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Abstract In order to develop biomedical shape memory alloys with high biocompatibility and magnetic resonance imaging compatibility, novel Zr-based alloys were designed and investigated to clarify their microstructure and mechanical properties. This study, for the first time, reports that Zr-Nb-Sn alloys exhibit shape memory effect and superelasticity by adjustment of Nb and Sn contents. The stress induced martensitic transformation from a β phase to an α″ phase was confirmed to occur in Zr-Nb-Sn alloys. The basic crystallographic characteristics of stress-induced martensitic transformation, such as crystal structure of martensite phase and orientation relationships between the two phases, in Zr-Nb-Sn alloys were examined. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The effects of Al addition on the superelastic properties of Ti-Zr-Nb alloys were investigated. Ti-18Zr-(12-16)Nb-(0-4)Al (at.%) alloys were prepared by the Ar arc melting method and superelastic properties, transformation temperature, and transformation strain were investigated quantitatively. The superelastic strain increased by the addition of Al and a large recovery strain of 6.1% was observed in a Ti-18Zr-13.5Nb-3Al alloy. The large superelastic strain of Al-added alloys was found to be due to a large transformation strain and a strong recrystallization texture. [ABSTRACT FROM AUTHOR]

The microstructure and deformation behavior of Ti-23Nb-2Zr-0.7Ta-(0.3–1.8)O (at.%) alloys were investigated in order to elucidate deformation mechanism of Gum metal in relation to the martensitic transformation. The long range stress induced martensitic transformation was suppressed with increasing oxygen concentration, resulting in a higher apparent yield stress and narrower stress hysteresis. The increase in the oxygen concentration facilitated the formation of nano-domain structures and suppressed the formation of athermal ω phase. The unique temperature dependence of the deformation behavior was discussed on the basis of the stability of β phase and athermal ω phase. [ABSTRACT FROM AUTHOR]

In this study, in order to fully elucidate the origin of nano-domain structure and abnormal thermal expansion behavior of Gum metals, the effects of oxygen concentration, β phase stability and cold rolling on the microstructure and thermal expansion properties were investigated in (Ti– x Nb–2Zr–0.7Ta)– y O (at.%) ( x = 21–25; y = 0.3–1.8) alloys. Normal thermal expansion behaviors with positive linear thermal expansion coefficients were observed in all the annealed alloys irrespective of oxygen and Nb concentrations. The thermal expansion behavior of as-rolled alloys was strongly affected by the concentration of oxygen and Nb. A negative thermal expansion coefficient was observed in the as-rolled alloys with lower oxygen and Nb concentrations. The thermal expansion coefficient increased with increasing oxygen and Nb concentrations. The amount of nano-domains increased with increasing oxygen concentration while decreased with increasing Nb concentration. The growth of a preferential nano-domain variant was observed in as-rolled alloys and became more prominent with decreasing oxygen and Nb concentrations. The reverse transformation from the nano-domain structure to the β phase occurred with increasing temperature. Thermal expansion behavior of as-rolled alloys resulted from a competing relationship between the contribution of lattice distortion of nano-domain structure and the contribution of lattice vibration of the β phase. [ABSTRACT FROM AUTHOR]

The mechanical properties and shape memory behavior of Ti–26Nb–(0–1.0)B and Ti–27Nb alloys were investigated. The stress for inducing martensite and the critical stress for slip deformation were greatly affected by increasing B content up to 0.1 at%, and less affected by further increase in B content up to 0.7 at%. The martensitic transformation start temperature decreased suddenly with a slope of 350 K/at% B when B content was less than 0.1 at% and decreased with a smaller slope of 15 K/at% B with further increase in B content up to 1.0 at%. In all samples tested, only superelasticity was observed at room temperature. TEM investigations revealed that the solubility of B in Ti–26Nb alloy was around 0.1 at%. In spite of the low solubility limit, B was effective to stabilize the superelastic behavior by effectively increasing the critical stress for slip deformation. On the other hand, while the stress for inducing martensite increased effectively by the addition of 1.0 at% Nb to the Ti–26Nb alloy, the critical stress for slip deformation remained almost constant and hence superelasticity was deteriorated. [ABSTRACT FROM AUTHOR]

While the stress for inducing the martensitic transformation in a Ti–27Nb alloy changed only slightly on aging, the stress at which the martensite phase starts to plastically deform and the ultimate tensile strength increased on aging due to the formation of thermal ω phase or α phase. An anomalous temperature dependence of the shape memory behavior was also observed and explained by the fact that the thermal ω phase encouraged the formation of the athermal ω phase during cooling. [ABSTRACT FROM AUTHOR]

A new class of Ti–50Zr base biomedical superelastic alloys was developed in this study. The (Ti–Zr)–Mo–Sn alloys exhibited a shape memory effect and superelastic property by adjusting Mo and Sn contents. The (Ti–Zr)–1.5Mo–3Sn alloy revealed the most stable superelasticity among (Ti–Zr)–(1–2)Mo–(2–4)Sn alloys. The superelastic recovery strain showed a strong dependence on heat treatment temperature after cold working in the (Ti–Zr)–1.5Mo–3Sn alloy. The superelastic recovery strain increased as the heat treatment temperature increased although the critical stress for slip decreased. The (Ti–Zr)–1.5Mo–3Sn alloy heat treated at 1073 K exhibited excellent superelastic properties with a large recovery strain as large as 7% which is due to the strong {001} β <110> β recrystallization texture. [ABSTRACT FROM AUTHOR]

The effects of Nb content and heat treatment temperature on the superelastic properties of Ti–24Zr–(8–12)Nb–2Sn alloys, developed for potential biomedical applications, were investigated by tensile testing. The Ti–24Zr–10Nb–2Sn alloy exhibited excellent superelastic properties: its superelastic recovery strain increased with increasing heat treatment temperature and reached 7%, which is twice that of other Ni-free Ti-base superelastic alloys. This was explained by the evolution of the strong {0 0 1}〈1 1 0〉 recrystallization texture. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the origin of twinning, which is a unique twinning mode in metastable β-Ti alloys, was investigated. The possible twinning modes in metastable β-Ti alloys are derived by considering lattice instability in addition to the theory of the crystallography of deformation twinning. In order to consider lattice instability in the body-centered-cubic (bcc) structure, a modulated structure was proposed. We then clarify how the modulated structure facilitates twinning preferentially compared to other possible twinning modes by evaluating the magnitude of twinning shear and the complexity of shuffle. It was found that the lattice instability makes twinning, which is a major twinning mode observed in alloys with stable bcc structure, unlikely to occur because the lattice modulation causes additional shuffling to form the twin. It was also found that twinning is the most possible twinning mode in the modulated structure in terms of the magnitude of twinning shear and the complexity of shuffle. The new formation model of the twin presented in this study can explain how the lattice instability preferentially activates the twinning in metastable β-Ti alloys. [Copyright &y& Elsevier]

The effects of Sn content on the stress hysteresis and superelastic properties of Ti–15Nb–3Mo–(0–1.5)Sn were investigated. The stress hysteresis decreased with increasing Sn content due to the suppression of athermal ω phase formation. The addition of Sn was also very effective at increasing the superelastic recovery strain. Due to the dual effect of Sn, which both decreases the M s and suppresses the athermal ω phase, the stress for inducing martensitic transformation decreased with increasing Sn content up to 1at.%, then increased on further addition. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure and martensitic transformation behavior of (Ti–23Nb)–1.0O (at.%) alloy have been closely and systematically investigated by transmission electron microscopy (TEM). Diffuse streaks along the directions with intensity maxima at 1/2 positions between basal spots are observed in selected area diffraction patterns obtained from the β phase. The streaks correspond to six variants of transverse lattice modulation caused by randomly distributed oxygen atoms and their local strain fields. Nanosized modulated domains (nanodomains) in the β phase are confirmed in dark field micrographs. These nanodomains act as local barriers to martensitic transformation, thereby suppressing long-range martensitic transformation in the (Ti–23Nb)–1.0O alloy. We suggest a new mechanism for superelasticity in oxygen-containing β-Ti–Nb alloys based on the results of in situ X-ray diffraction measurements and in situ TEM observations. [Copyright &y& Elsevier]

Ti alloys have attracted continuing research attention as promising biomaterials due to their superior corrosion resistance and biocompatibility and excellent mechanical properties. Metastable β-type Ti alloys also provide several unique properties such as low Young's modulus, shape memory effect, and superelasticity. Such unique properties are predominantly attributed to the phase stability and reversible martensitic transformation. In this study, the effects of the Nb and Zr contents on phase constitution, transformation temperature, deformation behavior, and Young's modulus were investigated. Ti–Nb and Ti–Nb–Zr alloys over a wide composition range, i.e., Ti–(18–40)Nb, Ti–(15–40)Nb–4Zr, Ti–(16–40)Nb–8Zr, Ti–(15–40)Nb–12Zr, Ti–(12–17)Nb–18Zr, were fabricated and their properties were characterized. The phase boundary between the β phase and the α′′ martensite phase was clarified. The lower limit content of Nb to suppress the martensitic transformation and to obtain a single β phase at room temperature decreased with increasing Zr content. The Ti–25Nb, Ti–22Nb–4Zr, Ti–19Nb–8Zr, Ti–17Nb–12Zr and Ti–14Nb–18Zr alloys exhibit the lowest Young's modulus among Ti–Nb–Zr alloys with Zr content of 0, 4, 8, 12, and 18 at.%, respectively. Particularly, the Ti–14Nb–18Zr alloy exhibits a very low Young's modulus less than 40 GPa. Correlation among alloy composition, phase stability, and Young's modulus was discussed. [ABSTRACT FROM AUTHOR]

Ti–Ni–Pd shape memory alloys are promising candidates for high-temperature actuators operating at above 373 K. One of the key issues in developing high-temperature shape memory alloys is the degradation of shape memory properties and dimensional stabilities because plastic deformation becomes more pronounced at higher working temperature ranges. In this study, the effect of the Ti:(Ni + Pd) atomic ratio in TixNi70−xPd30 alloys with Ti content in the range from 49 at.% to 52 at.% on the martensitic transformation temperatures, microstructures and shape memory properties during thermal cycling under constant stresses were investigated. The martensitic transformation temperatures decreased with increasing or decreasing Ti content from the stoichiometric composition. In both Ti-rich and Ti-lean alloys, the transformation temperatures decreased during thermal cycling and the degree of decrease in the transformation temperatures became more pronounced as the composition of the alloy departed from the stoichiometric composition. Ti2Pd and P phases were formed during thermal cycling in Ti-rich and Ti-lean alloys, respectively. Both Ti-rich and Ti-lean alloys exhibited superior dimensional stabilities and excellent shape memory properties with higher recovery ratio and larger work output during thermal cycling under constant stresses when compared with the alloys with near-stoichiometric composition. [ABSTRACT FROM AUTHOR]

The temperature dependence of nano-sized lattice modulation (nanodomains) introduced by oxygen atoms and martensitic transformation behavior of the (Ti–23Nb)–1.0O alloy were systematically investigated. Nanodomains in the β phase became unstable at higher temperatures and disappeared >373 K, where time-dependent heating-induced forward transformation (HIFT) was confirmed in the (Ti–23Nb)–1.0O alloy. The thermally activated process of HIFT in the (Ti–23Nb)–1.0O alloy was the diffusion of oxygen atoms in nanodomains. Anomalous temperature dependence of shape memory behavior was also confirmed in the (Ti–23Nb)–1.0O alloy, and its mechanism was successfully explained by the diffusion of oxygen atoms during tensile deformation. [ABSTRACT FROM AUTHOR]

(Ti–Zr–Hf) (97− x) – x Nb–3Sn (Ti:(Zr + Hf) = 1:1) alloys were designed for biomedical superelastic alloys with magnetic resonance imaging (MRI) compatibility. The effects of Zr and Hf on constituent phases, superelasticity, shape memory effect and magnetic susceptibility were clarified. The specific Nb content for superelasticity and shape memory effect was also explored. It was found that the (Ti–Zr)–6.5Nb–3Sn alloy exhibited a superelastic recovery strain of 5.2 % with lower magnetic susceptibility compared to conventional Ti–Ni and Ti–Nb alloys. The magnetic susceptibility decreased with increasing Hf content. The (Ti–Hf)–9Nb–3Sn alloy showed a superelastic recovery strain of 0.9 % and very low magnetic susceptibility less than half of that of the conventional alloys. Based on the lattice parameters of these alloys, it was found that these alloys exhibited larger transformation lattice strain between a β phase and an α " phase than those of the Ti–Nb alloys, suggesting high potential to exhibit a large superelastic recovery strain. According to orientation analyses of constituting grains, the larger superelastic recovery strain in the (Ti–Zr)–6.5Nb–3Sn alloy than that of the (Ti–Hf)–9Nb–3Sn alloy is considered to be caused by a strong recrystallization texture of {001} β <110> β. By optimization of heat-treatment temperature, superelastic recovery strain of 3.6 % was achieved in the (Ti–0.5Zr–0.5Hf)–7.5Nb–3Sn alloy. [ABSTRACT FROM AUTHOR]

Abstract: The effect of room temperature (RT) aging on the superelasticity of Ti–Nb–Mo-based superelastic alloys is investigated. The results show that annealing at relatively low temperatures such as 973K after severe cold rolling results in poor resistance to the effect of RT aging. The transformation stress increases considerably due to the formation of an isothermal ω phase at RT. Addition of Sn is partially effective in suppressing the RT aging effect in the specimens annealed at 973K. The RT aging effect is suppressed by increasing the annealing temperature, due to the annihilation of lattice defects or non-equilibrium vacancies introduced during cold rolling, which are responsible for accelerating the diffusion process, however, superelasticity is reduced by annealing at higher temperatures, due to a decrease in the critical stress for slip deformation (σ CSS). The specimen annealed at 1173K followed by aging at 773K exhibits stable superelasticity with a high resistance to the effect of RT aging. Annealing at 1173K causes the annihilation of lattice defects or non-equilibrium vacancies, while aging at 773K induces precipitation of the α phase, which in turn causes an increase in σ CSS, and further enhances the resistance to the RT aging effect by enriching the matrix with β-stabilizing elements. [Copyright &y& Elsevier]

• Isothermal formation of α′′ martensite in Ti–20Nb–0.7O alloy was observed. • Oxygen diffusion was the rate-determining process for isothermal α″ formation. • Imbalance in the nanodomain variants caused the formation of α′′ martensite. The isothermal martensitic transformation (IMT) behavior of Ti–20Nb–0.7O (mol %) alloy was investigated via X-ray diffractometry. Isothermal formation of the α″ martensite phase was observed, and the activation energy was determined using the time-temperature-transformation diagram coupled with differential scanning calorimetry. IMT in Ti–20Nb–0.7O alloy was explained by nanometer-sized local lattice modulations (nanodomains) and diffusion of oxygen atoms in the β phase. The latter was found to be the primary rate-determining process. [ABSTRACT FROM AUTHOR]

Constituent phases, shape memory and superelastic properties and microstructure in β -type Zr–(6–10.5)Nb–(2–5)Sn (at.%) alloys were investigated. Zr–(8–10.5)Nb–(2–4)Sn alloys exhibited shape memory effect and partial superelastic recovery. Zr–(8–9)Nb–5Sn alloys exhibited distinct superelasticity. The alloys with low Nb content (x < 8 at.%) exhibited an α " phase and a β phase. For the alloys with high Nb content (x ≥ 8 at.%), in addition to the matrix of a β phase, a β ′ phase with a body-centered tetragonal structure was observed. Lattice deformation strains were calculated based on the lattice parameters of the β phase and the α " phase. The distribution of the β ′ phase showed no significant Nb content dependence, whereas the addition of Sn caused a sparse distribution of the β ′ phase. In situ X-ray diffraction measurements under heating and cooling revealed that transformation between the β phase and the β ′ phase is reversible. The β ′ phase was absent in the deformation-induced α " phase, suggesting that β '→ α " phase transformation occurred during the deformation together with β → α " phase transformation. TEM observation revealed that dislocations remained in the deformation-induced α " phase closed to the β ′ phase. The presence of the β ′ phase was considered to be the cause for a larger stress hysteresis and a smaller superelastic recovery strain in the Zr-based superelastic alloys than those in Ti-based superelastic alloys. The effect of the β ' phase on the superelastic properties of Zr-based alloys was discussed in comparison with Ti-based alloys. [ABSTRACT FROM AUTHOR]

Novel superelastic alloys and shape memory alloys composed of multi-principal elements with non-toxic and low magnetic susceptibility were developed. Phase constitution, crystallographic characteristics, mechanical properties, superelasticity and shape memory effect were investigated in Ti–Zr–Hf–Nb–Sn alloys. According to X-ray measurements, compositions exhibiting β phase, α ' phase and α " phase were determined. The lattice parameters of the phases were also determined. Lattice deformation strains were calculated based on the lattice parameters. The Ti–Zr–Hf–Nb–Sn alloys exhibited high tensile strength in a range of 600–1000 MPa. Superelasticity and shape memory effect due to β → α " martensitic transformation and the reverse transformation were observed in the Ti–Zr–Hf–Nb–Sn alloys. Superelastic recovery strain of 2.1% was observed in a (TiZrHf)–8.5Nb–3Sn alloy. The (TiZrHf)–8.5Nb–3Sn alloy exhibited low magnetic susceptibility comparing with alloys applied in medical fields. Potential of large superelastic recovery strain of the alloys was discussed based on the crystallographic and microstructural characteristics. • Novel superelastic alloys with low magnetic susceptibility are developed. • Ti–Zr–Hf–Nb–Sn multicomponent alloys exhibit higher strength than Ti-based alloys. • The Ti–Zr–Hf–Nb–Sn alloys exhibit over 2% of superelastic recovery strain. • The shape recovery is due to β → α " martensitic transformation and its reversion. • The alloys potentially perform large recovery strain due to large transformation strain. [ABSTRACT FROM AUTHOR]