T x from 50 K to 65 K, indicating an improvement in the thermal stability [49]. Substituting 1 atom % Ti with Si in a Ti 42.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 alloy increases the critical diameter from 2 mm to 5 mm [99]. Because of the relatively small atomic size, the minor amount of Si may facilitate the formation of a denser atomic structure and stabilize the supercooled liquid. However, as Si exhibits large negative heats of mixing against most constituent elements of Ti-based BMGs (e.g., ∆H Si-Ti = −66 kJ/mol), an excessive addition of Si is detrimental to the GFA due to the formation of intermetallic compounds.

The Additions of Metallic Elements
The intermediate metallic atoms such as Fe, Ni, Cu, Co, and Nb have been widely used as alloying elements in various alloy systems. It was reported that only when the alloying quantities exceed 5 atom % have they been shown to be beneficial in bulk glass formation [123]. For Ti-based BMGs, this argument is supported by some examples listed in Table 3. For example, in order to improve the GFA of a Ti 41 Zr 25 Be 34 alloy, the alloying quantities of Fe, Al, Cu, Ni, or Cr have to be over 5 atom % [79,[87][88][89][90][91][92]. However, there are contrary examples. According to Guo et al.'s research [52], the optimum Ni content in the Ti 40 Zr 25 Cu 15−x Ni x Be 20 system is no more than 3 atom %. Moreover, the GFA of Ti-Zr-Cu-Pd BMG can be markedly improved by the addition of only 1 atom % Co, and the critical diameter is increased from 3 mm to 5 mm [117].
The addition of large atoms (e.g., Zr, Sn, and Sc) has been proved to be beneficial in increasing the atomic size difference and improving the GFA of Ti-based BMGs. For instance, Sn is an effective alloying element to developing Pd-and Be-free Ti-based BMGs with high GFA. By replacing 5 atom % of Cu with Sn in a Ti 41.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 Si 1 alloy, the supercooled liquid region can be extended by 29 K, and the critical diameter is also drastically enhanced from 2 mm to 6 mm [54]. Moreover, Sc has been found to be able to not only alleviate the harmful effect of oxygen impurity but also influence the formation of topological and chemical short-range orders. With the addition of 2 atom % Sc, the critical diameter of the Ti 47 Cu 40 Zr 7.5 Fe 2.5 Sn 2 Si 1 alloy is increased from 3 mm to 6 mm [57].
Among all the metallic elements, Be possesses the smallest atomic size. As shown in Table 1, the Be-containing Ti-based BMGs possess higher GFA compared with Be-free Ti-based BMGs, indicating that the addition of Be is effective in enhancing the GFA of Ti-based BMGs. Moreover, the addition of Be also increases the specific strength of Ti-based BMGs. As Be is a toxic element, it is hoped that the GFA of Ti-based BMGs can be significantly enhanced with minimal Be content. However, the Be content in Ti-based BMGs is always over 5 atom %. Little work has been done on the effect of the minor addition of Be (<2 atom %) on the GFA of Ti-based BMG forming systems.

The Additions of Rare-Earth Elements
Rare-earth elements possess unique physical and chemical properties, so they have been widely used as minor additions for metallic materials. Yttrium is the most widely used real earth element to tailor the properties of BMGs [133,134]. It was found that the addition of 0.5 atom % yttrium can effectively increase the GFA of Ti 40 Zr 25 Be 20 Cu 12 Ni 3 alloys and enables the formation of 5 mm-diameter glassy rods using low purity raw materials [106,127]. It is well known that oxygen is suspected to form titanium oxide at higher temperatures, which acts as crystallization nuclei. A small and proper amount of yttrium addition can scavenge oxygen from the supercooled liquid to suppress the precipitation of the Laves phase and lower the liquidus temperature. The large atomic size of yttrium also plays an important role in increasing the atomic packing efficiency and improving the GFA.

Prediction of GFA of Ti-Based BMGs
The search for novel BMGs is still mainly based on trial-and-error although a few empirical guides (e.g., Inoue's three rules) have been proposed. It is important to develop a simple and unified criterion that can characterize the GFA of BMGs. The GFA of BMGs is directly reflected by the critical cooling rate R c and the critical diameter t max . However, R c measurement is complex, while t max is affected by the preparation conditions. Therefore, researchers have established GFA parameters or criteria based on the characteristic temperatures (e.g., T g , T x , T m , and T l ), which can be determined by DSC, such as ∆T x , T rg , and γ, mentioned at the beginning of this section. These parameters are good in practicability but cannot intrinsically reflect the origin of GFA.
The GFA is an inherent property and should relate to the intrinsic factors of BMGs. Thus, it is important to determine the key factors influencing the GFA. Much research shows that atomic size differences between the constituent elements and the heat of mixing play important roles on the glass formation of BMGs [135][136][137]. Recently, it has been found that the electronegativity difference also influences the GFA of Al- [138,139], Fe- [140], and Ti-based BMGs [90][91][92][93]. In order to evaluate the GFA of Ti-based BMGs, Zhao et al. [93] calculated the parameters including the atomic size difference δ, electronegativity difference ∆x, and the enthalpy of mixing ∆H mix for typical Ti-based BMGs. As shown in Figure 8, the high GFA of Ti-based BMGs (t max ≥ 10 mm) requires an optimal combination of atomic size difference δ, electronegativity difference ∆x, and the enthalpy of mixing ∆H mix : 0.1176 ≤ δ ≤ 0.1333, 0.1194 ≤ ∆x ≤ 0.1837, and −23.81 kJ/mol ≤ ∆H mix ≤ −33.15 kJ/mol. These rules provide new insight and valuable guidance for the future development of Ti-based BMGs with higher GFA.
Metals 2016, 6, 264 2 of 59 element to tailor the properties of BMGs [133,134]. It was found that the addition of 0.5 atom % yttrium can effectively increase the GFA of Ti40Zr25Be20Cu12Ni3 alloys and enables the formation of 5 mm-diameter glassy rods using low purity raw materials [106,127]. It is well known that oxygen is suspected to form titanium oxide at higher temperatures, which acts as crystallization nuclei. A small and proper amount of yttrium addition can scavenge oxygen from the supercooled liquid to suppress the precipitation of the Laves phase and lower the liquidus temperature. The large atomic size of yttrium also plays an important role in increasing the atomic packing efficiency and improving the GFA.

Prediction of GFA of Ti-Based BMGs
The search for novel BMGs is still mainly based on trial-and-error although a few empirical guides (e.g., Inoue's three rules) have been proposed. It is important to develop a simple and unified criterion that can characterize the GFA of BMGs. The GFA of BMGs is directly reflected by the critical cooling rate Rc and the critical diameter tmax. However, Rc measurement is complex, while tmax is affected by the preparation conditions. Therefore, researchers have established GFA parameters or criteria based on the characteristic temperatures (e.g., Tg, Tx, Tm, and Tl), which can be determined by DSC, such as ∆Tx, Trg, and γ, mentioned at the beginning of this section. These parameters are good in practicability but cannot intrinsically reflect the origin of GFA.
The GFA is an inherent property and should relate to the intrinsic factors of BMGs. Thus, it is important to determine the key factors influencing the GFA. Much research shows that atomic size differences between the constituent elements and the heat of mixing play important roles on the glass formation of BMGs [135][136][137]. Recently, it has been found that the electronegativity difference also influences the GFA of Al- [138,139], Fe- [140], and Ti-based BMGs [90][91][92][93]. In order to evaluate the GFA of Ti-based BMGs, Zhao et al. [93] calculated the parameters including the atomic size difference δ, electronegativity difference ∆x, and the enthalpy of mixing ∆Hmix for typical Ti-based BMGs. As shown in Figure 8, the high GFA of Ti-based BMGs (tmax ≥ 10 mm) requires an optimal combination of atomic size difference δ, electronegativity difference ∆x, and the enthalpy of mixing ∆Hmix: 0.1176 ≤ δ ≤ 0.1333, 0.1194 ≤ ∆x ≤ 0.1837, and −23.81 kJ/mol ≤ ∆Hmix ≤ −33.15 kJ/mol. These rules provide new insight and valuable guidance for the future development of Ti-based BMGs with higher GFA.

Elastic Properties
Compared with crystalline Ti alloys, Ti-based BMGs exhibit outstanding mechanical propertie ch as large elastic limit, higher specific strength, and higher hardness [141][142][143]. Consequentl -based BMGs can be exploited for structural applications. However, as other BMGs, Ti-base Gs also have drawbacks that make their use for engineering applications appear challenging.
. Elastic Properties Figure 9 summarizes the relationship between the Young's modulus and tensile fractur ength of typical BMGs together with classic engineering materials [144]. Table 4  Compared wit nventional Ti crystalline alloys, Ti-based BMGs possess a lower Young's modulus but highe cture strength and larger elastic strain (close to 2.0%). The elastic strain energy of Ti-BMGs ger than 20.0 MJ/m 2 (e.g., Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 BMG), which is more than eight times that o e best spring steel [148]. There is also a correlation between the elastic properties (e.g., Young odulus E and Poisson's ratio ν) and room temperature plasticity of BMGs. It was reported th Gs with a higher Poisson's ratio (e.g., >0.32) may possess larger plasticity [149,150]. In this sens -based BMGs can be classified as "ductile" because of the relatively high Poisson's ratio.

Strength and Ductility
As a typical class of light metal-based BMGs, the development of Ti-based BMGs has been closely linked with high specific strength. Table 4 lists the density and compressive mechanical properties of typical Ti-based BMGs together with other BMGs and crystalline lightweight alloys [40,41,45,47,49,[52][53][54][55][56][57]72,[79][80][81][82]84,[86][87][88][89][90][91][92][93][94][95][99][100][101][102][103][104][105][106]110,126,[145][146][147]. As shown in Table 4, Ti-based BMGs exhibit high yield strengths of 1700-2300 MPa, which is the same level as that of Zr-based BMGs and much higher than that of other light metal-based BMGs (e.g., Mg-and Al-based BMGs). For comparison, the yield strength of the Ti-6Al-4V alloy is 825-895 MPa, which is only about half that of Ti-based BMGs. Although the density of Ti is higher compared with Mg and Al, the specific strength σ c of Ti-based BMGs is significantly higher, especially Be-containing Ti-based BMGs that possess densities of less than 5 g/cm 3 and high specific strength over 4 × 10 5 N·m/kg. Figure 10 shows the Ashby diagram of strength versus density for engineering materials [29]. It is clear that Ti-based BMGs possess higher specific strength than that of most other engineering materials. In order to further enhance the specific strength of Ti-based BMGs, the content of heavy constituent elements should be decreased without sacrificing the GFA. Unlike Fe-and Mg-based BMGs, which are completely brittle and fracture via fragmentation even under compression [151][152][153], most Ti-based BMGs exhibit a certain plastic strain and fracture mainly by shearing. This is mainly because of the relatively high Poisson's ratio of Ti and main constituent elements (e.g., Cu, Zr, and Pd). However, compared with Pdand Zr-based BMGs, the room temperature plasticity of Ti-based is still relatively low.

Strength and Ductility
As a typical class of light metal-based BMGs, the development of Ti-based BMGs has been closely linked with high specific strength. Table 4 lists the density and compressive mechanical properties of typical Ti-based BMGs together with other BMGs and crystalline lightweight alloys [40,41,45,47,49,[52][53][54][55][56][57]72,[79][80][81][82]84,[86][87][88][89][90][91][92][93][94]95,99,[100][101][102][103][104][105][106]110,126,[145][146][147]. As shown in Table 4, Ti-based BMGs exhibit high yield strengths of 1700-2300 MPa, which is the same level as that of Zr-based BMGs and much higher than that of other light metal-based BMGs (e.g., Mg-and Al-based BMGs). For comparison, the yield strength of the Ti-6Al-4V alloy is 825-895 MPa, which is only about half that of Ti-based BMGs. Although the density of Ti is higher compared with Mg and Al, the specific strength σc of Ti-based BMGs is significantly higher, especially Be-containing Ti-based BMGs that possess densities of less than 5 g/cm 3 and high specific strength over 4 × 10 5 N·m/kg. Figure 10 shows the Ashby diagram of strength versus density for engineering materials [29]. It is clear that Ti-based BMGs possess higher specific strength than that of most other engineering materials. In order to further enhance the specific strength of Ti-based BMGs, the content of heavy constituent elements should be decreased without sacrificing the GFA. Unlike Feand Mg-based BMGs, which are completely brittle and fracture via fragmentation even under compression [151][152][153], most Ti-based BMGs exhibit a certain plastic strain and fracture mainly by shearing. This is mainly because of the relatively high Poisson's ratio of Ti and main constituent elements (e.g., Cu, Zr, and Pd). However, compared with Pd-and Zr-based BMGs, the room temperature plasticity of Ti-based is still relatively low. Some scholars have also investigated the tensile properties of Ti-based BMGs. Figure 11 shows stress-strain curves of Ti41.5Cu42.5Ni7.5Zr2.5Hf5Si1 BMG obtained by tension and compression tests [99]. It was found that the tensile strength is slightly lower than the compressive strength. Moreover, although this Ti-based BMG exhibits certain ductility under compression, no plastic deformation is observed during the tension test. At room temperature, the plastic deformation of BMGs is dominated by localized shear bands. Under compression, the propagation of the primary shear band is restricted by the friction and confinement at the sample-loading platen surface. Then, multiple shear bands form and interact, leading to the substantial development of plastic deformation. However, this effect is not available under tension. Under the unconfined tensile loading, once a shear band penetrates the sample and bears the entire load, catastrophic fracture Some scholars have also investigated the tensile properties of Ti-based BMGs. Figure 11 shows stress-strain curves of Ti 41.5 Cu 42.5 Ni 7.5 Zr 2.5 Hf 5 Si 1 BMG obtained by tension and compression tests [99]. It was found that the tensile strength is slightly lower than the compressive strength. Moreover, although this Ti-based BMG exhibits certain ductility under compression, no plastic deformation is observed during the tension test. At room temperature, the plastic deformation of BMGs is dominated by localized shear bands. Under compression, the propagation of the primary shear band is restricted by the friction and confinement at the sample-loading platen surface. Then, multiple shear bands form and interact, leading to the substantial development of plastic deformation. However, this effect is not available under tension. Under the unconfined tensile loading, once a shear band penetrates the sample and bears the entire load, catastrophic fracture will immediately occur. Therefore, it is more difficult to obtain tensile ductility for BMGs including Ti-based BMGs. Few Ti-based BMG matrix composites have exhibited tensile plasticity [154][155][156]. Developing Ti-based BMGs with tensile ductility is still a challenge. Ti-based BMGs. Few Ti-based BMG matrix composites have exhibited tensile plasticity [154][155][156]. Developing Ti-based BMGs with tensile ductility is still a challenge. It is worthy noticing that sample size plays an important role on the plasticity of Ti-based BMGs. Huang et al. reported a "smaller is softer" phenomenon in Ti40Zr25Ni3Cu12Be20 BMG under compression [157]. According to their experiments, Ti-based BMG samples with a smaller diameter exhibit a larger plasticity, while the strength does not markedly change. Similar results have also been reported in other BMGs [158][159][160][161][162][163]. The size effect has been explained from the viewpoints of free volume content [157,162,163], flaw sensitivity [159], plastic zone [160], and elastic energy dissipation [161]. Except for the sample diameter, the aspect ratio is another important size factor that affects the ductility of BMGs [164]. By decreasing the aspect ratio of BMG samples, the geometrical constrain effect becomes more obvious and the compressive plasticity can be dramatically improved.
It was also found that the compressive mechanical properties of Ti-based strongly depends on the service temperature. A transition from ductile to brittle behavior in Ti40Zr25Ni3Cu12Be20 BMG at cryogenic temperatures has been reported by Huang et al. [165]. Because of the cryogenic surroundings, the diffusion of the atoms slows down, and the nucleation and growth of nanocrystals is suppressed, resulting in an improvement of both strength and plasticity.

Fracture Toughness
The lack of room temperature plasticity has been considered as the Achilles' heel of BMGs. In this section, typical techniques for improving the room temperature plasticity of Ti-based BMGs have been introduced.

Poisson's Ratio Control Strategy
Because of the correlation between the Poisson's ratio and plasticity, BMGs with good plasticity can be developed via composition optimization based on Poisson's ratio control strategy [166,167]. A simple but effective way is replacing one of the constituent elements with alloying elements that possess a higher Poisson's ratio. Gong et al. investigated the alloying effect on the compressive plasticity of Ti-Zr-Be BMGs [87][88][89][90][91][92][93][94]. Among the constituent elements in the Ti-Zr-Be system, Be possesses a very low Poisson's ratio of 0.032, while Zr possesses a high Poisson's ratio of 0.34. Therefore, it is not surprising that replacing Be by alloying elements (e.g., Fe, Cu, Ni, and Al) in the Ti-Zr-Be BMG improves the plasticity, while the substitution of alloying elements to Zr degrades room temperature plasticity. Park et al. [53] also reported that partial substitution of Zr by It is worthy noticing that sample size plays an important role on the plasticity of Ti-based BMGs. Huang et al. reported a "smaller is softer" phenomenon in Ti 40 Zr 25 Ni 3 Cu 12 Be 20 BMG under compression [157]. According to their experiments, Ti-based BMG samples with a smaller diameter exhibit a larger plasticity, while the strength does not markedly change. Similar results have also been reported in other BMGs [158][159][160][161][162][163]. The size effect has been explained from the viewpoints of free volume content [157,162,163], flaw sensitivity [159], plastic zone [160], and elastic energy dissipation [161]. Except for the sample diameter, the aspect ratio is another important size factor that affects the ductility of BMGs [164]. By decreasing the aspect ratio of BMG samples, the geometrical constrain effect becomes more obvious and the compressive plasticity can be dramatically improved.
It was also found that the compressive mechanical properties of Ti-based strongly depends on the service temperature. A transition from ductile to brittle behavior in Ti 40 Zr 25 Ni 3 Cu 12 Be 20 BMG at cryogenic temperatures has been reported by Huang et al. [165]. Because of the cryogenic surroundings, the diffusion of the atoms slows down, and the nucleation and growth of nanocrystals is suppressed, resulting in an improvement of both strength and plasticity.

Fracture Toughness
The lack of room temperature plasticity has been considered as the Achilles' heel of BMGs. In this section, typical techniques for improving the room temperature plasticity of Ti-based BMGs have been introduced.

Poisson's Ratio Control Strategy
Because of the correlation between the Poisson's ratio and plasticity, BMGs with good plasticity can be developed via composition optimization based on Poisson's ratio control strategy [166,167]. A simple but effective way is replacing one of the constituent elements with alloying elements that possess a higher Poisson's ratio. Gong et al. investigated the alloying effect on the compressive plasticity of Ti-Zr-Be BMGs [87][88][89][90][91][92][93][94]. Among the constituent elements in the Ti-Zr-Be system, Be possesses a very low Poisson's ratio of 0.032, while Zr possesses a high Poisson's ratio of 0.34. Therefore, it is not surprising that replacing Be by alloying elements (e.g., Fe, Cu, Ni, and Al) in the Ti-Zr-Be BMG improves the plasticity, while the substitution of alloying elements to Zr degrades room temperature plasticity. Park et al. [53] also reported that partial substitution of Zr by Ti in the Ti-Cu-Ni-Be system increases the plastic strain from 0.7% to 8.3%. The addition of Zr is beneficial for increasing the Poisson's ratio and the shear transformation zone volume, resulting in larger plasticity.

Nanocrystallization
Producing BMG composites reinforced by ductile crystalline phases is believed to be an effective way to overcome the poor room temperature ductility of BMGs [168,169]. Table 5 summarizes the composition, synthesis method, and mechanical properties of typical Ti-based BMG composites [153,[170][171][172][173][174][175][176][177][178][179][180]. It is found that most developed Ti-based BMG composites are of an in situ β-phase dendrite-reinforced type, as β-Ti possesses good ductility and a low modulus compared with hexagonal α-Ti. In general, Ti-based BMG composites exhibit better plasticity compared with Ti-based BMGs. For instance, Hofmann et al. developed Ti-Zr-V-Cu-Be BMG-matrix composites reinforced by a dendritic phase with room temperature tensile ductility over 10% [154,155]. However, the preparation process of BMG-matrix composites is always very complex [181]. The processing parameters should be precisely controlled to obtain a perfect microstructure. Therefore, the preparation of BMG composites puts a greater demand on the equipment. For example, a copper mold casting is the most widely used method of preparing BMGs. However, the microstructure of BMG composites prepared via copper mold casting is not always uniform because of the cooling rate difference. Zhang et al. [178] introduced a new technology named Bridgman solidification, which ensures the uniform microstructure of the prepared BMG composites by precisely controlling the processing parameters, but the corresponding equipment is more complex than traditional arc-melter. Moreover, the enhancement of ductility is always obtained at the price of lower strength. By introducing nanocrystals in situ formed in the glassy matrix, the propagation of primary shear band is disturbed when it reaches the nanocrystals, which act as high energy barriers; then, the primary shear band may be forced to be deflected and branched to initiate new shear bands. As a result, the ductility of BMGs can be improved without sacrificing strength. For example, Park et al. [121] reported a Ti 40 Zr 29 Be 14 Cu 9 Ni 8 BMG with a large plastic strain of~7%. In order to find out the reason of the superior plasticity, they investigated the microstructure and crystallization behavior of this alloy with a high resolution transmission electron microscope (HRTEM) and via differential scanning calorimetry (DSC). It was found that, during the deformation, the nuclei transform to precipitations with a size of several nanometers and disperse into the amorphous matrix, resulting in an improvement in ductility. There are several different ways to introduce nanocrystallization in BMG samples. The first method is composition design. For instance, the addition of alloying elements may promote nanocrystallization of BMGs during the deformation process. Typical examples are Nb for Ti-Zr-Cu-Pd alloys [50] and Si for Ti-Zr-Cu-Ni-Sn alloys [104]. Another widely used method is annealing treatment. Jun et al. [182] reported that the compressive strain of Ti 43.3 Zr 21.7 Ni 7.5 Be 27.5 BMG can be significantly enhanced to 42% after sub-T g annealing. Nanocrystalline phases formed during the annealing were believed to be responsible for the rise of the compressive plastic strain. Moreover, as structural relaxation also occurs during the annealing process, the free volume content of the amorphous matrix decreases, which maintains the high strength of the annealed BMG samples.

Pre-Plastic Deformation
For conventional crystalline alloys, after plastic deformation, the microstructure can be changed (e.g., dislocation density), and the flow behaviors can be quite different. For BMGs, pre-plastic deformation is supposed to introduce microstructural inhomogeneities and induce controlled stress distributions or activate multiple shear bands, which are beneficial to enhancing the room temperature plasticity [183,184]. Huang et al. utilized prior compressive plastic deformation to tune the room temperature plasticity of Ti 40 Zr 25 Ni 3 Cu 12 Be 20 BMG [185]. It was found that, with the increase of prior plastic strain, the plastic strain of the deformed BMG samples first increases and then decreases with the optimized prior plastic strain of 10%. The improvement of compressive plasticity was explained in view of the free volume content. Park et al. [186] investigated the effect of strain-induced internal state modulation created by cold rolling on the compressive plasticity of Ti 40 Zr 25 Ni 8 Cu 9 Be 18 BMG. The compressive plastic strain can be dramatically improved from 1.5% to 14.5% with a 50% thickness reduction (as shown in Figure 12). Because of rolling, a network-like structure of hard and soft regions was found to be introduced uniformly in the BMG samples, which is beneficial to the uniform distribution of multiple shear bands.

Pre-Plastic Deformation
For conventional crystalline alloys, after plastic deformation, the microstructure can be changed (e.g., dislocation density), and the flow behaviors can be quite different. For BMGs, pre-plastic deformation is supposed to introduce microstructural inhomogeneities and induce controlled stress distributions or activate multiple shear bands, which are beneficial to enhancing the room temperature plasticity [183,184]. Huang et al. utilized prior compressive plastic deformation to tune the room temperature plasticity of Ti40Zr25Ni3Cu12Be20 BMG [185]. It was found that, with the increase of prior plastic strain, the plastic strain of the deformed BMG samples first increases and then decreases with the optimized prior plastic strain of 10%. The improvement of compressive plasticity was explained in view of the free volume content. Park et al. [186] investigated the effect of strain-induced internal state modulation created by cold rolling on the compressive plasticity of Ti40Zr25Ni8Cu9Be18 BMG. The compressive plastic strain can be dramatically improved from 1.5% to 14.5% with a 50% thickness reduction (as shown in Figure 12). Because of rolling, a network-like structure of hard and soft regions was found to be introduced uniformly in the BMG samples, which is beneficial to the uniform distribution of multiple shear bands. Figure 12. Compressive stress-strain curves of as-cast and cold-rolled Ti40Zr25Ni8Cu9Be18 BMG samples. Reproduced with permission from [186]. Copyright 2012, Elsevier.

Surface Treatment
Surface treatment has been widely applied to improve the room temperature ductility of BMGs by retarding the initiation and propagation of the shear band [187]. It has been reported that the room temperature plasticity of Zr-based BMGs can be improved by various surface modification technologies including shot peening [188] and surface coating [189,190]. As Ti-based BMGs possess properties similar to Zr-based BMGs, these strategies can also be adopted to improve the room temperature plasticity of Ti-based BMGs. Fan et al. [191] proposed a novel technology called surface mechanical attrition treatment (SMAT) to improve the plasticity of BMGs. By surface crystallization, isolated crystallite islands are formed in the top surface layer, which act as the obstacles to restrict the localization of shear bands and avoid the development of cracks. With the optimization of SMAT processing parameters, the plastic strain of Ti40Zr25Ni3Cu12Be20 BMG can be enhanced to 3.78%, which is nearly four times that of the untreated sample.

Fracture Toughness
For engineering materials, damage tolerance is a very important mechanical design parameter. As BMGs usually possess high strength but a lack of plasticity, fracture toughness Kc, which assesses a material's resistance to crack propagation and can be measured by the energy needed to

Surface Treatment
Surface treatment has been widely applied to improve the room temperature ductility of BMGs by retarding the initiation and propagation of the shear band [187]. It has been reported that the room temperature plasticity of Zr-based BMGs can be improved by various surface modification technologies including shot peening [188] and surface coating [189,190]. As Ti-based BMGs possess properties similar to Zr-based BMGs, these strategies can also be adopted to improve the room temperature plasticity of Ti-based BMGs. Fan et al. [191] proposed a novel technology called surface mechanical attrition treatment (SMAT) to improve the plasticity of BMGs. By surface crystallization, isolated crystallite islands are formed in the top surface layer, which act as the obstacles to restrict the localization of shear bands and avoid the development of cracks. With the optimization of SMAT processing parameters, the plastic strain of Ti 40 Zr 25 Ni 3 Cu 12 Be 20 BMG can be enhanced to 3.78%, which is nearly four times that of the untreated sample.

Fracture Toughness
For engineering materials, damage tolerance is a very important mechanical design parameter. As BMGs usually possess high strength but a lack of plasticity, fracture toughness K c , which assesses a material's resistance to crack propagation and can be measured by the energy needed to cause fracture, is a more important indicator of mechanical performance compared with yield strength [192]. The fracture toughness of BMGs strongly depends on the alloy composition. For instance, some brittle BMGs (e.g., Mg-and Fe-based BMGs) exhibit an ideally brittle behavior (K c < 10 MPa·m 1/2 ) [193,194], while Pd-based BMGs are remarkably tough (K c~2 00 MPa·m 1/2 ) [195]. Regarding the fracture toughness of Ti-based BMGs, it has been reported that the K c of Ti 50 Ni 24 Cu 20 B 1 Si 2 Sn 3 alloy is 50 MPa·m 1/2 [153]. Gu et al. [196] investigated the effects of changes in sample dimensions and the stress state on the fracture toughness of Ti 40 Zr 25 Cu 12 Ni 3 Be 20 BMG. It was found that the measured fracture toughness ranges from 98.6 to 126.3 MPa·m 1/2 . The fatigue pre-cracked Ti 40 Zr 25 Cu 12 Ni 3 Be 20 sample exhibited a slightly higher toughness than that of the notched sample. The notched plate sample and the notched rod sample were also found to have different fracture toughness values. In summary, the fracture toughness of Ti-based BMGs is higher than that of brittle BMGs but lower than that of Pd-BMGs, which is comparable to those for age-hardened Al-based alloys (24-36 MPa·m 1/2 ), commercial Ti crystalline alloys (K c = 54-98 MPa·m 1/2 ), and 4340 high strength steels (K c~5 0 MPa·m 1/2 ). However, it should be noticed that, even for the same alloy, a wide scatter in K c has been reported. The cooling rate during the preparation process, the stress state, the impurity inclusions, the sample size, and the sharpness of notch/crack fabrication are all possible reasons. In order to reduce the extrinsic effect, Chen et al. [197] proposed a novel sample preparation method for fracture toughness test via the thermoplastic forming of BMGs and Si photolithography. Using this strategy, they measured the notch toughness of 86 ± 3 MPa·m 1/2 for a Ti 41 Zr 25 Be 28 Fe 6 BMG. The small scatter is believed to reflect an intrinsic origin rather than extrinsic sample preparation effects.

Fatigue Properties
The fatigue behavior is one of the dynamic mechanical properties and also a very important characteristic for the applications of BMGs as structural materials. Yamaura et al. [198] and Fujita et al. [199] have investigated the fatigue behavior of Ti 40 Zr 10 Cu 34 Pd 14 Sn 2 and Ti 41.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 Si 1 alloys, respectively. Table 6 lists the fatigue properties of Ti-based BMGs together with other typical BMGs and Ti-6Al-4V alloy [198][199][200][201][202]. Compared with the Ti-6Al-4V alloy, Ti 40 Zr 10 Cu 34 Pd 14 Sn 2 BMG, which is a typical representative of biomedical Ti-based BMG, exhibits higher fatigue strength although the fatigue ratio is lower. Moreover, it is worthy noticing that the Ti 41.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 Si 1 BMG shows a fatigue ratio of 0.79, which is significantly higher than that of other BMGs such as Zr-, Cu-, Co-, and Fe-based BMGs listed in Table 6. The fatigue strength of Ti 41.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 Si 1 BMG is up to 1610 MPa, which is much larger than that of Zr-based BMGs and close to the value of Ni-based BMGs. The superior fatigue properties of Ti 41.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 Si 1 BMG are attributed to the dispersion of nano-scaled crystalline particles in the glassy matrix. The initiation and the growth of shear bands are constrained by the nanocrystals. The shear bands branch, kink, and rotate, resulting in the reduction in the shear stress value near their tips. There have been few reports on the effect of frequency on the fatigue properties of Ti-based BMGs, and this will need to be further studied. Table 6. Fatigue-endurance limits and fatigue ratios of various BMGs [198][199][200][201][202].

Thermoplastic Formability
BMGs usually present superplasticity in their supercooled liquid region with behavior similar to a Newtonian viscosity of conventional glass materials. Because of this unique property, thermoplastic forming (TPF) and patterning have been widely used to precisely fabricate BMG parts on length scales ranging from the nanometer scale to several centimeters [203][204][205]. There are two key factors influencing the thermoplastic formability of BMGs: the temperature-dependent viscosity and the temperature-dependent crystallization time. It was already known that the temperature dependence of the viscosity and the temperature dependence of crystallization time among BMGs vary significantly. Thus, the thermoplastic formability of BMGs strongly depends on the alloy composition. Schroers et al. [206] introduced an experimental method to characterize the thermoplastic formability of different BMGs using the maximum diameter to which the BMG can be deformed for a standardized set of processing parameters. It was found that the S parameter, which is defined based on the characteristic temperatures (S = ∆T x /T l − T g ), is the best indicator for thermoplastic formability of BMGs so far. Table 7 summarizes the S values of developed Ti-based BMGs together with some typical Zr-, Pd-, Pt-, Au-, Fe-, and Mg-based BMGs [39,40,47,54,56,79,84,86,95,[206][207][208][209][210][211][212]. It is found that Ti-based BMGs possess relatively small S values compared with other BMGs, implying a relatively low thermoplastic formability. Moreover, as titanium is a very active element, during the heating process in air, the oxidation is more serious compared with Pd-, Au-, and Pt-based BMGs, which is harmful to the thermoplastic forming [213][214][215]. In order to perform the thermoplastic forming of Ti-based BMGs, especially in air, it is necessary to first attempt composition optimization in order to develop novel Ti-based BMGs with better thermoplastic formability and oxidation resistance. Be alloying may be an effective way as Be-containing Ti-based BMGs possess relatively higher S value (as shown in Table 7). Other strategies, e.g., vibrational loading [216] or introducing a wetting layer [217], are also recommended. Table 7. Thermoplastic formability of typical Ti-based BMGs together with various BMG forming alloys [39,40,47,54,56,79,84,86,95,[206][207][208][209][210][211][212].

The Corrosion Resistance of Ti-Based BMGs
Due to the formation of stable and protective surface oxide film, titanium alloys are inert and resistant to corrosion. Because of the potential application as structural and biomedical materials, the corrosion behavior of Ti-based BMGs in different solutions such as acid, alkaline, salt, and simulated body solutions have been investigated. Compared with conventional Ti alloys, Ti-based BMGs always exhibit better corrosion resistance in different kinds of solutions because of the unique amorphous structure. Figure 13 [110] shows the potentiodynamic polarization curves of Ti 46 Cu 27.5 Zr 11.5 Co 7 Sn 3 Si 1 Ag 4 BMG and Ti-6Al-4V alloys in PBS, 0.9 wt. % NaCl, 1 mol/L HCl, and 1 mol/L NaOH solutions, respectively. It was found that, compared with the commercial Ti-6Al-4V alloy, Ti 46 Cu 27.5 Zr 11.5 Co 7 Sn 3 Si 1 Ag 4 BMG possesses a higher corrosion potential and lower corrosion current density, implying a better corrosion resistance. In general, the corrosion-penetration rates (CPRs) of less than~76 µm/year are considered acceptable for chemical and industrial applications. According to Morrison et al.'s research [218], the CPR of Ti 43.3 Zr 21.7 Ni 7.5 Be 27.5 BMG in a PBS electrolyte at 37 • C is 2.9 ± 2.6 µm/year, which is well within the expected range for corrosion resistant materials and equivalent to, or better than, Zr-based BMGs and 316L stainless steel. Except for PBS, Ti-based BMGs also exhibit good bio-corrosion resistance in other simulated body fluids such as SBF, Ringer's solution, and Hanks' solution [219]. resistant to corrosion. Because of the potential application as structural and biomedical materials, the corrosion behavior of Ti-based BMGs in different solutions such as acid, alkaline, salt, and simulated body solutions have been investigated. Compared with conventional Ti alloys, Ti-based BMGs always exhibit better corrosion resistance in different kinds of solutions because of the unique amorphous structure. Figure 13 [110] shows the potentiodynamic polarization curves of Ti46Cu27.5Zr11.5Co7Sn3Si1Ag4 BMG and Ti-6Al-4V alloys in PBS, 0.9 wt. % NaCl, 1 mol/L HCl, and 1 mol/L NaOH solutions, respectively. It was found that, compared with the commercial Ti-6Al-4V alloy, Ti46Cu27.5Zr11.5Co7Sn3Si1Ag4 BMG possesses a higher corrosion potential and lower corrosion current density, implying a better corrosion resistance. In general, the corrosion-penetration rates (CPRs) of less than ~76 μm/year are considered acceptable for chemical and industrial applications. According to Morrison et al.'s research [218], the CPR of Ti43.3Zr21.7Ni7.5Be27.5 BMG in a PBS electrolyte at 37 °C is 2.9 ± 2.6 μm/year, which is well within the expected range for corrosion resistant materials and equivalent to, or better than, Zr-based BMGs and 316L stainless steel. Except for PBS, Ti-based BMGs also exhibit good bio-corrosion resistance in other simulated body fluids such as SBF, Ringer's solution, and Hanks' solution [219].  Chemical composition also plays an important role on the corrosion resistance of Ti-based BMGs. Qin et al. [220] investigated the effects of different alloying elements on the corrosion behavior of Ti 47.5 Cu 42.5 Ni 7.5 Zr 2.5 BMG in a 0.14 kmol/m 3 NaCl solution and a 0.2 kmol/m 3 phosphate buffer solution with 0.14 kmol/m 3 Cl − ions. It was found that the addition of Nb or Ta significantly improves corrosion resistance. The passive current density of the Nb-or Ta-containing BMGs is between 10 −2 and 10 −3 A/m, which is one order of magnitude lower than that of the base alloy. The addition of Nb or Ta facilitates the enrichment of Ti, and certain amounts of Nb or Ta existing in the surface film, resulting in higher corrosion resistance. The corrosion resistance of the (Ti 40 Zr 10 Cu 38 Pd 12 ) 97 Nb 3 BMG has also been shown to be higher than that of the Ti 40 Zr 10 Cu 38 Pd 12 alloy due to the presence of Nb. The pitting resistance is enhanced by the addition of Nb because of the improvement of the passive layer properties.
The effect of crystallization on the corrosion resistance of Ti-based BMGs has also been studied. Qin et al. [221] investigated the corrosion behavior of the Ti 40 Zr 10 Cu 36 Pd 14 alloy in Hanks' solution in three different conditions: in the as-cast fully amorphous condition, after annealing it for 10 min at 723 K to obtain BMG matrix composite, and after fully crystallizing the sample at 823 K for 10 min. As shown in Figure 14, it was found that the as-cast and partially crystalline samples have lower passive current densities located~10 −2 A/m 2 , markedly lower than that of the commercial Ti-6Al-4V alloy and pure Ti. However, the fully crystallized sample exhibits a much lower pitting potential, implying the least corrosion resistance. The enhancement of the pitting potential of the partially nanocrystalline alloy annealed at 723 K may be due to the formation of the Ti 3 Cu 4 phase. This results in the enrichment of Pd in the matrix, which is helpful in forming a protective passive film. Moreover, because of the large number of interface defects that are expected at the nano scale, the breakdown of the passive film is uniform, which allows the partially crystallized alloy to maintain passivity. Correspondingly, the crystalline phases of the fully crystallized sample are Ti 3 Cu 4 , Ti 2 Pd, and Ti 2 Pd 3 with a larger size. The lower corrosion resistance is mainly attributed to the serious micro-galvanic corrosion between the Cu-rich and Pd-rich phases. In conclusion, both the size and composition of the crystalline phase play important roles in controlling the corrosion behavior of Ti-based alloys.
due to the presence of Nb. The pitting resistance is enhanced by the addition of Nb because of the improvement of the passive layer properties.
The effect of crystallization on the corrosion resistance of Ti-based BMGs has also been studied. Qin et al. [221] investigated the corrosion behavior of the Ti40Zr10Cu36Pd14 alloy in Hanks' solution in three different conditions: in the as-cast fully amorphous condition, after annealing it for 10 min at 723 K to obtain BMG matrix composite, and after fully crystallizing the sample at 823 K for 10 min. As shown in Figure 14, it was found that the as-cast and partially crystalline samples have lower passive current densities located ~10 −2 A/m 2 , markedly lower than that of the commercial Ti-6Al-4V alloy and pure Ti. However, the fully crystallized sample exhibits a much lower pitting potential, implying the least corrosion resistance. The enhancement of the pitting potential of the partially nanocrystalline alloy annealed at 723 K may be due to the formation of the Ti3Cu4 phase. This results in the enrichment of Pd in the matrix, which is helpful in forming a protective passive film. Moreover, because of the large number of interface defects that are expected at the nano scale, the breakdown of the passive film is uniform, which allows the partially crystallized alloy to maintain passivity. Correspondingly, the crystalline phases of the fully crystallized sample are Ti3Cu4, Ti2Pd, and Ti2Pd3 with a larger size. The lower corrosion resistance is mainly attributed to the serious micro-galvanic corrosion between the Cu-rich and Pd-rich phases.
In conclusion, both the size and composition of the crystalline phase play important roles in controlling the corrosion behavior of Ti-based alloys.

Biocompatibility of Ti-Based BMGs
Titanium and its alloys are believed to be excellent implant materials in the fields of trauma and orthopedic surgery. Compared with conventional Ti alloys, Ti-based BMGs are more suitable for biomedical applications for the following reasons [222,223]: (1) high strength and hardness, which may lead to good loadbearing capability and high wear resistance; (2) low Young's (elastic)

Biocompatibility of Ti-Based BMGs
Titanium and its alloys are believed to be excellent implant materials in the fields of trauma and orthopedic surgery. Compared with conventional Ti alloys, Ti-based BMGs are more suitable for biomedical applications for the following reasons [222,223]: (1) high strength and hardness, which may lead to good loadbearing capability and high wear resistance; (2) low Young's (elastic) modulus, which implies better load transfer to the surrounding bone and a potential for mitigating stress-shielding; (3) excellent corrosion resistance, resulting in reduced ion release in the human body environment; and (4) excellent thermoplastic formability that allows for the production of precise and versatile geometries on different length scales, which is of great interest for biomaterials processing.
Young's modulus is one of the most important performance indicators of biomedical materials. According to Table 4, the Young's modulus value of Ti-based BMGs is 80-110 GPa, which is smaller than commercial biomedical materials such as biomedical 316L stainless steel (230 GPa), Co-Cr alloys (230 GPa), and the Ti-6Al-4V alloy (110)(111)(112)(113)(114). However, the Young's modulus of human bone tissue is only 20 GPa, which is still much smaller than that of Ti-based BMGs. Some developed β-type titanium alloys (e.g., Ti-Nb-Ta-Zr) possess relatively small Young's modulus values of~40 GPa [224], which are close to that of human bone tissue. In order to minimize the stress shielding effect, it is still necessary to decrease the Young's modulus value of biomedical Ti-based BMGs. In order to improve the GFA, Ti-based BMGs always contain several alloying elements. As the Young's modulus of BMGs shows a rough correlation with a weighted average of the Young's modulus for the constituent elements [225], the elements with a low Young's modulus value should be selected preferentially as constituent elements for biomedical Ti-based BMGs. On the other hand, the biological toxicity of different alloying elements has also been considered. Calin et al. investigated the biological safety of the constituent elements in Ti-based BMGs. Based on their research, it was concluded that Ti, B, Mg, Si, P, Ca, Sr, Zr, Nb, Mo, Pd, In, Sn, Ta, Pt, and Au are biocompatible elements, while harmful elements include Be, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ag [223]. In this sense, Ti-based BMGs for biomedical use should only contain biocompatible and low-modulus elements. According to this principle, a series of Ti-Zr-Si-(Nb, Ta) metallic glasses that contain no harmful elements has been successfully developed [226,227]. However, the GFA of this class of Ti-based alloys is too poor to form BMG samples. No Ti-based BMGs that do not contain either Ni, Cu, or Be has been reported. It is known that Cu ions are necessary nutrients for the human body, but excess Cu ions may lead to biological toxicity. An amount of 10-12 mg per day may possibly be the maximally safe concentration, which is suggested by the World Health Organization. According to Huang et al.'s research, after 1 day of immersion in a cell culture medium (DMEM), the concentration of Cu ions released from Zr 50 Cu 43 Al 7 BMGs was below 50 ppb [228]. The addition of Cu in Ti-based BMGs results in no bio-toxicity. Similarly, a small amount of Fe, Ag, or both may also be added to Ti-based BMGs. Thus, Ti-Zr-Cu-Pd-(Sn, Nb) [47,49,86] and Ti-Zr-Cu-Fe-Sn-Si-(Ag) BMGs [56,57], potential biomaterials, were successfully discovered.
Biocompatibility is described as the ability of the material to exist in contact with tissues of the human body without causing a non-acceptable degree of harm. As a typical class of non-degradable BMGs, the biocompatibility of Ti-based BMGs can be mainly related to its cell-biological activity in the body environment. Wang et al. [229] studied the biocompatibility of Ti 41.5 Zr 2.5 Hf 5 Cu 37.5 Ni 7.5 Si 1 Sn 5 BMG and pure Ti via in vitro cell response and in vivo animal implants. It was found that, although the cell viability is relatively lower due to the release of Cu ions, Ti 41.5 Zr 2.5 Hf 5 Cu 37.5 Ni 7.5 Si 1 Sn 5 BMG shows good compatibility from in vivo evaluation for one month implantation compared with pure Ti. As shown in Figure 15, both the pure Ti and Ti 41.5 Zr 2.5 Hf 5 Cu 37.5 Ni 7.5 Si 1 Sn 5 BMG are well integrated with the bone tissue, and the gap between the bone tissue is no more than 5 µm. However, because of the high content of Cu and the addition of allergenic element Ni, Ti 41.5 Zr 2.5 Hf 5 Cu 37.5 Ni 7.5 Si 1 Sn 5 is not an ideal biomaterial, and long-term implantation is still needed for further investigation. Liu et al. [55] investigated the cytocompatibility of Ti-Zr-Cu-Fe-Sn-Si BMGs and the Ti-6Al-4V alloy via adopted mouse MC3Ts-E1 pre-osteoblast. The results demonstrated that the cell viabilities in the Ti-Zr-Cu-Fe-Sn-Si BMG extracts are slightly higher than that in the Ti-6Al-4V alloy extract. Kokubun et al. [230] conducted a thorough in vivo evaluation of biocompatibilities of Ti 40 Zr 10 Cu 34 Pd 14 Sn 2 BMG. They implanted bars of Ti 40 Zr 10 Cu 34 Pd 14 Sn 2 BMG in the femoral bone of rats. A typical macroscopic view of the Ti 40 Zr 10 Cu 34 Pd 14 Sn 2 bar at 12 weeks after implantation is shown in Figure 16. No inflammatory reaction, implant dislocation, or loosening was observed, implying that Ti 40 Zr 10 Cu 34 Pd 14 Sn 2 BMG has an excellent biocompatibility and integration to bone tissue. As shown in Figure 17, the histological images revealed that both the BMG sample and the Ti sample were well covered by surrounding bone tissue. No abnormal finding in surrounding bone tissue was observed, and no component ion diffusion was detected up to 3 months post-implantation. tissue. As shown in Figure 17, the histological images revealed that both the BMG sample and the Ti sample were well covered by surrounding bone tissue. No abnormal finding in surrounding bone tissue was observed, and no component ion diffusion was detected up to 3 months post-implantation. Figure 15. Implantation of Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 BMG and pure Ti samples. (a) BMG sample; (b) representative X-ray images for the implants; (c,d) representative histological images stained by methylene blue after 1 month of implantation [229]. Reproduced with permission from [229]. Copyright 2013, Elsevier.  (b) representative X-ray images for the implants; (c,d) representative histological images stained by methylene blue after 1 month of implantation [229]. Reproduced with permission from [229]. Copyright 2013, Elsevier.
Metals 2016, 6, 264 7 of 59 tissue. As shown in Figure 17, the histological images revealed that both the BMG sample and the Ti sample were well covered by surrounding bone tissue. No abnormal finding in surrounding bone tissue was observed, and no component ion diffusion was detected up to 3 months post-implantation. Figure 15. Implantation of Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 BMG and pure Ti samples. (a) BMG sample; (b) representative X-ray images for the implants; (c,d) representative histological images stained by methylene blue after 1 month of implantation [229]. Reproduced with permission from [229]. Copyright 2013, Elsevier.   Based on the above experimental results, the possibility and efficacy of its use for bone implant is preliminarily confirmed. However, further long-term implant study is still recommended. Another issue is that the number of developed Ti-based BMGs for biomedical applications is very limited. It is still necessary to explore novel biomedical Ti-based BMGs with improved GFA and good biocompatibility.

Applications of Ti-Based BMGs
Ti-based BMGs have been applied to fabricate a sensing tube in a Coriolis flow meter, which is used to measure the Coriolis force of a liquid or gas that is flowing inside the pipe subjected to reinforced oscillation (as shown in Figure 18) [231,232]. A Ti-based BMG with the composition of Ti50Cu25Ni15Zr5Sn5 was selected to fabricate the metallic glass tubes using a copper mold suction casting technique. The sensitivity of the Coriolis flow meter using the Ti-based glassy alloy pipe was reported to be 28-53 times higher than that of a conventional Coriolis flow meter using a SUS316 pipe. The significant improvement in sensitivity allows the possibility of the use of a new type of Coriolis flow meter in various industries such as fossil-fuel, chemical, environmental, semiconductor, and medical science fields. Objective magnification of upper images is ×4, and that for lower images is ×20. Reproduced with permission from [230]. Copyright 2015, IOS press. Based on the above experimental results, the possibility and efficacy of its use for bone implant is preliminarily confirmed. However, further long-term implant study is still recommended. Another issue is that the number of developed Ti-based BMGs for biomedical applications is very limited. It is still necessary to explore novel biomedical Ti-based BMGs with improved GFA and good biocompatibility.

Applications of Ti-Based BMGs
Ti-based BMGs have been applied to fabricate a sensing tube in a Coriolis flow meter, which is used to measure the Coriolis force of a liquid or gas that is flowing inside the pipe subjected to reinforced oscillation (as shown in Figure 18) [231,232]. A Ti-based BMG with the composition of Ti 50 Cu 25 Ni 15 Zr 5 Sn 5 was selected to fabricate the metallic glass tubes using a copper mold suction casting technique. The sensitivity of the Coriolis flow meter using the Ti-based glassy alloy pipe was reported to be 28-53 times higher than that of a conventional Coriolis flow meter using a SUS316 pipe. The significant improvement in sensitivity allows the possibility of the use of a new type of Coriolis flow meter in various industries such as fossil-fuel, chemical, environmental, semiconductor, and medical science fields. Figure 18. An outer appearance of the self-made Coriolis flow meter using the Ti-Cu-Ni-Zr-Sn BMG pipe. Reproduced with permission from [232]. Copyright 2011, Elsevier.
Ti-based BMGs are also potentially useful in many other serious applications. It was reported that Zr-based BMGs have been used to fabricate micro-geared motor parts [231][232][233][234], pressure sensors [231,235,236], watch cases [204,237], cell phone cases [204,237], and sports goods (e.g., golf plate) [238]. Replacing Zr-based BMGs by Ti-based BMGs is beneficial for enhancing the specific strength and reducing the cost. Because of the good corrosion resistance and biocompatibility, Ti-based BMGs are also suitable for medical components such as prosthetic implants and surgical instruments (e.g., the surgical razor and micro-surgery scissors). Aerospace engineering is another important applied field of Ti-based BMGs. Ti-based BMGs possess outstanding mechanical properties such as high specific strength, high hardness, and large elastic elongation. Moreover, although BMGs are metastable in thermodynamics, Ti-based BMGs exhibit high space environment applicability. According to Wang et al.'s experimental results [239], the microstructure, thermodynamics, and mechanical properties of Ti-based BMGs are all relatively stable after simulated thermal cycling treatment in vacuum in a temperature range of −196 °C to 150 °C. The effect of atomic oxygen (AO) on the BMGs has been also studied in a plasma type ground-based AO effect simulation facility. The results show that the structure on the surfaces of Ti-based BMGs samples do not change much, suggesting that Ti-based BMGs have high AO erosion resistance [240]. The combination of good mechanical properties and high space environment applicability implies that Ti-based BMGs possess a potential for applications in aerospace environments.

Conclusions and Future Research Directions
Ti-based BMGs have attracted much attention due to the combination of unique properties such as high specific strength and good anti-corrosion properties. In recent decades, many Ti-based BMGs have been successfully developed, and their GFA, mechanical properties, corrosion resistance, and biocompatibility have been investigated. In this paper, the development of Ti-based BMGs is reviewed. In terms of GFA, a maximum diameter for glass formation of over 50 mm is now achieved in (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 and Ti32.8Zr30.2Cu9Fe5.3Be22.7 alloys. In the aspect of mechanical properties, Ti-Be-based BMGs possess very high specific strength over 4 × 10 5 N·m/kg, which is more than twice that of the Ti-6Al-4V alloy. Because of the relatively high Poisson's ratio of titanium, Ti-based BMGs are classified as "ductile," and most developed Ti-based BMGs possess a certain plastic strain under compression. Ti-based BMGs also exhibit excellent corrosion resistance in acid, alkaline, and salt solutions compared with crystalline Ti alloys and typical engineering materials such as 316L stainless steel. Ti-Zr-Cu-Pd-(Sn, Nb) BMGs do not contain noxious elements and show good GFA together with a high potential for biomedical applications.
Although rapid progress has been made, several challenges for future applications of Ti-based BMGs still exist, including the need to (1) understand the glass-forming mechanisms and further improve the GFA of Ti-based BMG especially non-toxic, low-modulus, and biocompatible Ti-based BMGs; (2) develop a complete understanding of the processing-structure-mechanical property Figure 18. An outer appearance of the self-made Coriolis flow meter using the Ti-Cu-Ni-Zr-Sn BMG pipe. Reproduced with permission from [232]. Copyright 2011, Elsevier.
Ti-based BMGs are also potentially useful in many other serious applications. It was reported that Zr-based BMGs have been used to fabricate micro-geared motor parts [231][232][233][234], pressure sensors [231,235,236], watch cases [204,237], cell phone cases [204,237], and sports goods (e.g., golf plate) [238]. Replacing Zr-based BMGs by Ti-based BMGs is beneficial for enhancing the specific strength and reducing the cost. Because of the good corrosion resistance and biocompatibility, Ti-based BMGs are also suitable for medical components such as prosthetic implants and surgical instruments (e.g., the surgical razor and micro-surgery scissors). Aerospace engineering is another important applied field of Ti-based BMGs. Ti-based BMGs possess outstanding mechanical properties such as high specific strength, high hardness, and large elastic elongation. Moreover, although BMGs are metastable in thermodynamics, Ti-based BMGs exhibit high space environment applicability. According to Wang et al.'s experimental results [239], the microstructure, thermodynamics, and mechanical properties of Ti-based BMGs are all relatively stable after simulated thermal cycling treatment in vacuum in a temperature range of −196 • C to 150 • C. The effect of atomic oxygen (AO) on the BMGs has been also studied in a plasma type ground-based AO effect simulation facility. The results show that the structure on the surfaces of Ti-based BMGs samples do not change much, suggesting that Ti-based BMGs have high AO erosion resistance [240]. The combination of good mechanical properties and high space environment applicability implies that Ti-based BMGs possess a potential for applications in aerospace environments.

Conclusions and Future Research Directions
Ti-based BMGs have attracted much attention due to the combination of unique properties such as high specific strength and good anti-corrosion properties. In recent decades, many Ti-based BMGs have been successfully developed, and their GFA, mechanical properties, corrosion resistance, and biocompatibility have been investigated. In this paper, the development of Ti-based BMGs is reviewed. In terms of GFA, a maximum diameter for glass formation of over 50 mm is now achieved in (Ti 36.1 Zr 33.2 Ni 5.8 Be 24.9 ) 91 Cu 9 and Ti 32.8 Zr 30.2 Cu 9 Fe 5.3 Be 22.7 alloys. In the aspect of mechanical properties, Ti-Be-based BMGs possess very high specific strength over 4 × 10 5 N·m/kg, which is more than twice that of the Ti-6Al-4V alloy. Because of the relatively high Poisson's ratio of titanium, Ti-based BMGs are classified as "ductile", and most developed Ti-based BMGs possess a certain plastic strain under compression. Ti-based BMGs also exhibit excellent corrosion resistance in acid, alkaline, and salt solutions compared with crystalline Ti alloys and typical engineering materials such as 316L stainless steel. Ti-Zr-Cu-Pd-(Sn, Nb) BMGs do not contain noxious elements and show good GFA together with a high potential for biomedical applications.
Although rapid progress has been made, several challenges for future applications of Ti-based BMGs still exist, including the need to (1) understand the glass-forming mechanisms and further improve the GFA of Ti-based BMG especially non-toxic, low-modulus, and biocompatible Ti-based BMGs; (2) develop a complete understanding of the processing-structure-mechanical property relations of Ti-based BMGs and propose new strategies for achieving the room temperature tensile ductility of Ti-based BMGs; (3) address the formability and oxidation resistance of Ti-based BMGs in their supercooled region and improve the thermoplastic formability of Ti-based BMGs; and (4) explore new application areas for Ti-based BMGs.