Masaki Tahara

Wide-range temperature dependences of Brillouin scattering properties in polymer optical fiber

We investigate the temperature dependences of the Brillouin scattering properties in a perfluorinated graded-index (PFGI-) polymer optical fiber (POF) in a wide temperature range from −160 to 125 °C. The temperature dependences of the Brillouin frequency shift, linewidth, and Stokes power are almost linear at lower temperature down to −160 °C; while they show nonlinear dependences at higher temperature. These behaviors appear to originate from the partial glass transition of the polymer material.

SHAPE MEMORY EFFECT AND CYCLIC DEFORMATION BEHAVIOR OF Ti–Nb–N ALLOYS

The effect of 1 at.% N addition on the shape memory effect of Ti–Nb alloys was investigated. The shape memory effect and superelasticity were observed in Ti–(18–20)Nb–1.0N and Ti–(21–25)Nb–1.0N alloys, respectively. The Ti–23Nb–1.0N alloy exhibits the best superelastic properties with almost perfect recovery and small stress hysteresis among N added ternary alloys. The effect of cyclic deformation on the stability of superelasticity was investigated for the Ti–23Nb–1.0N alloy by loading and unloading cyclic tensile tests up to 500 cycles with a constant maximum applied strain of 2.5%. It was confirmed that the stability of superelasticity was improved by the addition of N due to the increase of the critical stress for slip.

Martensitic Transformation and Related Properties of AuTi-FeTi Pseudobinary Alloys

The effects of Fe addition on martensitic transformation and mechanical properties of AuTi were investigated in this study. It was found that B2 parent phase is stabilized by the Fe addition and that AuTi can contain at least 20mol%Fe. The lattice deformation strain evaluated from θ-2θ X-ray diffraction analysis (XRD) is not significantly changed by the Fe addition. The decrease in Ms evaluated by differential scanning calorimetry (DSC) is-40K/mol%Fe. Tensile tests revealed that, with increasing Fe content, the yield stress decreases up to about 13mol%Fe, largely increases up to 15mol%Fe and then decreases gradually. By taking into account XRD and DSC results, these behaviors are judged to correspond to reorientation of martensite variants, stress induced martensitic transformation and slip deformation of parent phase, respectively. The values of dσSIMT/dCFe and dσSIMT/dT are evaluated to be-170MPa/mol%Fe and-4.3MPa/K, respectively. The elongation is degraded with increasing Fe content from 8% in AuTi (0mol%Fe, martensite phase) to 2% in AuTi-20mol%Fe (parent phase) depending on the apparent phase.

Effect of Zr Addition on Martensitic Transformation in TiMoSn Alloy

The effects of Zr addition on martensitic transformation and the lattice parameters of α” (orthorhombic) martensite and β (bcc) phase were investigated in Ti-3mol%Mo-6mol%Sn based alloys containing up to 4mol%Zr using θ-2θ X-ray diffraction measurement (XRD) and differential scanning calorimetry (DSC). It was found by XRD that orthorhombic α” martensite phase is formed when Zr content is 0 to 2mol% while bcc β phase also existed in the alloy containing 2 to 4mol%Zr. Based on the lattice parameters in α” martensite and β parent phases evaluated, the transformation strains between α” and β phase calculated become slightly small with increasing Zr content. DSC revealed that, with increasing Zr content, reverse martensitic transformation start and finish temperatures decreased down to 410K with a rate of-30K/mol%Zr. It is concluded in the Ti-Mo-Sn alloy system that Zr addition stabilizes β phase and that Zr addition is effective to control martensitic transformation temperature without changing the transformation strains largely.

Effect of Heat Treatment Condition on Texture in Ti-Mo-Al-Zr Shape Memory Alloy

Effect of solution-treatment condition on texture of a Ti-Mo-Al-Zr shape memory alloy was investigated by X-ray pole figure measurement. Ti-Mo-Al-Zr alloy is a Ni-free and β-Ti based biomedical shape memory alloy. Unlike Ti-Nb system alloys, {110}β <001>β recrystallization texture was developed by the solution-treatment above β-transus as the reduction rate increases. However, this texture disappeared by the solution-treatment below β-transus and {531}β <134>β recrystallization texture was strongly developed. It was found that precipitation of hcp α phase has a significant effect on recrystallization texture in this alloy.

Plastic deformation behaviour of single-crystalline martensite of Ti-Nb shape memory alloy

Abstractβ-Ti alloys have attracted considerable attention as new biomedical shape memory alloys. Given the critical importance of the plastic deformation in the martensite phase for the shape memory effect and superelasticity, we investigated here the plastic deformation behaviour of a single crystal of α″ (orthorhombic) martensite of Ti-27 mol%Nb shape memory alloy obtained by the stress-induced martensitic transformation of a single crystal of the parent β phase. Four operative plastic deformation modes were observed, including two dislocation slips and two twinnings. To the best of our knowledge, two of these plastic deformation modes (one dislocation slip and one twinning) were discovered for the first time in this study. The identified slip and twinning systems in the martensite phase have corresponding slip and twinning systems in the parent β phase with which they share many similarities. Therefore, we believe that the plastic deformation of the α″ martensite is inherited from that of the parent β phase.

Determination of Preferred Morphology of Self-Accommodating Martensite in Ti-Nb-Al Shape Memory Alloy Using Optical Microscopy

The preferred morphology of self-accommodation (SA) microstructure in a Ti-Nb-Al shape memory alloy was investigated by the evaluation of the frequency distribution of the habit plane variant (HPV) clusters using in-situ optical microscopy. The observed HPV clusters were classified into two different types; one is the cluster connected by the {111}o type I twin (Type I) and the other is connected by the <211>o type II twin (Type II). The total fractions of the Type I and Type II clusters were 52% and 48%, respectively. The incompatibility at junction planes (JPs) of the two clusters was almost the same among these clusters. However, most of the larger martensite plates (> 50μm) formed Type I cluster at the later stage of the reverse martensitic transformation, i.e., at the early stage of the forward transformation upon cooling. The ratio of the fraction of Type I and II is almost 2:1 at the early stage of the forward transformation.

Phase Constitution and Martensitic Transformation Behavior of Au-51Ti-18Co Biomedical Shape Memory Alloy Heat-Treated at 1173K to 1373K

Phase constitution and martensitic transformation behavior were investigated for a Au–51Ti–18Co alloy heat-treated at 1173 K to 1373 K for 3.6 ks. The Au–51Ti–18Co alloy was fabricated by Ar arc-melting technique and subsequently by hot-forging at 1423 K for 10.8 ks. X-ray diffraction analysis revealed that B2 parent phase, B19 martensite phase and AuTi3 simultaneously appeared regardless of the heat-treatment temperatures. By increasing the heat-treatment temperature, the volume fraction of AuTi3 was slightly reduced. Besides, the lattice transformation strain which was calculated from the precisely-determined lattice parameters was evaluated to be 7 % in the Au–51Ti–18Co alloy in all the heat-treated conditions. This value is comparable to that of NiTi practical alloys. From differential scanning calorimetry (DSC) analysis, reverse martensitic transformation temperature was slightly increased with the heat-treatment temperature. From the lattice transformation strain point of views, the Au–51Ti–18Co has a large potential for novel biomedical shape memory alloy.

Role of Interstitial Oxygen Atom on Martensitic Transformation of Ti-Nb Alloy

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.

Mechanical and Superelastic Properties of Au-51Ti-18Co Biomedical Shape Memory Alloy Heat-Treated at 1173 K to 1373 K

The effect of heat treatment temperature from 1173 K to 1373 K for 3.6 ks on mechanical and superelastic properties of an Ni-free Au-51Ti-18Co alloy (mol%) was investigated. The stress for inducing martensitic transformation (SIMT) and the critical stress for slip deformation (CSS) slightly decrease with increasing the heat–treatment temperature. Regardless of heat–treatment temperature, good superelasticity was definitely recognized with the maximum shape recovery ratio up to 95 % and 4 % superelastic shape recovery strain. As the mentioned reasons, the Au-51Ti-18Co alloy is promising for practical biomedical applications.