Investigation of the martensitic microstructure and magnetostructural coupling in rapidly solidified Mn-Ni-Fe-Ge rod samples

The rapidly solidified Fe-free and 15 at. % Fe-substituted Mn-Ni-Ge-based samples in the rod shape were fabricated with different diameters by the copper mould spraying casting method. Firstly, the lath martensite-like variants was strikingly observed in these compounds by the field emission scanning electron microscope (FESEM), especially in the annealed Mn-Ni-Fe-Ge samples after the mechanical and chemical treatments. It is also found that the martensite (TiNiSi-type with orthorhombic structure) coexists with the austenite (Ni₂In-type with hexagonal structure) around room temperature in the Mn-Ni-Fe-Ge compounds by the FESEM and transmission electron microscope. Resultantly, the thermomagnetic measurements shows that the single, more intimate magnetostructural transformation is achieved around room temperature for the annealed samples compared to the as-cast rods. - Highlights: • The Mn-Ni-Fe-Ge rod samples were prepared by the copper mould spraying casting method. • The lath martensitic-like variants are observed in MnNiGe-based alloys. • TEM and SAED results support strongly the co-existence of the martensite and austenite. • The magnetostructural coupling is strengthened after heat treatment.