Micro-structural analysis of self-accommodating martensites in Cu–11.92 wt%Al–3.78 wt%Ni shape memory alloy

Morphologies and configurations of the self-accommodating martensites in Cu–11.92 wt%Al–3.78 wt%Ni shape memory alloy were studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From micro-structural observations, it is clearly observed that the self-accommodating martensites have typical plate group configuration. Each martensite plate group consists of four variants A, B, C, and D. Usually, variants A and C (or B and D) form characteristic spear morphology and variants A and D (or B and C) form fork morphology. Variants A and C are twin-related with respect to the image mirror plane of the 2H martensite. Additionally, TEM observations reveal that two kinds of close-packed martensite structures, 2H (image) and 18R (image) martensites, coexist in the quenched alloy. These structures occur by the localized formation of four variants of a plate group in a self-accommodating manner to minimize the total shape change.