Twin microstructure in 5M Ni-Mn-Ga alloys

A very interesting feature of the martensitic transformation is the microstructure it produces. The initial microstructure of 5M Ni-Mn-Ga alloys is a typical self-accommodated martensite containing a mixture of variants. The 5M Ni-Mn-Ga single and polycrystals investigated consist of macro and micro twins. The macro twins are observed as lamellae separating fine twins which frequently cross the coarse variants in a zig-zag manner with mirror symmetry. However, materials with a very large or small grain size show some deviation from this configuration indicating that the arrangement of martensitic variants is significantly affected by grain size. Moreover, fine-grained material does not completely transform to martensite giving rise to some residual austenite. The results are discussed with respect to grain size, deformation mode and phase transformations.