Effect of high undercooling on Co–Ni–Ga ferromagnetic shape memory alloys

Co45Ni25Ga30 ferromagnetic shape memory alloys were treated by glass fluxing combined with superheating cycling. The effect of high undercooling on solidified microstructure and transformation temperatures was investigated. The martensite lath is obviously refined and a mass of sub-grains are produced with the increase of undercooling. Undercooling rapid solidification introduces a number of dislocations and large internal stress, which give rise to the production of sub-grains in recovery. During the following annealing process, these sub-grains can gradually recrystallize and grow up with an amount of γ′ phase precipitated. Meanwhile, transformation temperatures of undercooled alloys are greatly elevated compared with those of as cast, which results from the large internal stress, and gradually reduce with the heating time.

Research highlights▶ The martensite variants are refined with the increase of undercooling. ▶ A mass of sub-grains are induced by undercooling rapid solidification. ▶ The produce of sub-grains is attributed to the rearrangement of dislocations. ▶ The sub-grains can recrystallize and grow up during the following annealing. ▶ The transformation temperatures are elevated with undercooling and reduce with heating time.