Transformation behavior of prism shaped shape memory alloy single crystals

A methodology of successive compression loads to different faces of a prism shaped SMA single crystal is proposed as an efficient approach to investigate twinning deformation processes in martensites. It is applied to study twinning processes in 2H martensite phase in Cu–Al–Ni single crystals. The method involves an experimental evaluation of compression stress–strain curves, a measurement of complete shape changes due to transformation and twinning processes, and a three surface trace analysis for characterization of planar interfaces. The experiments are supported by theoretical calculations of shape change, analysis of the possible twinning modes and orientation dependence of twinning stresses. It was found that the large difference between the resolved shear stresses for type II (7 MPa) and compound twinning (0.7 MPa) and their orientation dependencies control the stress–strain responses of the 2H martensite in uniaxial compression. Type I twinning was not observed in compression. It is reported that the reversion of the martensite variant single crystal to the austenite upon heating proceeds very fast along a rather complex reverse transformation path at a temperature T = ∼As + 40 K.