Effects of High-Pressure Torsion on Mechanical Properties of Biocompatible Ti–6Al–7Nb Alloy

Abstract

Ti–6Al–7Nb alloys have been widely used in the medical field, particularly in artificial hip joints, spinal fixators, and dental implants, owing to their light weight, low toxicity, and superior corrosion resistance. Grain refinement through a severe plastic deformation process under high pressure, such as high-pressure torsion (HPT) or high-pressure sliding, is widely employed for strengthening metallic materials. This overview presents the recent advances in the effect of HPT on the mechanical properties of the Ti–6Al–7Nb alloy. This alloy was grain-refined through HPT under applied pressures of 2 and 6 GPa, and the results revealed that the alloy subjected to HPT processing at 6 GPa exhibited a higher strength. To inhibit the decrease in the total elongation of the alloy, the number of revolutions in the HPT process was set to moderate. The tensile properties achieved after HPT processing were found to be dependent on the initial microstructure before the HPT treatment. Furthermore, an alloy with a bimodal equiaxed and acicular structure was subjected to grain refinement via the HPT process. The results revealed that fragmentation of the acicular structure during HPT further increased the strength. Moreover, the HPT-processed Ti–6Al–7Nb alloy exhibited superplasticity. It was thus confirmed that grain refinement by HPT is an effective method for strengthening the Ti–6Al–7Nb alloy, which is advantageous for medical applications.

1. Introduction

Metallic biomaterials are widely used in medical devices, particularly implants, owing to their high mechanical reliability. The corrosion resistance of metallic implants is a highly desired and critical feature. Austenitic stainless steel,^1,2) Co–Cr alloys,^3–5) pure Ti,^6,7) and Ti alloys^8–12) are some of the most widely used non-precious metal alloys. A passive surface oxide film maintains the corrosion resistance in these alloys. In particular, Ti alloys are suitable for preparing orthopedic and dental implants owing to their high corrosion resistance, weight, strength, and osseointegration. These advantageous properties have resulted in the widespread use of pure Ti and Ti alloys as alternatives to stainless steel and other materials.

Ti–6Al–7Nb alloys have been widely used in artificial hip joints, spinal fixators, and dental implants owing to their light weight, low toxicity, and superior corrosion resistance.¹³⁾ Because of the supposed cytotoxicity of vanadium in the human body, the Ti–6Al–7Nb alloy was developed by Semlitsch to replace the Ti–6Al–4V alloy in biomedical applications.^14–16) Since V is a congener of Nb, the microstructure and mechanical properties of the Ti–6Al–7Nb alloy are similar to those of the Ti–6Al–4V alloy.¹⁷⁾ The microstructure of the alloy comprises two phases, α and β, which provide mechanical properties suitable for medical applications.

Smaller and lighter medical devices can be obtained by increasing the strength of metallic biomaterials. Grain refinement through a severe plastic deformation process under pressure, such as high-pressure torsion (HPT)^18,19) or high-pressure sliding (HPS),^20,21) is an effective method for strengthening metallic materials. Several studies have been conducted on the HPT processing of different biomedical metallic materials, such as Ti–29Nb–13Ta–4.6Zr,^22–25) Ti–6Al–4V,^26–38) Ti–6Al–7Nb,^39–45) Ti–Nb,^46,51) Ti–15Mo,^47–50) and Co–Cr–Mo,^4,52–54) Mg⁵⁵⁾ alloys. Grain refinement not only improves the mechanical properties but also the biocompatibility.⁵⁶⁾ Chen et al. reported that subjecting the biomedical Co–Cr–Mo alloy to HPT and subsequent short-time annealing processing enhanced its fatigue properties and cytocompatibility.⁴⁾ In recent years, severe plastic deformation has been applied to high-entropy alloys for medical applications. An ultra-high hardness of 880 HV and good biocompatibility were achieved in a TiAlFeCoNi alloy after HPT.⁵⁷⁾ HPT has also been employed to obtain titanium-protein composites as novel biomaterials. The results revealed that the Ti–5 vol% bovine serum albumin nanocomposites showed good biocompatibility with nano-level grain refinement of Ti.⁵⁸⁾ Superplasticity is the ability of a polycrystalline material to exhibit very high tensile elongations before failure, and in general, a finer grain size improves the superplastic flow. Superplastic forming is an attractive option for manufacturing complex-shaped components in the medical and biomaterial fields, particularly in the dental field, such as in the fabrication of denture bases.⁵⁹⁾ α+β-type Ti alloys, including Ti–6Al–7Nb, are known to exhibit superplasticity⁶⁰⁾ when the ratio of the α-phase to β-phase is appropriate and when the alloy has a fine grain structure. Kulikowski et al. reported that the Ti–6Al–7Nb alloy exhibited a superplasticity of 300% at 1173 K in IMI367.⁶¹⁾

The properties of HPT-processed Ti–6Al–7Nb alloys have been previously reported. However, because Ti–6Al–7Nb is used for medical purposes, studies on HPT processing are limited, and the mechanical properties of the HPT-processed Ti–6Al–7Nb alloy still need to be fully understood. Therefore, this overview reports the recent advances in the mechanical properties of the HPT-processed Ti–6Al–7Nb alloy.

2. Mechanical Properties of the HPT-Processed Ti–6Al–7Nb Alloy

Pinheiro et al. reported that, when the as-cast Ti–6Al–7Nb alloy was subjected to HPT, the grain size was reduced to 97 nm after 3 revolutions and 107 nm after 5 revolutions.⁴⁴⁾ The Vickers hardness increased from 190 HV to approximately 340 HV by applying HPT under a pressure of 5 GPa. It was thus concluded that HPT could enable the grain refinement of the Ti–6Al–7Nb alloy.

The processing of Ti–6Al–7Nb alloy (ASTM F1295) disks with a diameter of 10 mm and thickness of 0.8 mm through HPT has been previously reported.⁴²⁾ The HPT conditions were 1, 5, and 20 revolutions under pressures of 2 and 6 GPa and a rotational speed of 1 rpm. The initial microstructure was an equiaxed structure of the α- and β-phases, and the volume fraction of the α-phase was ∼94%.

Figure 1 shows that in the HPT-processed Ti–6Al–7Nb alloy disks, the Vickers microhardness increases with an increase in the number of revolutions and pressure. The hardness before HPT was 325 HV, which increased to 375 HV at five revolutions under 6 GPa and 365 HV at five revolutions under 2 GPa. The tensile properties were also enhanced by HPT processing (Fig. 2), and a maximum tensile strength of 1250 MPa was achieved after 20 revolutions at 6 GPa. The grain sizes of the α phase were reduced to approximately 300 and 100 nm by HPT processing at 2 and 6 GPa, respectively, and consequently, the grain size of the alloy after HPT processing at 6 GPa was finer than that at 2 GPa. Although the tensile properties improved, total elongation decreased. A total elongation of 19% was observed at 6 GPa for both 1 and 5 revolutions, which was almost the same as that before HPT (22%); however, it was significantly reduced to 2% after 20 revolutions. The elongation of titanium alloys has to be maintained to the maximum possible extent to increase the durability of the material. For a good balance of tensile properties for both strength and elongation, the tensile properties of the Ti–6Al–7Nb alloy were improved through HPT by applying a pressure of 6 GPa and a moderate number of revolutions.

Fig. 1

Vickers microhardness as a function of the distance from the disk center of the Ti–6Al–7Nb alloy before and after HPT processing under 2 and 6 GPa.⁴²⁾

Fig. 2

Tensile properties of the Ti–6Al–7Nb alloy before and after HPT processing under 2 and 6 GPa.⁴²⁾

The hardness results of the study performed by Pinheiro et al. are different from our results because the alloys employed in their study⁴⁴⁾ were cast to different microstructures in the initial state. Although the grain size was similar, the hardness after HPT processing differed.

In addition, the Vickers microhardness of the Ti–6Al–7Nb alloy after HPT processing under 6 GPa increased from ∼305 HV before HPT processing to 361.7, 377, and 397.1 HV for 1, 5, and 15 revolutions, respectively, and the grain size was finer than 100 nm.⁶²⁾ The disk dimensions for HPT were 10 mm (diameter) and 0.8 mm (thickness). Our results revealed a maximum hardness value of 395 HV after 20 revolutions. The hardness values after 15 and 20 revolutions were similar. The minimum grain size of the alloy after HPT was also consistent. These results indicated that hardness was saturated after 5 revolutions. The initial microstructure, Vickers hardness, and grain size are summarized in Table 1.

Table 1 Initial microstructure, Vickers microhardness, and grain size of α-phase Ti–6Al–7Nb alloy after HPT processing.^41–45,62)

3. Mechanical Properties after HPT with Heat Treatment

Janeček et al. reported that applying heat treatment before HPT increased the Vickers microhardness of the alloy from 330 to 400 HV after HPT.⁴⁵⁾ The Ti–6Al–7Nb alloy ELI (IMI 367) was subjected to heat treatment at 1258 K and aging treatment at 973 K for 4 h before HPT to obtain a bimodal microstructure consisting of the equiaxed primary α phase and a lamellar structure of the α and β phases. The results of this study revealed that the hardness became saturated after 3 revolutions.

Before HPT processing, three types of heat treatment were applied to the Ti–6Al–7Nb alloy. (1) Solution treatment (at 1258 K for 1 h; air cooling) and aging (at 973 K for 4 h; air cooling) (STA); (2) solution treatment at a temperature lower than the β-transus temperature and quenching (at 1258 K for 1 h; quenching in ice water), T_β = 1278 K⁴⁵⁾ (STQ < T_β); (3) STQ > T_β: solution treatment (at 1308 K (higher than T_β) for 1 h; quenching in ice water). The HPT disks were 10 mm in diameter and 0.8 mm in thickness, and the HPT conditions were 1, 5, and 20 revolutions under a pressure of 6 GPa with a rotational speed of 1 rpm. An as-received sample of the alloy without heat treatment was used for comparison.

The initial microstructures of the as-received, STA-treated, STQ < T_β-treated, and STQ < T_β-treated samples were equiaxed structures of the α and β phases (grain size and volume fraction of the α phase were ∼5 µm and 94%, respectively), a bimodal microstructure comprising an equiaxed α phase and lamellar α+β phases (grain size and volume fraction of the α phase were ∼10 µm and ∼20%, respectively), a bimodal microstructure comprising the equiaxed α-phase and acicular martensitic α′-phase (grain size and volume fraction: ∼5 µm and ∼20%, respectively), and only an acicular α′-phase, respectively, as shown in Fig. 3.⁴¹⁾ The hardness values before HPT processing were 310, 340, and 355 HV in the STA, STQ < T_β, and STQ > T_β, respectively. In all samples, the Vickers microhardness increased with an increase in the number of revolutions with HPT processing (Fig. 4),⁴¹⁾ and the highest values of 395, 400, 395, and 395 HV were achieved after 20 revolutions for the as-received, STA, STQ < T_β, and STQ > T_β specimens, respectively. A larger volume fraction of the acicular martensitic structure of the α′-phase in the initial microstructure resulted in an earlier increase in the hardness with increasing number of revolutions of the HPT process. However, the hardness after 20 revolutions was almost the same for all samples. The tensile strength increased by HPT processing in all initial microstructures (Fig. 5).⁴¹⁾ The total elongation decreased after HPT as the number of revolutions increased. Although the tensile strength was high in most samples after more than five revolutions, the elongation significantly decreased, demonstrating brittle fracture. However, the total elongations of the as-received, STA, and STQ < T_β samples after one revolution were almost identical to those before HPT processing. After one revolution, a good balance of a tensile strength of 1280 MPa and a total elongation of 22% was achieved in the STQ < T_β sample.

Fig. 3

IPF map of the initial microstructure of the Ti–6Al–7Nb alloy subjected to heat treatment before HPT processing.⁴¹⁾

Fig. 4

Vickers microhardness of the Ti–6Al–7Nb alloy (a) without heat treatment (as-received) and with heat treatment. (b) STA, (c) STQ < T_β, and (d) STQ > T_β before and after HPT processing.⁴¹⁾

Fig. 5

Tensile properties of the Ti–6Al–7Nb alloy before and after HPT processing without and with heat treatment.⁴¹⁾

The minimum grain size was achieved after 20 revolutions, which was ∼90 and ∼70 nm for the α phase in the as-received and STA specimens, respectively, and ∼70 and ∼80 nm for the α′-phase in the STQ < T_β and STQ > T_β specimens, respectively. Furthermore, the grain sizes of the β phase in the as-received and STA specimens were ∼65 and ∼40 nm, respectively, smaller than those of the α grains in both the as-received and STA specimens. For the STA, STQ < T_β, and STQ > T_β specimens after 5 and 20 revolutions, ultrafine grains were observed by TEM grains, and they were mainly refined from the lamella structures for STA specimens or acicular structures for the STQ < T_β and STQ > T_β specimens.

In the STQ > T_β specimen, the acicular structure of the α′-phase was fragmented into several grains. It has been previously reported that the fragmentation of the thin β-plate is an effective strategy for achieving uniform ultrafine grain.⁴⁷⁾ Thus, it was hypothesized that the acicular structure fragmentation could have resulted in grain refinement, which increased the hardness and tensile strength.

Hernández et al. reported⁶²⁾ the fabrication of a Ti–6Al–7Nb alloy with a bimodal structure of equiaxed grains and lamellar structures and with similar grain refinement of less than 100 nm of the α phase by HPT processing at 6 GPa at a rotational speed of 1 rpm for 5 revolutions. The bimodal structure of the equiaxed grains and the lamellar structures was obtained by heat treatment of the solution treatment at 960°C for 2 h, followed by quenching in liquid nitrogen at ∼−196°C. The HPT disk had a diameter of 10 mm and a thickness of 0.8 mm. These results revealed that the minimum grain size of the α phase was similar to our study’s. Since the hardness becomes saturated by straining using HPT, it was considered that the grain refinement had reached the limit.

4. Superplasticity

It is reported that the processing of a Ti–6Al–7Nb (ASTM F1295) alloy with an equiaxed α+β structure by HPT under a pressure of 6 GPa and a rotational speed of 1 rpm at 1 and 5 revolutions.³⁹⁾ The HPT disks were 10 mm in diameter and 0.8 mm in thickness. The tensile test was performed at 1073 K with initial strain rates of 2 × 10⁻², 2 × 10⁻³, and 2 × 10⁻⁴ s⁻¹.

After five revolutions, the size of the α grains was reduced to ∼100 nm. Figure 6 shows the stress–strain curves after tensile deformation at 1073 K at initial strain rates ranging from 2 × 10⁻⁴ to 2 × 10⁻² s⁻¹ for the HPT-treated samples up to N = 5.³⁹⁾ Deformation was accompanied by hardening in the initial phase and reached a peak, followed by gradual softening. As the strain rate decreased from 2 × 10⁻² to 2 × 10⁻⁴ s⁻¹, the peak stress significantly decreased. An elongation of 930% was achieved after tensile testing at an initial strain rate of 2 × 10⁻³ s⁻¹ (Fig. 7).³⁹⁾ A much larger elongation was attained at a lower testing temperature and a higher strain rate than that (300%) previously reported.⁶¹⁾ The superplastic behavior is most commonly understood by analyzing the results based on the following equation⁶³⁾ $\sigma = K\dot{\varepsilon }^{m}$, where σ is the flow stress, $\dot{\varepsilon }$ is the strain rate, K is the material constant, and m is the strain rate sensitivity. m can be derived from the slope of the double logarithmic plot (Fig. 8). From the graph, an m = 0.5 was achieved, which meets the requirement for superplasticity.⁶⁴⁾ An m = 0.4–0.7 was achieved for the α+β type Ti alloys with a β-phase ratio of less than 20%. In addition, an m = 0.5 was derived, which is good via grain boundary sliding^64,65) in agreement with the fine structure superplasticity, thus indicating that deformation occurred.

Fig. 6

Stress–strain curves at 1073 K for the Ti–6Al–7Nb alloy after HPT processing under 6 GPa at five revolutions.³⁹⁾

Fig. 7

Photograph of the tensile specimens after the tensile test at 1073 K for the HPT-processed Ti–6Al–7Nb alloy.³⁹⁾

Fig. 8

Peak stress as a function of the initial strain rate of the Ti–6Al–7Nb alloy after HPT processing under 6 GPa at five revolutions.³⁹⁾

Cubero et al.⁴³⁾ reported achieving a superplasticity of ∼580% at 1073 K with 2.0 × 10⁻³ s⁻¹ through heat-treatment quenching from 960°C in liquid nitrogen before HPT. A bimodal structure consisting of an equiaxed primary α phase and a lamellar structure of the α and β phases was obtained. These were not our study’s results due to the difference in the initial microstructure, i.e., the equiaxed or bimodal structure, and the difference in the ratio of the β-phase of the initial state.

5. Medical Applications

HPT achieved a good balance of the tensile properties of the Ti–6Al–7Nb alloy with almost the same elongation by grain refinement. However, the sample form for HPT is limited to disks and rings, and consequently, a larger sample of the alloy is required for medical applications such as dental implants. Alternatively, the HPS process can also reduce the grain size through a principle similar to that of HPT but applies to sheet or rod samples.

Watanabe et al. reported the successful application of HPS (pressure of 3 GPa, processing speed of 1 mm s⁻¹, and sliding distance of 10 mm) to a Ti–6Al–7Nb alloy with a width of 10 mm, length of 100 mm, and thickness of 1 mm, and grain refinement was achieved.²¹⁾ In addition, it has been reported that the grain size can be reduced to 200–300 nm, and a superplasticity of 790% can be achieved at 1123 K and an initial strain rate of 1 × 10⁻³ s⁻¹.⁶⁶⁾ These results indicated that grain refinement is also possible by applying HPS processing. The sample size should be increased in future studies, and the Ti–6Al–7Nb alloy should be strengthened.

Furthermore, for medical applications, it is necessary to focus on the corrosion resistance and biocompatibility of the alloy after processing. The cytocompatibility of Ti–6Al–7Nb alloys before and after HPT processing was evaluated using mouse preosteoblasts (MC3T3-E1).⁴⁰⁾ The results indicated that the Ti–6Al–7Nb alloy with ultrafine grains showed good cytocompatibility with the alloy before HPT processing; however, there were some differences in cell adhesion, proliferation, and osteogenic differentiation, which may hinder their medical applications. For medical applications, it is necessary to study the effects of grain refinement by HPS processing on corrosion resistance, biocompatibility, and fatigue properties.

6. Conclusion

This overview discussed the effects of HPT processing on the mechanical properties of the Ti–6Al–7Nb alloy for medical applications.

Finer grains and higher tensile strength were obtained upon HPT processing under 6 GPa compared with those at 2 GPa in the Ti–6Al–7Nb alloy with an equiaxed structure. The tensile properties of the Ti–6Al–7Nb alloy could be improved by applying a pressure of 6 GPa through moderate numbers of revolutions during the HPT processing.

After heat treatment with STA, STQ < T_β, and STQ > T_β, the mechanical properties of the alloy after HPT differed from those before HPT. For the STQ < T_β sample, the initial microstructure was a bimodal structure of equiaxed and acicular structures. A good balance of 1280 MPa and 22% of tensile properties were obtained after HPT for one revolution.

A grain-refined Ti–6Al–7Nb alloy upon HPT processing showed a superplasticity of 930% at 1073 K and an initial strain rate of 2 × 10⁻³ s⁻¹.

Acknowledgments

The author would like to thank Prof. Takao Hanawa (Tokyo Medical and Dental University) for the valuable discussion and advice on the experimental design. The author is also grateful to Prof. Zenji Horita (Kyushu Institute of Technology, Japan) for the samples and helpful discussions during the experiments. This work was supported by the Japan Science Promotion Society (Grant-in-Aid for Early-Career Scientists, Grant Number JP21K17007). This work was also supported by the Light Metal Educational Foundation, Inc. Part of this research is based on the Cooperative Research Project of the Research Center for Biomedical Engineering.

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