M. Tahara, H. Y. Kim, S. Miyazaki, University of Tsukuba, Tsukuba, Japan; H. Hosoda, Tokyo Institute of Technology, Yokohama, Japan
Ti-Nb base superelastic alloys have been developed for biomedical applications. The present authors have reported that the addition of Zr and Ta is effective to improve the superelastic properties of Ti-Nb alloys. They have also reported that nitrogen (N) addition is effective to increase the critical stress for slip (CSS) and maximum recovery strain (εmax) in a Ti-20Nb-4Zr-2Ta (at.%) alloy. In this study, the effect of annealing temperature on superelastic properties of a Ti-20Nb-4Zr-2Ta-0.6N alloy was investigated to improve superelastic properties. The Ti-20Nb-4Zr-2Ta-0.6N (at.%) alloy was fabricated by an arc melting method and then homogenized at 1273K for 7.2ks. The ingot was cold rolled with a reduction up to 98.5% in thickness and heat treated at various temperatures between 773 and 1173 K for 3.6 ks. The mechanical properties and superelastic behavior were investigated by tensile tests. Microstructure was investigated by the X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The specimen annealed at 773 K fractured just after yielding. The specimens annealed in the temperature range between 873 and 1173 K exhibited two-stage yielding and superelastic behavior. The β transus temperature was determined to be between 973 and 1073 K. The stress for inducing the martensitic transformation (σSIM) decreased with increasing annealing temperature, taking a minimum-value at 973 K, then increased with further increasing annealing temperature. It is considered that the decrease of σSIM is due to the recovery and recrystallization, and the increase of σSIM is due to the solid solution hardening by N. Although CSS exhibited similar values, εmax increased from 2.5 to 4.0% with increasing annealing temperature from 873 to 1173 K. All specimens annealed above 873 K exhibited stable superelastic behavior at room temperature.
Summary: Recently, Ni-free superelastic alloy have been developed as biomedical materials. The present authors have reported that nitrogen (N) addition is effective to increase the critical stress for slip and maximum recovery strain in a Ti-20Nb-4Zr-2Ta (at.%) alloy. In this study, the effect of annealing temperature on Ti-20Nb-4Zr-2Ta-0.6N alloy was investigated. The specimens were annealed at various temperatures between 773 K and 1173 K for 3.6 ks. All specimens annealed above 873 K exhibited stable superelastic behavior at room temperature.
