Microstructures and phase transformations in as-aged Mn2.04NiGa Heusler alloy

The room-temperature microstructures and phase transformation of Mn2.04NiGa alloy were investigated by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and high-angle annular dark field scanning-TEM (HAADF-STEM). The Mn2.04NiGa alloy annealed at 1073 K for 72 h is mainly austenite phase with Hg2CuTi-type structure. After 623 K/72 h ageing, nano-scaled MnNiGa precipitates considered as Ni2In-type hexagonal structure were found by TEM. A crystalline orientation relationship between precipitates and austensite can be described by (2¯2¯4)A//(01¯10)H, [11¯0]A//[21¯1¯0]H. These precipitates are accompanying with Mn-rich martensite lamellae. Both non-modulated and nearly 16-layered modulated structures are generally in coexistence in one martensite lath. A crystalline orientation relationship between the ageing-induced martensite and austenite is (111)A//(110)M, [01¯1]A//[11¯2]M.