Systematic analysis of the crystal structure, chemical ordering, and microstructure of Ni-Mn-Ga ferromagnetic shape memory alloys

摘要

Ni-n-Ga based ferromagnetic shape-memory alloys (FSMAs) have shown great promise as an active material that yields a large output strain over a range of actuation frequencies. The maximum strain has been reported to be 6% in the tetragonal martensitic phase and up to 10% in the orthorhombic phase. There has been a large body of work exploring the engineering properties of these alloys but less extensive work in the understanding of the underlying structure and its connection to the material properties. This is particularly true for the off-stoichiometry compositions that are of most practical interest. The crystal structure of Ni-Mn-Ga ferromagnetic shape-memory alloys is extremely sensitive to composition. Several martensitic structures including tetragonal (5-layer), orthorhombic (7-layer) and non-modulated tetragonal have been identified. A systematic exploration of the composition-structure relationship has been performed using x-ray diffraction on samples taken from several single crystals with different compositions. A room temperature phase diagram has been constructed delineating the fields where the tetragonal and orthorhombic martensites are found.