In situ transmission electron microscopy studies of directionally solidified Ni–Mn–Ga ferromagnetic shape memory alloys

Transmission electron microscopy has been used to examine the microstructure and transformation behaviour of four non-stoichiometric Ni–Mn–Ga oriented polycrystalline alloys grown by seedless Bridgman solidification. Despite similar average compositions, the observed microstructures and transformation temperatures of these alloys varied significantly. For alloy A with nominal composition Ni50.1Mn28.2Ga21.7 (at.%), the Heusler ordered (L21) austenite phase was observed to exhibit characteristic “tweed” strain contrast at room temperature. On cooling, sharp satellite reflections associated with the formation of an intermediate phase appeared at ∼200 K, prior to a martensitic transformation at ∼190 K which generated a microstructure consisting predominately of seven-layer martensite with regions of five-layer martensite. For alloys B (nominal Ni49.3Mn28.6Ga22.1) and C (nominal Ni49.3Mn28.8Ga21.9), the martensitic transformation occurred slightly below room temperature and the TEM images revealed a fine-scale intermixture of austenite with five-layer, seven-layer and non-modulated martensite. The microstructure of alloy D (nominal Ni49.3Mn28.8Ga21.9), with a transformation temperature somewhat above room temperature was composed predominately of seven-layer martensite. The results indicate that small microscale variations in composition and/or parent phase grain size have a significant effect on the transformation behaviour of directionally solidified Ni–Mn–Ga alloys.