Papers by Author: Shuichi Miyazaki

Abstract: The effect of oxygen content on the stability of α′′ martensite phase of the Ti-20mol%Nb alloy was investigated by differential scanning calorimetry. The reverse martensitic transformation temperature increased with increasing oxygen content, and this means that α′′ martensite phase was stabilized by oxygen addition. The mechanism of α′′ martensite stabilization by oxygen atoms was discussed based on the changes in the lattice parameters of α′′ martensite.

Abstract: Effect of rolling rate on texure of Ti-Mo-Al-Zr shape memory alloy was investigated using X-ray diffraction pole figure measurement and electron backscattering pattern (EBSP) analysis to optimize the thermomechanical processing. Ti-Mo-Al-Zr alloy is a candidate Ni-free shape memory alloy to replace Ti-Ni alloy in medical applications. The alloy was single phase of β before rolling and then α''-martensite was induced during the cold-rolling. Unlike the recrystallization texture in Ti-Nb alloy system, {110}ββ recrystallization texture of β-phase was developed by the solution-treatment especially in the specimens with the reduction rate higher than 90%. This texture has never been observed in Ti-Nb based alloys and can exhibit a larger recovery strain in both tension and compression.

Abstract: The stability of shape memory properties was investigated by thermal cycling tests in Ti-23Ta-(2-4)Sn alloys. The martensitic transformation temperature decreased considerably after aging at 573 K for the Ti-23Ta-2Sn alloy. The decrease in the martensitic transformation temperature by the aging effect was suppressed by the addition of 3at.%Sn and more to the Ti-23Ta alloy. The Ti-23Ta-3Sn alloy exhibited better cyclic stability compared with a Ti-32Ta alloy which has a similar transformation temperature.

Abstract: The effect of reduction rate on the deformation texture of cold-rolled Ti-26mol%Nb-3mol%Al shape memory alloy was investigated. The alloy is the parent phase (β: bcc) at room temperature (RT) and the martensite start temperature is much lower than RT; no residual martensite was detected after cold-rolling. The reduction rate, r, was varied in the range of 60 ~ 99%. Texture evolution was as following; γ-fiber à {001}<110> à {001}<110> + {112}<110> (α-fiber). The strength of {001}<110> was maximized at about r = 97%. The recrystallization texture is expected to be controlled by the reduction rate; optimization of r is supposed to be promising to obtain the {001}<110> recrystallization texture that is preferred for superelastic deformation.

Abstract: In order to develop new nickel-free biomedical Ti-based alloys, effect of silver additions on mechanical properties of Ti-5Cr (mol%) alloy was investigated. Cold workability of Ti-5Cr alloy was 5% in thickness reduction and the cold rolling reduction was improved to be 38% by 2mol% Ag addition and 96% by 4mol%Ag addition. The improvement was due to β phase stabilization. From the XRD results, α’ martensite was the dominant phase in Ti-5Cr-2Ag alloy and β phase was the dominant phase in Ti-5Cr-4Ag alloy. By tensile tests, Ti-5Cr-4Ag alloy showed good strength of 447 MPa in ultimate tensile strength (UTS) and ductility of 13% in fracture strain. Ti-5Cr-4Ag showed higher hardness of HV398 than Ti-5Cr-2Ag with HV288. The hardening by increase of Ag is probably due to the solid solution strengthening. By the cyclic loading-unloading tensile tests with a constant strain increment, Ti-5Cr-4Ag showed pseoudoelastic behavior. Ti-5Cr-4Ag also showed shape memory effect with 57% in shape recovery ratio. It is concluded that Ti-5Cr-4Ag is hopeful as a new non-allergic shape memory material for biomedical applications.

Abstract: The self-accommodation microstructure of a -titanium shape memory alloy (SMA) was investigated by conventional transmission electron microscopy (CTEM) observation. There were two distinct minimum units for the self-accommodation microstructure. One was the V-shaped pair of two habit plane variants that were bounded by the {111}TypeI twin as reported in some previous studies. The other was the bundle of the two habit plane variants that were bounded by the <211>TypeII twin. The later one had not been recognized as the self-accommodation microstructure in -titanium SMAs.

Abstract: Effects of single- and multi-step aging on mechanical properties and shape memory properties of Ti-6Mo-8Al (mol%) biomedical shape memory alloy were studied using tensile tests at room temperature (RT). The solution-treated alloy at RT was two phase of bcc β and martensite α". Tensile tests revealed that the solution-treated alloy exhibited good shape memory effect. As for the single-step aging, (1) pseudoelastic shape recovery by unloading was observed after aging at 623K, (2) the alloy became brittle after aging at 773K due to ω embrittlement, and (3) strength was improved with small shape memory effect by aging at 1023K. On the other hand, after a multistep aging at 773K-1023K-1123K, the alloy was strengthened and showed perfect shape recovery. The improvement must be achieved by the formation of fine and uniform hcp α precipitates.

Abstract: Effect of nitrogen (N) addition on mechanical properties of Ti-Cr-Sn alloy was investigated in this study. Ti-7mol%Cr-3mol%Sn was selected and less than 0.5wt% of N were systematically added. The alloys were characterized by optical microscopy, X-ray diffraction analysis and tensile tests at room temperature. The apparent phase was β (bcc) phase, whereas the presence of precipitates was confirmed in 0.5wt%N-added alloy only which did not exhibit sufficient cold workability. The grain size was not largely affected by N addition being less than 0.5wt%. Tensile tests revealed that less than 0.5wt%N addition improves the strength which is due to the solution hardening by interstitial N atoms.

Abstract: The phase constituents of the Ti-Cr-Au ternary and Ti-Cr-Au-Zr quaternary systems were investigated using X-ray diffraction analysis and optical microscopy. The composition ranges were 3~10mol%Cr and 0~8mol%Au for the ternary system, and 0~42mol%Zr were added to Ti-5mol%Cr-4mol%Au. It was found that the β phase was stabilized by the addition of Cr and Zr, and that A15-type Ti3Au was formed when the Au content was 6mol% or higher. Besides, C14 Laves phase similar to Zr33Ti40Au27 was recognized in Ti-5mol%Cr-4mol%Au-42mol%Zr.

Abstract: In order to produce new β (bcc) Ti alloys for medical applications, effects of Mn substitution for Cr on phase constitution and mechanical properties of Ti-Cr-Sn alloys were investigated. All the Ti-7mol%(Cr, Mn)-3mol%Sn alloys investigated by XRD analysis were identified as β (bcc) alloys, and athermal ω phase was also detected in Ti-7mol%Mn-3mol%Sn . The lattice parameter of β was slightly decreased by Mn substitution. Besides, the Mn substitution for Cr raised the hardness and the strength while reduced the ductility of the Ti-Cr-Sn alloys. The hardening by Mn substitution must be due to ω precipitation. The hardening is discussed from the viewpoint of electron atom ratio (e/a) in comparison with Ti-Cr binary alloys in the literature.