The hexagonal-orthorhombic martensitic transformation in MnNiSi1-x(CoNiGe)x alloy: being triggered by pre-existing orthorhombic embryos

The thermal-elastic martensitic transformation from high-temperature Ni2In-type hexagonal structure to low-temperature TiNiSi-type orthorhombic structure has been widely observed in MnMX (M=Ni or Co, and X=Ge or Si) alloys. However, the answer to how the orthorhombic martensite nucleates and grows within the hexagonal parent is still unclear. This paper investigates the hexagonal-orthorhombic martensitic transformation in a Co and Ge co-substituted MnNiSi. One can find some orthorhombic laths embedded in the hexagonal parent at a temperature above the martensitic transformation start temperature (M s). With the cooling of the sample to M s, the laths extend broader, indicating that the martensitic transformation starts from these pre-existing orthorhombic laths. Microstructure observation suggests that these pre-existing orthorhombic laths are not originated from the hexagonal-orthorhombic martensitic transformation because of the difference in atomic occupations of doping elements in the hexagonal parent and pre-existing orthorhombic laths. The phenomenological crystallographic theory and experimental investigations prove that the pre-existing orthorhombic lath and generated orthorhombic martensite have the same crystallography relationship to the hexagonal parent. Therefore, the orthorhombic martensite can take these pre-existing laths as embryos and grow up. This work implies that the martensitic transformation in MnNiSi1-x(CoNiGe)x alloy is initiated by orthorhombic embryos.