Structural differences between single crystalline and polycrystalline NiMnGa-based alloys

Abstract

Three different martensitic crystal structures i.e. 10M, 14M, and NM of NiMnGa-based alloys, occurring in different forms, such as powders, polycrystalline melt-spun ribbons, and bulk single crystals were characterized in detail. The crystal structure of materials having the same chemical compositions and martensitic structures but existing in different forms were evaluated using high-energy synchrotron radiation, showing significant changes in lattice parameters. Additionally, the samples examined by high-resolution TEM imaging and electron diffraction showed local structure changes including variations of stacking fault sequence, and lattice distortion at the grain and twin boundaries. The changes in lattice parameters of unit cells as well as the intensity of modulation reflections have been discussed in terms of microstructure (single variant, multivariant state), internal stresses, dislocation density, atomic shuffling, periodic and partially periodic atom displacements. Moreover, the effect of heat treatment on microstrain level, dislocation density, and formation of martensitic structures was investigated.