2.1. Commercially Pure Ti
The first studies devoted to Ti-based materials potentially applicable in medicine were applied to commercial purity titanium (CP Ti) due to its high biocompatibility with living tissues [22
]. Unparalleled biocompatibility of Ti was the main interest of many clinical studies of medical devices and tools applied in traumatology, orthopedics, and dentistry. Unfortunately, CP Ti is characterized by reduced strength when compared to other metallic materials used in biomedical devices such as steels or cobalt-based alloys. Achieving higher strength level is possible by alloying or thermo-mechanical processing, but then the Ti-based materials usually lose their biometric response or fatigue performance. Therefore, SPD processing was considered as an alternative strategy proving that nanostructuring of CP Ti may become a novel approach to improve the mechanical properties of this material to achieve its high-performance [13
]. Apart from enhancing mechanical properties, this strategy is also advantageous in improving the biological response of the surface of the CP titanium based products [18
The first results on nanostructured CP Ti Grade 4 (O–0.34%, Fe–0.3%, C–0.052%, N–0.015%, all in wt.%, balance–Ti] were achieved by Valiev et al. aiming on manufacturing rods with significantly enhanced mechanical properties and superior biomedical response for the fabrication of dental implants [19
]. The processing route involved equal-channel angular pressing (ECAP) as an SPD technique [9
] followed by thermo-mechanical treatment by forging and, finally, drawing. Continuous SPD processing by ECAP-Conform (ECAP-C) and subsequent drawing, was capable of producing rods with the diameter of 7 mm and the length of 3 m with homogeneous ultrafine-grained (UFG) structure along the entire length of the rods [23
]. Furthermore, ECAP-Conform represents an economical SPD-based fabrication procedure for mass production of ‘nanoTi’.
After combined severe plastic deformation and thermo-mechanical processing, the grain size was significantly reduced from 25 µm in the initial Ti rods to 150 nm in the processed material. Figure 1
illustrates the effect of ECAP-C strain on the density of high-angle boundaries (HAB) and mechanical strength of CP Ti Grade 4 [21
shows the improved mechanical properties of CP Ti after nanostructuring by ECAP and subsequent thermomechanical treatment. The strength of the nanostructured titanium is doubled when compared to the conventional CP titanium. The increase in strength was achieved without reduction of ductility (total elongation to failure is above the limit of 10%), which is otherwise commonly observed after intensive drawing or rolling.
Fatigue tests of conventional and nanostructured CP Ti were conducted in air at room temperature in accordance with ASTM E 466-96 with the loading frequency of 20 Hz and R = 0.1. Table 1
shows that the fatigue strength of nanoTi [17
] after one million cycles is almost doubled when compared to the conventional CP titanium and even exceeds the fatigue performance of the Ti-6Al-4V alloy [22
]. Significant enhancement of fatigue properties and improved strength of nanostructured Ti allow us to produce smaller sizes of implants and therefore to reduce the extent of a surgical intervention (see also Section 3
CP Ti is known for its considerable biocompatibility which results from the presence of the protective oxide film. Titanium dioxide TiO2
forms naturally on the surface of CP Ti and represents a stable protective layer on that a mineralized bone matrix can be attached. This film is usually 5–10nm thick and biologically inert, thus it prevents a potentially negative reaction between the surrounding body environment and the metal [22
NanoTi with UFG structure containing high density of non-equilibrium grain boundaries achieved by SPD is also characterized by significantly increased internal energy of the material [3
]. This fact may result in considerable change in the morphology of the oxide film on the material surface. NanoTi with polished surface exhibits improved biological reaction of the surface as confirmed by recent studies in a series of experiments through cytocompatibility tests using mouse fibroblast cells [20
]. At the same time, additional improvement of biomedical properties of nanostructured titanium can be achieved by dedicated surface modifications such as chemical etching or bioactive coatings [17
2.2. Titanium Alloys
Two-phase (α + β) titanium alloys such as Ti-6Al-4V and Ti-6Al-7Nb continue to be the most important metallic materials in the dental and orthopedic fields due to their excellent mechanical properties and satisfactory biocompatibility [2
Several recent studies reported improved mechanical and functional properties of nanostructured titanium alloys.
Microstructure and mechanical properties of Ti-6Al-4V ELI (extra low interstitial alloys for medical applications) prepared by SPD are reported in [15
]. Round rods of the two-phase alloy with the diameter of 40 mm (Intrinsic Devices Company, San Francisco, CA, USA) and with chemical composition: Ti–base, Al–6.0%; V–4.2%; Fe–0.2%; O–0.11%; N–0.0025%; H–0.002%, C–0.001% (wt.%) had the grain size of about 8 µm in a cross-section and 20 µm in a longitudinal section. X-ray diffraction analysis proved that the volume fractions of α and β phases were approximately 85% and 15%, respectively. 250 mm length rods were processed in two steps. The rods were subjected to ECAP via route Bc at 600 °C and subsequently extruded, altogether with total strain of 4.2 [33
]. The extrusion steps were carried out at 300 °C with the last pass at room temperature for additional strengthening. The rods with the diameter of 18 mm and length up to 300 mm were produced. The rods were finally annealed in the temperature range from 200 °C to 800 °C for 1 h and subsequently cooled in air.
Transmission electron microscopy (TEM) studies showed that SPD leads to a complex UFG structure containing refined grains and subgrains with a mean size of about 300 nm.
Stress–strain curves for the initial coarse-grained and UFG material shown in Figure 2
demonstrate that the alloy after grain refinement by SPD underwent significant strengthening. Tensile elongation of the UFG material (curve 2) is reduced from 17% to 9%. Strength/ductility trade off, however, improved after subsequent annealing at 500 °C. The results of tensile tests correspond to the measurement of microhardness [32
In accordance with [10
], enhancement of the ductility in the UFG material by annealing is clearly associated with a decrease of internal elastic stress and dislocation density. Simultaneous additional strengthening of the alloy can be explained by the observed decrease in content of metastable β-phase after cooling from the annealing temperature. Its volume fraction in the UFG alloy annealed at 500 °C can be higher than before annealing, as shown in [10
], due to quenching from the annealing temperature. Despite no visible particles of any secondary phase, aging processes might have caused grain boundary segregations associated with additional improvement of the properties of the annealed UFG material [34
Fine tuning of mechanical properties by annealing after the SPD processing is limited mainly by grain growth occurring at elevated temperatures. Thermal stability of UFG structure of commercially pure Ti follows classical grain growth depending on temperature via Arrhenius equation [35
] and limited to approximately 450 °C [36
]. Nanostructured α + β exhibit enhanced thermal stability up to 550 °C [37
Fatigue properties of the Ti-6Al-4V ELI alloy with UFG structure were investigated. High strength and enhanced ductility (1370 MPa and 12%) after SPD processing and subsequent annealing at 500 °C; resulted in an enhancement of fatigue limit to 740 MPa after 107
cycles in comparison to 600 MPa in the initial coarse-grained condition (Figure 3
The fatigue limit of the Ti-6Al-4V alloy in UFG condition reported in [32
] tested by rotating bending was slightly higher than the values in [32
] proving that measured fatigue properties depend on the choice of the measurement technique.
Achieved results show that high strength can be achieved in UFG Ti-6Al-4V ELI alloy by processing by ECAP and subsequent thermo-mechanical treatment. Selection of SPD regimes and adjustment of processing parameters of SPD processing such as temperature, strain rate and strain allow us to manipulate the grain boundary structure and phase morphology in the two-phase UFG alloy. As the result, the best combination of strength and ductility can be achieved along with the improved fatigue endurance limit. Enhancement of strength and ductility of the biomedical Ti-6Al-7Nb alloy was reported in another comprehensive study [39
]. In comparison to Ti-6Al-4V, the Ti-6Al-7Nb alloy represents a better choice for biomedical use due to avoiding the toxic vanadium [40
]. This study shows that processing by ECAP and consequent thermo-mechanical treatment causing formation of UFG structure results in high strength (1400 MPa) and ductility (elongation of 10%). These achieved properties are attractive for designing, developing and manufacturing of high-performance medical devices and implants.
Considering that vanadium and partly also aluminum are rather toxic elements and, simultaneously, that reducing of the Young’s modulus is required for avoiding so-called stress-shielding [39
], the development of brand new biomedical alloys represents a current relevant challenge for researchers. A new generation of titanium alloys must provide improved strength, better biocompatibility, and lower Young’s modulus than Ti6Al4V alloy. Current research focuses on new alloying systems, in particular Ti-Nb and Ti-Mo.
Given the above mentioned requirements, the interest is drawn to titanium alloys containing high content of the β phase, because this phase is characterized by lower Young’s modulus in the range of 55–90 GPa, and thus exhibit lower stress shielding [39
]. Moreover, these Ti alloys are designed to contain only non-toxic constituents such as Nb, Mo, Zr, and Ta. On the other hand, these materials are characterized by comparatively low strength, because the lowest Young’s modulus is obtained only in solution treated single phase β-Ti alloys. Achieving low Young’s modulus and high strength simultaneously is a challenging task. Ageing treatments that induce a fine and uniform precipitation of ω and α phase components provides significant strengthening. On the other hand, this inevitably increases the Young’s modulus of the alloy [41
]. Only few studies present successful results in development of thermal treatments without detrimental effect on some of the relevant mechanical properties [44
Advancements in the areas of orthopedics and dentistry called for new strategies for development of new generation of β-Ti alloys with reduced Young’s modulus and high strength, which would be more suitable for such applications. Recently, SPD processing has been proposed to fabricate nanocrystalline β-Ti alloys with high strength, low modulus of elasticity and excellent biocompatibility [46
]. Nanostructuring of these alloys leads to improved strength due to grain refinement and substructure evolution [52
]. In particular, solution treated β-Ti Ti15Mo alloy, which is qualified for medical use, can be significantly refined by HPT as demonstrated in Figure 4
a. Grain size can be decreased well below 100 nm [53
]. Significant disadvantage, apart from limited size of HPT samples, is formation of deformation induced ω phase causing sharp increase of elastic modulus [54
]. Subsequent aging of UFG Ti15Mo alloy leads to two-phase α + β structure which is also characterized by increased modulus of elasticity [55
]. More promising is using Ti-Nb-Ta-Zr based alloys which are less prone to ω phase formation. Ti-29Nb-13Ta-5Zr alloy prepared by HPT exhibited increased yield stress from 550 to 800 MPa with unchanged elastic modulus [58
]. Significant microstructure refinement was recently also achieved in Ti-35Nb-6Ta-7Zr biomedical alloy by ECAP (Figure 4
Microstructural refinement in β-Ti alloys can be also enhanced by multiple twinning and/or martensitic transformation β → α’’ [60
]. The nanocrystalline β-Ti alloys also display excellent in vitro biocompatibility as shown by enhanced cell attachment and proliferation [48
]. These novel nanocrystalline β-Ti alloys have high chances to meet the challenge of next-generation implant material with significant prospects in load bearing biomedical applications.
2.3. Nanostructured NiTi Shape Memory Alloys
NiTi alloys exhibit unique mechanical behavior—shape memory effect (SME) and superelasticity, which arise from a transformation between martensite and austenite phases [61
]. NiTi alloys are important materials which are already used in advanced medical devices due to the above mentioned mechanical properties and, additionally, due to functional properties such as good biocompatibility and corrosion resistance in vivo [61
]. At the same time, new, advanced applications will require enhanced properties (higher strength, higher recovery strain and stress, etc.) of NiTi shape memory alloys.
During the past two decades, there has been interest in the application of SPD methods to NiTi alloys because the formation of nanocrystalline and UFG structures allows enhancing mechanical and functional properties in comparison to coarse grained materials [63
HPT processing of NiTi alloys leads to a transformation from crystalline to amorphous phase. Microstructural changes in deformed NiTi during thermal treatment are of key interest as they are responsible for the shape memory effect [65
]. During following thermal treatments nanocrystalline (NC) structure can be obtained in NiTi alloys via crystallization process (Figure 5
]. Nanocrystalline NiTi alloys with grain size about 20 nm demonstrate very high strength up to 2000 MPa [64
Equal channel angular pressing is another SPD processing technique applied for producing uniform UFG structure in bulk NiTi alloys. The ECAP processing of NiTi at 400–450 °C results in formation of UFG structure with grain size of about 200 nm (Figure 5
UFG structure formation leads to significant improvement of mechanical and functional properties of NiTi-based alloys [64
]. The ultimate tensile strength (UTS) of UFG NiTi alloy attains 1400 MPa, which is 50% higher than in CG alloys; and the yield stress (YS) increases after ECAP from 500 MPa to 1100 MPa (Figure 6
a). The functional shape-memory effect of NiTi after ECAP is also improved (Figure 6
b). The maximum completely recoverable strain εrmax
increases from 6% (in CG state) to 9% after ECAP and the maximum recovery stress σrmax
reaches 1120 MPa, which is twice more than the level of CG alloys (about 500 MPa) [69
]. UFG structure formation in Ni-rich NiTi alloys by ECAP results in an emergence of superelasticity at temperature close to the human body temperature. Superelasticity in UFG NiTi is characterized by a narrow mechanical hysteresis and low residual strain [71
The high-strength NC and UFG NiTi alloys with improved functional characteristics are very promising for medical applications in particular for manufacturing of stents, embolic protection filters, guide wires, and other peripheral vascular devices (see Section 4