The self-accommodated morphology of martensite in nanocrystalline NiTi shape memory alloys

The martensitic phase transformation of nanocrystalline NiTi causes a unique “herring-bone” microstructure of B19′ not encountered in the coarse grains. The constraints of the grain boundaries make a nanoscale compensation of the transformation strains necessary that is obtained by the autocatalytic formation of self-accommodated lamellae containing (0 0 1)m compound twins. The coherent junction planes of the twinned lamellae were analysed by high-resolution transmission electron microscopy. Alternating segments of the interface are composed of lattice planes of matching nanotwins that are structurally very similar and almost parallel. The compensation of transformation deformations is facilitated by invariant interfaces yielding energy minimization and strain compatibility. Using the geometrically nonlinear theory of the martensitic transformation, a systematic classification of the interfaces is proposed that is based on crystalline symmetry and predicts the observed geometry of both straight and curved junction planes of the “herring-bone” microstructure.