Internal structure of B19 martensite in AuTi shape memory alloy

Internal twin of B19 martensite in equiatomic AuTi binary alloy was examined by conventional transmission electron microscopy observation and the phenomenological theory of martensite crystallography (PTMC). The crystal structure of martensite was B 19 (orthorhombic) with the lattice parameters of 0.2944nm, 0.4900nm and 0.4633nm. Most of martensite plates were internally twinned by {111}typeI twin. < 211 > typeII twin was occasionally observed and (0111 compound-twin relationship was observed at boundaries between adjacent martensite plates. However, no martensite plate entirely twinned by the < 211 > typeII twinning or the 1011)compound twinning was observed. PTMC analysis showed that the invariant plane is formed only by the introduction of the internal twin of {111}typeI or < 211 > typeII twin in the present geometry of the transformation. The geometry of a typical martensite plate with internal twin of {111}typeI twin was in good agreement with that required for the formation of the habit plane with the invariant plane character. The observed {111}typeI twin is, therefore, considered to be the lattice invariant shear to minimize the elastic strain energy due to the transformation.