Martensitic Transformation Of Ni_2fega Ferromagnetic Shape-memory Alloy Studied Via Transmission Electron Microscopy And Electron Energy-loss Spectroscopy

摘要

The structural properties of Ni_2FeGa Heusler alloy synthesized by melt-spinning technique have been systematically studied by means of in situ heating and cooling transmission electron microscopy. It was found that the Ni_2FeGa alloy was annealed into a well-defined L2_1 structure at around 980 K, and complex micro-structural domains appeared along with lowering temperature. At room temperature (293 K), a rich variety of micromodulated domains were observed. The domain structures were aligned along the <110> or <100> directions resulting to complex tweed structures. Below martensitic transformation (MT) temperature (M_s, ～ 142 K), the cubic parent phase transformed into unmodulated martensitic variants and modulated martensitic variants. The variants were alternated along the <100> direction with various arrangements and steplike incommensurate boundaries. The modulated martensitic variants were composed of lamellar structures that have predominately a 5M modulation structure along the <110> directions. The electron energy-loss spectroscopy analysis of the low-loss region and the electron energy-loss near-edge fine structure revealed a visible change of the electronic structure along with MT, which can be well interpreted by means of intra-atomic or intraband charge redistribution due to spd orbital hybridization among the Ni-Fe-Ga atoms.