Transmission electron microscopy of twins in martensite in Ti-Pd shape memory alloy

Twins in the B19 martensite in the Ti-Pd shape memory alloy have been investigated by conventional transmission electron microscopy (CTEM) and electron diffraction. There were three twinning modes, i.e. {111} Type I, 〈121〉 Type II and {101} compound twins, in the martensite. The {111} Type I and 〈121〉 Type II twinnings which were conjugate to each other coexisted in a same martensite variant. The {111} Type I twins were dominantly observed and the 〈121〉 Type II twins were less frequently observed. The former twinning was considered to be a lattice invariant shear. The Type II twin plate appeared in two types of forms. The first one was directly connected to the Type I plate. In other words, the twin plate was inclined at crystallographically defined angle. The second one branched off from the Type I plate. Since there was no martensite variant consisting wholly of the 〈121〉 Type II twins throughout the present observations, the 〈121〉 Type II twins were considered to be a deformation twin due to the elastic interaction during the transformation. The {101} compound twinning was also considered to be a deformation twin which was introduced as result of elastic interactions during the transformation since the twin had an isolated fashion in the martensite variant consisting of {111} Type I twins.