Phase transformations in rapidly quenched Ni-Mn-Ga alloys

Structural and ferromagnetic transitions in shape memory Ni-Mn-Ga alloys and their martensitic structure and microstructure were found to be strongly influenced by rapid quenching from the liquid state and by subsequent aging. Detailed calorimetric and magnetic measurements, dynamic mechanical analysis, and transmission electron microscopy observations were performed to characterize the transformation behavior of thin ribbons compared to the bulk materials. The rapid quenching caused the decrease of the ferromagnetic, martensitic, and premartensitic transformation temperatures, as well as of the saturation magnetization, while the subsequent annealing brought about an increase of the depressed values. This influence was attributed to quenched-in short-range chemical disorder within sublattices, the ordering being improved by a vacancy migration mechanism upon subsequent annealing.