Crystallography of Martensite in TiAu Shape Memory Alloy

The twin structure, habit plane orientation, and morphology of B19 martensite in TiAu, which is a candidate shape memory alloy (SMA) for high-temperature and biomedical applications, were investigated by conventional transmission electron microscopy. Almost all internal twins were {111} type I twins as lattice-invariant deformation (LID). The < 211 > type II twin was scarcely observed in TiAu, unlike in TiPd and TiPt SMAs. The habit plane roughly corresponded to the twinning plane (K-1 plane) of the < 211 > type II twin because of the superb lattice parameter ratio of TiAu. As a result, an energy-minimizing microstructure referred to as "twins within twins'' appears as the major microstructure. The selection rules for the twinning of LID are also discussed considering the results of extensive studies on LID in SMAs.