Internal-stress-induced martensitic transformation of Co50Ni20Ga30 ferromagnetic shape memory alloys under high undercooling

Co50Ni20Ga30 ferromagnetic shape memory alloys with different undercoolings were prepared by glass fluxing combined with superheating cycling; the effect of undercooling on martensitic transformation in the alloy was studied. Results show that undercooling not only causes grain refinement but also introduces internal stress that rises with undercooling increasing and, when large enough, induces martensitic transformation in the alloy as external stress does. At the moderate undercooling of 179 K, martensitic transformation is observed in some parts of the austenitic matrix due to stress concentration; at the maximum undercooling of 298 K, the internal stress becomes as large as to cause complete martensitic transformation at room temperature. Differential scanning calorimeter proves that transformation temperatures are greatly elevated with undercooling increasing, and X-ray diffraction patterns confirm that the alloy exhibits different phase states at different undercoolings.