The Effect of Iron Content on Glass Forming Ability and Thermal Stability of Co–fe–ni–ta–nb–b–si Bulk Metallic Glass

In this study, change in glass forming ability (GFA) and thermal stability of Co–Fe-based bulk metallic glasses were investigated as a function of iron content. Cylindrical samples of alloys with diameters of up to 4 mm were synthesized by a suction casting method in an arc furnace. Structures and thermal properties of the as-cast samples were determined by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. It was found that the critical casting thickness of the alloys reduced as iron content was increased and cobalt content was decreased. It was determined that GFA parameters, reduced glass transition temperature (T g /T l) and δ (= T x /(T l − T g)), show a very good correlation with critical casting thickness values. It was also observed that changing iron content did not effect thermal properties of the alloys.

A comparison of boron contents of Co-and Fe-based bulk metallic glasses shows that, in general, boron contents of Co-based bulk metallic glasses [1,14,15] are higher than those of Fe-based bulk metallic glasses [2,4,6,7,13].In fact, the boron content of Co-based bulk metallic glasses can be as high as 37.5 atom % [14].However, the highest boron content of a Fe-based bulk metallic glass is 25 atom % [6].When compared in terms of cost, it is obvious that Fe-based bulk metallic glasses are more attractive than Co-based bulk metallic glasses.Therefore, replacing cobalt with iron in a Co-based bulk metallic glass containing high amount of boron without degrading its GFA will definitely make the resulting metallic glasses more cost-effective precursors to develop composites having ultrahigh hardness values.In addition, the cost of composites can be lowered further by using low-cost industrial raw materials such as ferro-boron, ferro-niobium, and ferro-tantalum.
In this study, we report the effect of iron content on GFA and the thermal properties of a Co-based bulk metallic glass, Co 41 Fe 20 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 , which has a critical casting thickness of 4 mm [26].
For this reason, cobalt is partially replaced with iron and Co 41 − x Fe 20 + x Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloys (where x = 10, 20, and 30) were synthesized.The critical casting thicknesses, the thermal properties, and the microhardnesses of the alloys were determined as a function of iron content.

Materials and Methods
Co-Fe-based alloy ingots with composition of Co 41 − x Fe 20 + x Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 (where x = 10, 20, 30) were prepared by arc melting the mixtures of pure Co (99.8 wt %), Fe (99.9 wt %), Ni (99.9 wt %), Ta (99.9 wt %), Nb (99.8 wt %), and Si (99.9 wt %) metals and pure crystalline B (98 wt %) in a Ti-gettered ultrahigh purity argon atmosphere.Master alloys were melted three times in order to ensure homogeneity.Cylindrical samples of the alloys with diameters up to 4 mm and a length of 40 mm were synthesized by suction casting method in a vacuum arc furnace.Structures of the samples were examined by X-ray diffraction (XRD, Shimadzu XRD-6000, Kyoto, Japan) with Cu Kα radiation.The glass transition temperatures (T g ), the crystallization temperatures (T x ), the solidus temperatures (T m ), and the liquidus temperatures (T l ) of the alloys were determined by differential scanning calorimetry (DSC, Netzsch STA 449 F3, Selb, Germany) at a heating rate of 0.33 K/s.Microhardness measurements were carried out with a Vickers microhardness tester (Shimadzu HMV 2 L, Kyoto, Japan) under a load of 2.94 N.For each alloy, microhardnesses of as-cast samples were measured.Twenty microhardness measurements were carried out for each sample, and the arithmetic mean of the measurements were considered as the microhardness of the alloy.In this study, we report the effect of iron content on GFA and the thermal properties of a Cobased bulk metallic glass, Co41Fe20Ni2Ta2.75Nb2.75B26.5Si5,which has a critical casting thickness of 4 mm [26].For this reason, cobalt is partially replaced with iron and Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5alloys (where x = 10, 20, and 30) were synthesized.The critical casting thicknesses, the thermal properties, and the microhardnesses of the alloys were determined as a function of iron content.

Materials and Methods
Co-Fe-based alloy ingots with composition of Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5(where x = 10, 20, 30) were prepared by arc melting the mixtures of pure Co (99.8 wt %), Fe (99.9 wt %), Ni (99.9 wt %), Ta (99.9 wt %), Nb (99.8 wt %), and Si (99.9 wt %) metals and pure crystalline B (98 wt %) in a Tigettered ultrahigh purity argon atmosphere.Master alloys were melted three times in order to ensure homogeneity.Cylindrical samples of the alloys with diameters up to 4 mm and a length of 40 mm were synthesized by suction casting method in a vacuum arc furnace.Structures of the samples were examined by X-ray diffraction (XRD, Shimadzu XRD-6000, Kyoto, Japan) with Cu Kα radiation.The glass transition temperatures (Tg), the crystallization temperatures (Tx), the solidus temperatures (Tm), and the liquidus temperatures (Tl) of the alloys were determined by differential scanning calorimetry (DSC, Netzsch STA 449 F3, Selb, Germany) at a heating rate of 0.33 K/s.Microhardness measurements were carried out with a Vickers microhardness tester (Shimadzu HMV 2 L, Kyoto, Japan) under a load of 2.94 N.For each alloy, microhardnesses of as-cast samples were measured.Twenty microhardness measurements were carried out for each sample, and the arithmetic mean of the measurements were considered as the microhardness of the alloy.

Results
XRD patterns of samples are given in Figure 1.The base alloy, Co41Fe20Ni2Ta2.75Nb2.75B26.5Si5,has a critical casting thickness of 4 mm.For the casting thickness of 5 mm, (Co,Fe)21Ta2B6 and (Co,Fe)2B phases form.The Co31Fe30Ni2Ta2.75Nb2.75B26.5Si5alloy has a critical casting thickness of 3 mm.For the casting thickness of 4 mm, the precipitation of (Co,Fe)2B phase was observed for this alloy.The critical casting thicknesses of alloys Co21Fe40Ni2Ta2.75Nb2.75B26.5Si5and Co11Fe50Ni2Ta2.75Nb2.75B26.5Si5were found to be 2 mm and 0.5 mm, respectively.For both of these alloys, the precipitation of the (Co,Fe)2B, (Co,Fe)16Ta6Si7, and Fe3Si phases was observed in the samples having diameters larger than the critical casting thicknesses.Thermal properties of the alloys were determined by DSC (Figure 2).During heating, all the DSC traces showed an endothermic event, which is the indication of the glass transition and followed Thermal properties of the alloys were determined by DSC (Figure 2).During heating, all the DSC traces showed an endothermic event, which is the indication of the glass transition and followed by exothermic reactions, which are the signs of crystallization of the glassy structure.In addition, the T l of the base alloy is 1443 K [26], and the T l of 30, 40, and 50 atom % iron-containing alloys are 1458, 1488, and 1532 K, respectively.Thermal properties of the alloys are given in Table 1.In addition, the microhardnesses of the were determined to be around 1200 H v .

Discussion
The critical casting thickness decreases as iron content is increased.It is quite obvious that the reduction in critical casting thickness results from the fact that the Tl of the alloys increases with iron content.For a constant Tg, if liquidus temperature increases, the minimum cooling rate necessary to obtain a completely amorphous structure also increases.As a result, the casting thickness must be decreased to achieve this cooling rate.Well-known GFA parameters, reduced glass transition temperature (Tg/Tl) [27] and δ (= Tx/(Tl − Tg)), show a very good correlation with the critical casting Table 1.Thermal properties (T g , T x , T l , T m ), parameters for GFA, critical casting thickness and microhardnesses of Co-Fe-Ni-Ta-Nb-B-Si alloys.

Discussion
The critical casting thickness decreases as iron content is increased.It is quite obvious that the reduction in critical casting thickness results from the fact that the T l of the alloys increases with iron content.For a constant T g , if liquidus temperature increases, the minimum cooling rate necessary to obtain a completely amorphous structure also increases.As a result, the casting thickness must be decreased to achieve this cooling rate.Well-known GFA parameters, reduced glass transition temperature (T g /T l ) [27] and δ (= T x /(T l − T g )), show a very good correlation with the critical casting thickness values (Figure 3).However, the correlation between the GFA parameter T x /(T g + T l ) and the critical casting thickness values is not as satisfactory as those of the others.
Metals 2017, 7, 7 4 of 6 thickness values (Figure 3).However, the correlation between the GFA parameter Tx/(Tg + Tl) and the critical casting thickness values is not as satisfactory as those of the others.(Co,Fe)2B, (Co,Fe)16Ta6Si7, and Fe3Si phases form during cooling.It is shown that (Co,Fe)2B phase forms at temperatures higher than temperatures at which the (Co,Fe)16Ta6Si7 phase forms [25].In other words, (Co,Fe)2B is the first phase that precipitates during cooling.Since the melting temperature of the Fe2B phase is higher than the Co2B phase, increasing iron content increases the melting temperature of the (Co,Fe)2B phase.As a result, the liquidus temperatures of the alloys increase.
Glass transition temperatures of the alloys do not change with iron content.However, crystallization temperatures increase slightly.Similarly, microhardnesses of the alloys remain almost constant.Cohesive energies of Fe-Fe and Co-Co bonds are 413 and 424 kJ/mol, respectively [28].It is quite reasonable to assume that cohesive energy of Fe-Co bond is close to these values.When the iron content of the alloys is increased, the number of Co-Co bonds decreases, but the number of Fe-Co bonds increases.Since cohesive energies of these bond are almost the same, the total cohesive energy of the amorphous structure remains constant.For this reason, the thermal properties and the microhardnesses of the alloys do not change.
The Co21Fe40Ni2Ta2.75Nb2.75B26.5Si5alloy has a reasonably high iron content and critical casting thickness.Therefore, it is believed that it can used as a precursor to develop composites with ultrahigh hardness values at a low cost.

Conclusions
The following conclusions can be reached from this study: 1.The critical casting thicknesses of the alloys decrease with iron content due to the increase in liquidus temperatures.(Co,Fe) 2 B, (Co,Fe) 16 Ta 6 Si 7 , and Fe 3 Si phases form during cooling.It is shown that (Co,Fe) 2 B phase forms at temperatures higher than temperatures at which the (Co,Fe) 16 Ta 6 Si 7 phase forms [25].In other words, (Co,Fe) 2 B is the first phase that precipitates during cooling.Since the melting temperature of the Fe 2 B phase is higher than the Co 2 B phase, increasing iron content increases the melting temperature of the (Co,Fe) 2 B phase.As a result, the liquidus temperatures of the alloys increase.
Glass transition temperatures of the alloys do not change with iron content.However, crystallization temperatures increase slightly.Similarly, microhardnesses of the alloys remain almost constant.Cohesive energies of Fe-Fe and Co-Co bonds are 413 and 424 kJ/mol, respectively [28].It is quite reasonable to assume that cohesive energy of Fe-Co bond is close to these values.When the iron content of the alloys is increased, the number of Co-Co bonds decreases, but the number of Fe-Co bonds increases.Since cohesive energies of these bond are almost the same, the total cohesive energy of the amorphous structure remains constant.For this reason, the thermal properties and the microhardnesses of the alloys do not change.
The Co 21 Fe 40 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloy has a reasonably high iron content and critical casting thickness.Therefore, it is believed that it can used as a precursor to develop composites with ultrahigh hardness values at a low cost.

Conclusions
The following conclusions can be reached from this study: 1.
The critical casting thicknesses of the alloys decrease with iron content due to the increase in liquidus temperatures.

2.
The critical casting thicknesses of the alloys show a very good correlation with reduced glass transition temperature, T g /T l and T x /(T l − T g ).

3.
Thermal properties and microhardnesses of the alloys do not change with iron content because of the fact that cohesive energies of the Co-Co-and Fe-Fe bonds are almost the same.

Figure 1 .
The base alloy, Co 41 Fe 20 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 , has a critical casting thickness of 4 mm.For the casting thickness of 5 mm, (Co,Fe) 21 Ta 2 B 6 and (Co,Fe) 2 B phases form.The Co 31 Fe 30 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloy has a critical casting thickness of 3 mm.For the casting thickness of 4 mm, the precipitation of (Co,Fe) 2 B phase was observed for this alloy.The critical casting thicknesses of alloys Co 21 Fe 40 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 and Co 11 Fe 50 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 were found to be 2 mm and 0.5 mm, respectively.For both of these alloys, the precipitation of the (Co,Fe) 2 B, (Co,Fe) 16 Ta 6 Si 7 , and Fe 3 Si phases was observed in the samples having diameters larger than the critical casting thicknesses.
T g and T x of the base alloy, Co 41 Fe 20 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 , are 891 and 947 K, respectively [21].The T g of Co 31 Fe 30 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloy is 890 K. Additionally, the T x of Co 31 Fe 30 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloy is 957 K, which is about 10 K higher than the T x of Co 41 Fe 20 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloy.The T g of Co 21 Fe 40 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 and Co 11 Fe 50 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloys are found to be 895 and 894 K, respectively.Moreover, the T x of Co 21 Fe 40 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 and Co 11 Fe 50 Ni 2 Ta 2.75 Nb 2.75 B 26.5 Si 5 alloys are determined as 960 and 964 K, respectively.

2 .
The critical casting thicknesses of the alloys show a very good correlation with reduced glass transition temperature, Tg/Tl and Tx/(Tl − Tg).3.Thermal properties and microhardnesses of the alloys do not change with iron content becauseof the fact that cohesive energies of the Co-Co-and Fe-Fe bonds are almost the same.

Figure 3 .
Figure 3. Relationship between the critical casting thickness (D max ) for the formation of a glassy phase and GFA parameters.(a) Reduced glass transition temperature (T g /T l ); (b) T x /(T l − T g ); (c) T x /(T g + T l ).