Experimental investigation of simultaneous creep, plasticity and transformation of Ti50.5Pd30Ni19.5 high temperature shape memory alloy during cyclic actuation

The influence of inelastic phenomena (i.e. plasticity and viscoplasticity) on the phase transformation and the cyclic actuation behavior of a Ti50.5Pd30Ni19.5 high temperature shape memory alloy (HTSMA) was investigated by thermomechanical testing. Standard creep and constant stress thermal cycling experiments were conducted to study the rate-independent and rate-dependent irrecoverable strain generation both individually and simultaneously with the phase transformation. Based on a preliminary power-law fit of the creep data from the creep tests, the varying creep rates and stress exponents suggest that the mechanism for creep deformation changes with both stress level and temperature within the likely operating range for the alloy. The load-biased thermal cycling tests show that the material performance can be significantly affected by rate independent plasticity strains generated during transformation, rate dependent viscoplastic strains as well as factors such as retained martensite accumulation. To consolidate these inelastic mechanisms, a combined phase transformation-deformation diagram was constructed to show the phase transformation along with the plastic and viscoplastic regions within the operational range of the HTSMA. (C) 2011 Published by Elsevier B.V.