Transformation behavior of two Ni–Mn–Ga alloys

Abstract

Abstract The transformation behavior of Ni 50.5 Mn 30.4 Ga 19.1 and Ni 53.7 Mn 26.4 Ga 19.9 was studied with ac magnetic susceptibility, differential scanning calorimetry, in-situ optical microscopy and X-ray diffractometry. The liquidus, solidus and L2 1 ⟶B2’ temperatures were detected for the alloys. The transformation enthalpies of melting, solidification and martensitic/reverse reactions were determined. In Ni 53.7 Mn 26.4 Ga 19.9 the magnetic transition was clearly below the parent to martensite phase transformation and did not have hysteresis. The transformation sequence is suggested as P param ⟶T param ⟶ T ferrom with reverse reactions in respective order. In Ni 50.5 Mn 30.4 Ga 19.1 the temperatures of the magnetic transition and structural phase transformations are close to each other and the Curie temperature shows a hysteresis of 9 K during cooling-heating cycle. Based on the combined measurements, the transformation sequences are assumed to be during cooling P param ⟶ 7M param ⟶ 7M ferrom ⟶ T ferrom and T ferrom ⟶ T param ⟶ 7M param ⟶ P param during heating. Consequently, the hysteresis in the magnetic transition is explained with the intermartensitic reaction and the difference of the Curie points of 7M and T phases.