Microstructure and phase evolution of Ni<inf>2</inf>FeGa heusler alloy extended to different degrees of undercooling

The Ni2FeGa Heusler alloy is synthesized by arc melting in argon atmosphere. It shows two phase microstructure, γ-phase (disordered fcc) and Austenite (ordered bcc, L21). Phase identification and microstructural characterization were carried out using XRD, SEM and TEM. Solidification at various undercoolings upto 215 °C was performed using flux undercooling technique. B2O3 was used as the flux that provides an inert atmosphere and isolates the molten pool from the quartz tube. The solidified microstructure of the undercooled samples were analyzed and the result indicates γ-phase to be the primary phase to form. The samples are also textured. XRD patterns indicate different texture at different undercoolings. Possible mechanisms for such changes will be discussed. The competitive nucleation mechanism can not also be ruled out as the SEM micrographs show the globular morphology of γ-phase likely due to defragmentation of primary dendrites. Thermal analysis by DSC shows incongruent melting of Ni2FeGa Heusler alloy which strengthen the argument of poor nucleation ability of L21 ordered intermetallic austenite phase as compared to primary γ-phase. Up to achieved undercooling limits, γ-phase forms as the primary phase competitively with the L21 ordered phase. Studies indicate that competitive nucleation mechanism is a likely mechanism to explain the phase selection. © (2014) Trans Tech Publications, Switzerland.