Crystallography and morphology of twins in equiatomic TiPt martensite

Twins in the B19 (2H) martensite in Ti-Pt shape memory alloy have been investigated by transmission electron microscopy. There are three twinning modes evident, i.e. {111}_2H Type I, 〈121〉_2H Type II and {101}_2H compound twins. In addition to the 2H martensite, various long period stacking structures are observed such as 21̄12̄ stacking and incommensurate like modulated martensites. It Is frequently observed that two martensite structures derived from the identical closed packed plane of the parent phase coexist in the same area, namely, 'two-in-one' morphology. The crystallographic aspects of the 'two-in-one' morphology are also discussed.