Electron microscopy studies of twin morphologies in B19′ martensite in the Ti-Ni shape memory alloy

Twins in the B19′ martensite in the Ti-Ni shape memory alloy have been investigated by conventional transmission electron microscopy (CTEM) and electron diffraction. The following five twinning modes were observed. The 〈011〉 Type II twin was dominantly observed among those and confirmed to be a lattice invariant shear since most martensite variants consisted of the 〈011〉 Type II twinning. The {111-} Type I twinning was morphologically and characteristically divided into three types, i.e. a deformation twin, a variant accommodation twin and a lattice invariant shear. (100) and (001) compound twinnings were considered to be a deformation twin. The 011 Type I twinning was inevitably reconfirmed and considered to be a lattice invariant shear rather than a deformation twin since it extended over the whole martensite plate. The formation order and mechanism of each twinning mode were qualitatively discussed in view of the elastic interaction accumulated during the martensitic transformation.