The microstructure of a Ni–Mn–Ga alloy in the martensitic phase was investigated using transmission electron microscopy. Inter-variant twin boundaries were observed separating non-modulated tetragonal martensite variants. In addition, intra-variant boundary structures, referred to here as “conjugation boundaries”, were also observed. We propose that conjugation boundaries originate at the transformation interface between austenite and a nascent martensite variant.

In the alloy studied, deformation twinning was observed, consistent with being the mode of lattice-invariant deformation, and this can occur on either of two crystallographically equivalent conjugate View the MathML source twinning systems: conjugation boundaries separate regions within a single variant in which the active modes were distinct.

The defect structure of conjugation boundaries and the low-angle of misorientation across them are revealed in detail using high-resolution microscopy. We anticipate that the mobility of such boundaries is lower than that of inter-variant boundaries, and is therefore likely to significantly affect the kinetics of deformation in the martensitic phase.

This article originally appeared in Acta Materialia 71 (2014), pages 255-263.