Phase Transition and Texture Evolution in the Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys Studied by a Neutron Diffraction Technique

The phase transition and influence of the applied stress on the texture evolution in the as-cast Ni-Mn-Ga ferromagnetic shape-memory alloys were studied by the time-of-flight (TOF) neutron diffraction technique. The neutron diffraction experiments were performed on the General Purpose Powder Diffractometer (Argonne National Laboratory). Inverse pole figures were determined from the neutron data for characterizing the orientation distributions and variant selections of polycrystalline Ni-Mn-Ga alloys subjected to different uniaxial compression deformations. Texture analyses reveal that the initial texture for the parent phase in the as-cast specimen was composed of \( {\left\{ {{\text{001}}} \right\}}{\left\langle {{\text{100}}} \right\rangle } \), \( {\left\{ {{\text{001}}} \right\}}{\left\langle {{\text{110}}} \right\rangle } \), \( {\left\{ {{\text{011}}} \right\}}{\left\langle {{\text{100}}} \right\rangle } \), and \( {\left\{ {{\text{011}}} \right\}}{\left\langle {{\text{110}}} \right\rangle } \), which was weakened after the compression deformation. Moreover, a strong preferred selection of martensitic-twin variants (\( {\left\{ {{\text{110}}} \right\}}{\left\langle {{\text{001}}} \right\rangle } \)and \( {\left\{ {{\text{100}}} \right\}}{\left\langle {{\text{001}}} \right\rangle } \)) was observed in the transformed martensite after a compression stress applied on the parent phase along the cyclindrical axis of the specimens. The preferred selection of variants can be well explained by considering the grain/variant-orientation-dependent Bain-distortion energy.