Effect Of Heat Treatments On Martensitic Transformations Of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

The effects of thermal cycles and heat treatment on the martensitic transformation' characteristics and crystal structures of Ni-Mn-Ga polycrystal alloys have been studied by means of resistivity measurement, thermal expansion measurement, optical microscopy and X-ray diffraction.The polycrystal Ni_51.4Mn_27.8Ga_20.8 exhibits the excellent thermal stability. In the first several thermal cycles the martensitic transformation temperatures decrease slightly, and then keep constant in further thermal cycles. Transformation sequences and temperatures obtained by resistvity measurement and thermal expansion measurement are different, because resistvity and thermal expansion are different physical characteristics of alloys.The thermal elastic features of martensitic transformations of Ni-Mn-Ga alloys are improved obviously by heat treatment. With increasing heating temperature, M_s and Af of Ni_49.7Mn_29.1Ga_21.2 alloy decrease, M_f and A_s increase, and thermal hysteresis of transformation decrease. This may be caused by disappearing of crystal defect and internal stress in the heat treatment process. As heat treatment time increases, M_s of Ni_50.5Mn_28.4Ga_21.1 alloy decrease and thermal hysteresis decrease. M_s of Ni_51.2Mn_28.0Ga_20.8 alloy increase with increasing cooling rate because of the increasing of internal heat stress induced by cumulation of dislocation.Heat treatment can change crystal structures of Ni-Mn-Ga polycrystal martensite. There is 5M+T dual-phase martensitic microstructure in the as-cast alloys. When Ni_50.5Mn_28.4Ga_21.1 and Ni_51.2Mn_28.0Ga_20.8 are heated at 673K and 873K, respectively, the martensite structure of both alloys are changed from 5M + T to 7M + T; as Ni_50.7Mn_27.2Ga_22.1 is treated at 1073K, 5M+T is changed to 7M.Heat treatment can change crystal structures of Ni-Mn-Ga polycrystal martensite. There is 5M+T dual-phase martensitic microstructure in the as-cast alloys. When Ni_50.5Mn_28.4Ga_21.1 and...