Crystal structure and phase transformation behaviors in two
Ni-Mn-Ga
ferromagnetic shape memory alloys (FSMAs) with compositions of
Ni48Mn30Ga22 and
Ni53Mn25Ga22 (at. %) as a function of temperature were investigated by in situ neutron diffraction experiments. Neutron diffraction technique proves to be highly efficient in characterizing structural transformation in
Ni-Mn-Ga
FSMAs, which consist of nearby elements in the periodic table. Our neutron results show that
Ni48Mn30Ga22 has a cubic, L21 Heusler structure from 373 to 293 K. Its crystal structure changes into a seven-layered orthorhombic martensitic structure when cooled to 243 K, and no further transformation is observed upon cooling to 19 K. Neutron diffraction results also show that
Ni53Mn25Ga22 has a tetragonal I4/mmm martensitic structure from 20 to 403 K. A pre-transformation around room temperature is observed from an abrupt jump in unit-cell volume of
Ni53Mn25Ga22, which corresponds with an endothermic peak detected in a heated DSC curve.