In situ and post-mortem diffraction contrast transmission electron microscopy was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It was proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {110}B2 planes parallel to {001}B19′ compound twin planes. The findings were compared with transmission electron microscopic results reported in the literature for NiTi and other shape memory alloys. It was suggested that the type of dislocation multiplication mechanism documented in the present study was generic and that it could account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.

On the Multiplication of Dislocations during Martensitic Transformations in NiTi Shape Memory Alloys. T.Simon, A.Kröger, C.Somsen, A.Dlouhy, G.Eggeler: Acta Materialia, 2010, 58[5], 1850-60