Crystallography and morphology of twins in 9R martensite in Ti–Pd–Fe shape memory alloy

Abstract

Twins in 9R martensite in Ti 50.0Pd 43.0Fe 7.0 shape memory alloy are investigated by conventional transmission electron microscopy. There are four twinning modes, i.e., {1 1 4 } 9R Type I, 〈 5 9 1〉 9 R Type II, {1 0 5} 9R compound and {0 0 1} 9R compound twins. The {1 1 4 } 9R Type I and the 〈 5 9 1〉 9 R Type II twins are considered to be lattice invariant shear in the 9R martensite continuously coexisting with 2H martensite. On the other hand, those two twins and {1 0 5} 9R compound twin can be regarded as variant accommodation twins in the region only consisting of the 9R martensite. Irrational nature of Type II twin boundary, i.e., A:B type pair, is also observed in the edge-on state by high resolution electron microscopy (HREM). The boundary is gradually and randomly curved with strain contrast.