Influence of Thermal-Mechanical Processing on Martensitic and R-Phase Transformations of Ni-Ti and Ni-Ti-Hf Shape Memory Alloys

摘要

A Ti-rich binary NiTi shape memory alloy (SMA) and two ternary SMAs having thecompositions Ni49Ti51-xHf with 1at% and 3at% Hf wereinvestigated. The influence of Hf content, coldworking (CW) and heat treatment temperature (HT)on the martensitic and R-phase transformations wasthermally analyzed. Results show that martensitictransformation temperature (TT) changes with CW,HT as well as Hf content. Different mechanisms canbe responsible for the observed behavior. Thesemechanisms may involve interaction between phasetransformations, dislocations and internalstresses. The effect of Hf substitution is likelydue to the atomic size difference between Ti andHf. A similar study was conducted to examineRphase transformation. Effort was made to identifythe factors responsible for the observed behavior in Rphase transformation.