Tribological behaviour of biomedical Ti–Zr-based shape memory alloys

The tribological behaviour of Ti–30Zr, Ti–20Zr–10Nb and Ti–19Zr–10Nb–1Fe alloys was investigated using reciprocating friction and wear tests. X-ray diffraction(XRD) results indicate that Ti–30Zr, Ti–20Zr–10Nb and Ti–19Zr–10Nb–1Fe alloys are composed of hexagonal a'-martensite, orthorhombic a'-martensite and bcc β phases,respectively. Ti–30Zr alloy has the highest hardness of HV(273.1 ± 9.3), while Ti–20Zr–10Nb alloy exhibits the lowest hardness of HV(235.2 ± 20.4) among all the alloys.The tribological results indicate that Ti–30Zr alloy shows the best wear resistance among these alloys, corresponding to the minimum average friction coefficient of 0.052 and the lowest wear rate of 6.4x10~(-4)mm3·N~(-1)·m~(-1). Ti–20Zr–10Nb alloy displays better wear resistance than Ti–19Zr–10Nb–1Fe alloy, because the iron oxide is easy to fall off and less Nb_2O_5 films form on the worn surface of the latter.Delamination and abrasive wear in association with adhesive wear are the main wear mechanism of these alloys.     

Shape memory properties of Ti–Nb–Mo biomedical alloys

Mo is added to Ti–Nb alloys in order to enhance their superelasticity. The shape memory properties of Ti–(12–28)Nb–(0–4)Mo alloys are investigated in this paper. The Ti–27Nb, Ti–24Nb–1Mo, Ti–21Nb–2Mo and Ti–18Nb–3Mo alloys exhibit the most stable superelasticity with a narrow stress hysteresis among Ti–Nb–Mo alloys with Mo contents of 0, 1, 2 and 3 at.%, respectively. The ternary alloys reveal better superelasticity due to a higher critical stress for slip deformation and a larger transformation strain. A Ti–15Nb–4Mo alloy heat-treated at 973 K undergoes (2 1 1)〈1 1 1〉-type twinning during tensile testing. Twinning is suppressed in the alloy heat-treated at 923 K due to the precipitation of the α phase, allowing the alloy to deform via a martensitic transformation process. The Ti–15Nb–4Mo alloy exhibits stable superelasticity with a critical stress for slip deformation of 582 MPa and a total recovery strain of 3.5%.     

Microstructures and shape memory characteristics of dual-phase Co–Ni–Ga high-temperature shape memory alloys

The influence of microstructure on mechanical properties and shape memory characteristics of Co–Ni–Ga high-temperature shape memory alloys were investigated in this study. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were employed to detect the microstructures. We found that these alloys were composed of dual phases, a non-modulated tetragonal L1₀ martensite and a face-centered cubic (fcc) γ phase. The martensite was twinned and well self-accommodated. The γ phase was a Co-based solid solution with 30% lower hardness than martensite. Although the fracture mode was intergranular, the strength and plasticity of the alloys increased markedly with the increasing volume fraction of the γ phase. The presence of the γ phase in grain boundaries rather than in the martensite is favorable to shape memory recovery. This was revealed by the maximum shape recovery strain over 5.0% that was obtained in the Co₄₆Ni₂₅Ga₂₉ alloy, with the γ phase formed mainly in grain boundaries.     

Microstructure,mechanical properties and shape memory effect of Ni–Mn–Ga–B high-temperature shape memory alloy

The effects of boron addition on the microstructure, transformation temperature, mechanical properties and shape memory effect of (Ni₅₄Mn₂₅Ga₂₁)_100−xB_x alloys were investigated. The results showed that the martensitic transformation start temperatures M_s decreased monotonically from 465 K for x = 0–278 K for x = 3. Boron addition refined the grain and significantly enhanced the mechanical properties. The compressive fracture strain of 22.3% and reversible strain of 6.8% were obtained in (Ni₅₄Mn₂₅Ga₂₁)_99.5B_0.5 alloy.     

Comparison of high temperature shape memory behaviour for ZrCu-based,Ti–Ni–Zr and Ti–Ni–Hf alloys

New ZrCu-based high temperature shape memory alloys with M_s close to 500 K are under development. The shape memory behaviour of this material is compared to those of Ti–Ni–Zr and Ti–Ni–Hf alloys. The optimal compositions show a shape recovery of not less than 3% at temperatures above 470 K.     

Isothermal and athermal martensitic transformations in Ni–Ti shape memory alloys

Ni–Ti shape memory alloys are known to demonstrate three possible transformation paths between B2 and B19′ phases, B2–R–B19′, B2–B19′ and B2–B19–B19′, depending on their composition and thermo-mechanical treatment. In this work the isothermal kinetics of accumulation of martensite/austenite for all types of martensitic transformations in Ni–Ti and Ni–Ti–X (X = Fe or Cu) has been studied by means of resistance measurements during interruption of cooling/heating scans. Experimental results show that all transformations to the B19′ phase (B2–B19′, R–B19′, B19–B19′) demonstrate a substantial isothermal accumulation of martensite during isothermal dwelling between the martensitic transformation start and finish temperatures. The reverse transformations B19′–R and B19′–B19 are also classified as isothermal. The isothermal accumulation of austenite detected during the reverse B19′–B2 transformation is much less intense, at least partially due to the low sensitivity of resistance to the martensite fraction variation during the reverse transformation, and remains comparable with the resolution of the experimental set-up. The transformations between the B2 and R as well as between the B2 and B19 phases are athermal. Analysis of the entire set of possible transformations in β Ni–Ti systems allows one to conclude that isothermal transformations possess a much broader hysteresis and transformation range compared with athermal ones. Since the hysteresis of the transformation is related to the friction forces acting on interfaces this fact, and also observation of the isothermal effects during reverse martensitic and intermartensitic transformations, strongly support the interpretation of the observed isothermal effects in Ni–Ti as due to the diffusionless but thermally activated motion of interfaces during transformation. The difference between the transformation to B19′ martensite (isothermal) and all others (athermal) is attributed to a distinction in strain accommodation.     

Small-angle neutron scattering of precipitates in Ni–Ti shape memory alloys

Small-angle neutron scattering was performed on polycrystals of Ni–(46–49) at.% Ti quenched in ice water from the solid solution. The presence of small precipitates of a radius of about 1 nm was found for Ni–(46, 47 and 48) at.% Ti. Assuming a composition of Ni₄Ti₃ of the precipitates, their volume fraction varies from 7% to 0.3%. No precipitates are found if the Ti content is closer to stoichiometric NiTi. The formation of these precipitates already during quenching seems to suppress the formation of martensite. Ni–(47.9 and 48.5) at.% Ti were further aged for 1 h at 553 K, and small-angle scattering shows a fully established precipitate microstructure. The particles have a radius of about 1.5 nm and a mean interparticle distance of 4.8–5.8 nm. From the integrated small-angle scattering curves, a volume fraction of Ni₄Ti₃ particles of about 20% is obtained.     

Influence of Zr content on microstructure and mechanical properties of implant Ti–35Nb–4Sn–6Mo–xZr alloys

The influence of Zr content on the microstructure and mechanical properties of implant Ti–35Nb–4Sn–6Mo–xZr (x=0, 3, 6, 9, 12, 15; mass fraction) alloys was investigated. It is shown that Ti–35Nb–4Sn–6Mo–xZr alloys appear to have equiaxed single β microstructure after solution treatment at 1023 K. It is found that the grains are refined first and then coarsened with the increase of Zr content. It is also found that Zr element added to titanium alloys has both the solution strengthening and fine-grain strengthening effect, and affects the lattice parameters. With increasing the Zr content of the alloys, the strength increases, the elongation decreases, whereas the elastic modulus firstly increases and then decreases. The mechanical properties of Ti–35Nb–4Sn–6Mo–9Zr alloy are as follows: σ_b=785 MPa, δ=11%, E=68 GPa, which is more suitable for acting as human implant materials compared to the traditional implant Ti–6Al–4V alloy.     

Activation of magnetic shape memory effect in Ni–Mn–Ga alloys by mechanical and magnetic treatment

Mechanical and magnetic training decreased considerably the twinning stress of five-layered modulated, approximately tetragonal martensite (5M) of Ni–Mn–Ga and Ni–Mn–Ga doped with Fe, and of seven-layered modulated, approximately orthorhombic Ni–Mn–Ga martensite (7M). Repeated compressions along two perpendicular directions of single crystal specimens of 5M and 7M reduced the twinning stress approximately threefold. Further reduction of twinning stress of 7M was achieved by the rotation of the specimen in 1 T magnetic field several times. The training resulted in the appearance of the magnetic shape memory effect or giant magnetic-field-induced strain in agreement with a simple energy-based model of the effect. The origin of the decrease in twinning stress is discussed.     

Structure and mechanical properties of as-cast Ti–Si alloys

In the present work, the microstructure and mechanical properties of as-cast Ti–Si alloys with a Si content ranging from 1 to 12.5 wt% prepared using a dental cast machine were investigated and compared with commercially pure titanium (c.p. Ti). X-ray diffraction (XRD) for phase analysis was conducted using a diffractometer. Three-point bending tests were performed to evaluate the mechanical properties of all specimens and their microstructure and fractured surfaces were observed using scanning electron microscopy (SEM). Experimental results indicated that the diffraction peaks of the Ti–Si alloys matched those of α-Ti and Ti₅Si₃. All the Ti–Si alloys had higher bending strengths and bending moduli than those of c.p. Ti. For example, the bending strength of Ti–5Si was about 2.6 times that of c.p. Ti, and both Ti–10Si and Ti–12.5Si had the highest bending moduli, which were about 1.8 times higher than that of c.p. Ti. Additionally, Ti–1Si exhibited ductile properties and Ti–3Si and Ti–5Si had a combination of brittleness and ductility. When the Si content was 7.5 wt% or greater, the alloys showed brittle properties. Judging from the results of the mechanical properties and deformation behavior, Ti–1Si, Ti–3Si, and Ti–5Si can be considered highly feasible alloys for prosthetic dental applications if other properties necessary for dental casting are obtained.     

The compositional stability of the P-phase in Ni–Ti–Pd shape memory alloys

The precipitation of the P-phase in Ni–Ti–Pd and Ni–Ti–Pt shape memory alloys has been shown to dramatically increase the martensitic transformation temperature and strength in Ni-rich ternary alloys, yet little is known about the phase's compositional stability. Therefore, the compositional limits of the P-phase have been systematically studied by varying the Pd and Ni content while maintaining the general P-phase Ti₁₁(Ni + Pd)₁₃ stoichiometry. Each alloy was solutionized at 1050 °C followed by water quenching, and aging at 400 °C for 100 h. Four distinct phases were identified by electron and x-ray diffraction: Ti₂Pd₃, B2 NiTi, P- and P1-phases. The latter precipitate phases became more stable with increasing Ni at the expense of the Pd content. Atom probe tomography revealed the P-phase composition to be 45.8Ti–29.2Ni–25Pd (at.%) or Ti₁₁(Ni₇Pd₆) as compared to the P1-phase 44.7Ti– 45.8Ni–9.4Pd (at.%) or Ti₅Ni₅Pd.     

Effect of ternary alloying elements on the shape memory behavior of Ti–Ta alloys

The effect of ternary alloying elements (X = V, Cr, Fe, Zr, Hf, Mo, Sn, Al) on the shape memory behavior of Ti–30Ta–X alloys was investigated. All the alloying elements decreased the martensitic transformation temperatures. The decrease in the martensitic transformation start (M_s) temperature due to alloying was affected by the atomic size and number of valence electrons of the alloying element. A larger number of valence electrons and a smaller atomic radius of an alloying element decreased the M_s more strongly. The effect of the alloying elements on suppressing the aging effect on the shape memory behavior was also investigated. It was found that the additions of Sn and Al to Ti–Ta were effective in suppressing the effect of aging on the shape memory behavior, since they strongly suppress the formation of ω phase during aging treatment. For this reason the Ti–30Ta–1Al and Ti–30Ta–1Sn alloys exhibited a stable high-temperature shape memory effect during thermal cycling.     

TEM study of structural and microstructural characteristics of a precipitate phase in Ni-rich Ni–Ti–Hf and Ni–Ti–Zr shape memory alloys

The precipitates formed after suitable thermal treatments in seven Ni-rich Ni–Ti–Hf and Ni–Ti–Zr high-temperature shape memory alloys have been investigated by conventional and high-resolution transmission electron microscopy. In both ternary systems, the precipitate coarsening kinetics become faster as the Ni and ternary element contents (Hf or Zr) of the bulk alloy are increased, in agreement with the precipitate composition measured by energy-dispersive X-ray microanalysis. The precipitate structure has been found to be the same in both Hf- and Zr-containing ternary alloys, and determined to be a superstructure of the B2 austenite phase, which arises from a recombination of the Hf/Zr and Ti atoms in their sublattice. Two different structural models for the precipitate phase were optimized using density functional theory methods. These calculations indicate that the energetics of the structure are not very sensitive to the atomic configuration of the Ti–Hf/Zr planes, thus significant configurational disorder due to entropic effects can be envisaged at high temperatures. The precipitates are fully coherent with the austenite B2 matrix; however, upon martensitic transformation, they lose some coherency with the B19′ matrix as a result of the transformation shear process in the surrounding matrix. The strain accommodation around the particles is much easier in the Ni–Ti–Zr-containing alloys than in the Ni–Ti–Hf system, which correlates well with the lower transformation strain and stiffness predicted for the Ni–Ti–Zr alloys. The B19′ martensite twinning modes observed in the studied Ni-rich ternary alloys are not changed by the new precipitated phase, being equivalent to those previously reported in Ni-poor ternary alloys.     

Phase transformation behavior and shape memory characteristics of Ti−Ni−Si alloys

Transformation behavior, microstructures and shape memory characteristics of Ti−(50−X)Ni−XSi (X=2, 4, 6 at.%) and (50−X)Ti−Ni−XSi (X=2, 5, 7, 10 at.%) alloys were investigated by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, electrical resistivity measurements and constant load thermal cycling tests. Ti₅Si₃, Ni₁₆Ti₆Si₇ and Ni₄Ti₄Si₇ were formed in Ti−(50−X)Ni−XSi alloys, while Ti₅Si₄, Ni₃Si, Ni₃Ti₂ and Ni₃Ti₂Si were found in (50−X)Ti−Ni−XSi alloys. The total amount of silicides increased with increasing Si content, irrespective of Si content. The B2→B19 transformation occurred in Ti−(50−X)Ni−XSi alloys, and their transformation temperatures appeared to be almost constant. Transformation elongation associated with the B2→B19 transformation decreased with increasing Si content. In contrast to Ti−(50−X)Ni−XSi alloys, a transformation accompanied with structural change did not occur in (50−X)Ti−Ni−XSi alloys.