Superelasticity and microstructure of Ti 50+x Pd 30 Ni 20-x high-temperature shape memory alloys

Abstract

In off stoichiometric Ti<SUB>50+x</SUB>Pd<SUB>30</SUB>Ni<SUB>20+x</SUB> alloys, it has been confirmed that B2-B19 phase transformations take place as temperature changes, and found that the phase transformation temperatures decrease with the increase of Ti content deviation from 50 at. %. Shape memory effect (SME) was determined at room temperature and at temperature higher than the austenite finish temperature, respectively. The room temperature SME is evaluated as a total strain of 7.2% with a recovery rate of 100%, and weakens with the increase of tensile strain. SME at high temperature indicates that stress-induced martensite remains in the alloys after unloading. Complete linear superelasticity has been observed by training the specimen under loading-unloading cycling. The shapes of superelastic cycles for specimens of different phases are different, and superelastic strain can be changed according to the load level of training. The yield stress for specimen in austenitic state is much higher than that in martensitic state, and the two kinds of specimens show different strain-hardening ability. The fracture surface shows overall characteristics of brittle failure for specimens both in austenitic and martensitic state. The microstructures of the specimens were also investigated. It is revealed that thermomechanical treatment and training process are necessary steps for preparation the superelastic alloy.