High-temperature in-situ microscopy during stress-induced phase transformations in Co49Ni21Ga30 shape memory alloy single crystals

This paper reports on the role of repeated stress-induced martensite transformations at different temperatures (referred to as "training") on the pseudoelasticity and microstructural evolution of as-grown Co49Ni21Ga30 [001]-oriented shape memory alloy single crystals under compression. The training resulted in a reduction in transformation stress levels and a stable microstructure with a multi-variant configuration reducing the observed transformation strains. Training also improved the recoverability giving way to a large pseudoelastic temperature window of 400 degrees C. In-situ microscopy revealed the martensite stabilization to be due to pinning of moving interfaces especially at temperatures above 120 degrees C. An insight into the evolution of microstructure and stress strain behavior in terms of stress hysteresis changes with test temperature is provided, and the possible operant mechanisms are discussed.