Microstructural evolution during isothermal aging in Ni-rich Ti-Zr-Ni shape memory alloys

Abstract

Microstructural evolution during isothermal aging at temperatures ranging from 673 K to 773 K was investigated by transmission electron microscopy (TEM) for Ni-rich Ti-Zr-Ni alloys. It was revealed that the aging is a two-stage process: the first stage is characterized by circular diffuse scattering in electron diffraction patterns, most likely attributed to short range ordered structure with no significant microstructural change. Second stage is characterized by a fine coherent precipitation followed by the coarsening. The precipitates had a lenticular shape and their habit was {100}B2. The electron diffraction pattern of the sample containing the precipitates exhibited satellite spots at 1/3(110)B2 and 1/4(210)B2 positions. High resolution TEM observations and fast Fourier transformation analysis revealed that the 〈100〉B2 electron diffraction pattern was composed of the reflection from 4 variants of the precipitates. The two-stage microstructural evolutions coincide well with the previously reported changes in mechanical properties and martensitic transformation behavior. © 2007 The Japan Institute of Metals.