総説・解説等

Recently developed β-Ti alloy Gum Metal is one of attractive materials for biomedical applications due to its excellent combination of low Young's modulus and high strength. Gum Metal also exhibits other unique properties such as non-linear elasticity, plastic deformation without work-hardening. These multifunctional properties in Gum Metal are quite intriguing characteristics not only for practical applications but also for fundamental studies. However the mechanism is not fully understood yet. In this research, in order to obtain new information for special plastic deformation behavior of Gum Metal, the deformation texture and microstructure of Ti-(19-29)Nb-2Zr-0.7Ta-1.2O alloys including typical composition of Gum Metal were investigated by XRD measurement and TEM observation. The major deformation texture of Ti-(21-29)Nb-2Zr-0.7Ta-1. 2O alloys was {001}〈110〉 type. However, the maximum orientation density I max in orientation distribution functions (ODFs) strongly depends on Nb content and the maximum value was obtained at Ti-23Nb-2Zr-0.7Ta-1. 2O, indicating that the strongest deformation texture was formed in Gum Metal composition. TEM observation implied that the strong deformation texture formation in Gum Metal is due to its unique deformation mechanism. © 2008 The Japan Institute of Metals.

We have studied the effect of disorder on the martensitic transformation (MT), especially on the hysteretic behavior of Ti-Nb-O alloy. This system allows arbitrary tuning of the amount of disorder by the introduction of interstitial oxygen atoms into elastically soft host lattice [Ti-26Nb (at. %) alloy], which results in the formation of randomly oriented nanometer-sized martensite as confirmed by dark-field transmission electron microscopy images. In the clean system without interstitial oxygen doping, we observe a sharp hysteresis loop with plateau region in both a stress-strain curve and a strain-temperature curve, whereas a slim hysteresis loop without plateau appears by introducing disorder. The rounding of hysteretic behavior can be understood in random-field Ising model regime, providing the possibility of a new physical picture for MT in a dirty system, where the small size of martensite homogeneously nucleates and percolates the whole system.