M. J. Drexel, Confirmd LLC, San Carlos, CA; G. S. Selvaduray, San Jose State University, San Jose, CA; A. R. Pelton, Nitinol Devices & Components, Fremont, CA
The purpose of this paper is to report on the effects of cold work on the aging response of standard Ti-50.8Ni (at.%) wire. As-drawn wire of 30% or 50% cold work was heat treated in a salt bath between 300-550°C at times of 2-180 min. The resultant thermal response and mechanical properties showed the effects of dynamic evolution of the microstructure in the traditional shape setting temperature range. The results of this study demonstrate that the interplay of the thermally activated processes produce differences in the heat treated 30% and 50% cold worked wire. The differences in the austenite finish temperature (Af) primarily occur at times below 60 min. and for temperatures between 350-500°C. The observed general increases in Af support the nucleation and growth phenomena expected when aging nitinol. The recovery processes are shown to dominate the precipitation hardening effects resulting in significant decreases in tensile strength with temperature and time. A greater reduction in tensile strength was observed in the 50% cold work sample due to its higher stored internal energy prior to aging.
Summary: An investigation into the effect of cold work and heat treatment parameters on the aging response of standard Ti-50.8Ni (at.%) wire was performed. As-drawn wire of 30% or 50% cold work was heat treated in a salt bath between 300-550°C at times of 2-180 min. The wire’s thermomechanical response was characterized through bend and free recovery as well as tensile testing. The dominant thermally activated processes dictated the final properties.
