Ni-Mn-Ga定向凝固合金7M马氏体微观组织形成依据研究OA

The deep understanding of the martensitic microstructure in directionally solidified Ni-Mn-Ga alloys is crucial for optimizing the magnetic field-induced strain obtained through magnetic field-induced martensite variant reorientation.Although previous studies have systematically characterized the typical micro-configurations and orientation relationships of the strongly textured seven-layer modulated(7M)martensite,the underlying formation basis of these microstructural features remains insufficiently under-stood.In this work,the characteristics of microstructural morphology and orientation correlation of the 7M martensite in directionally solidified Ni₅₀Mn₃₀Ga₂₀ alloy were systematically characterized using scan-ning electron microscopy(SEM)and electron backscatter diffraction(EBSD).On this basis,the forma-tion basis of the typical microstructure was analyzed based on the transformation orientation relationship between austenite and martensite phases.Furthermore,the twinning relations between martensitic variants were theoretically predicted and classified using the phenomenological theory of martensite crystallography(PTMC).The fundamental reason for the existence of six variant groups and a total of 24 variants within a single austenite grain lies in the multiple transformation paths corresponding to the six sets of{101}A habit planes of the cubic austenite under the transformation routes of Pitsch orientation relationship.The spatial arrangement of lamellar interfaces within different variant groups can manifest as horizontal,verti-cal,or inclined types,which originates from the geometric position difference of the{101}A orientation relationship planes relative to the directional solidification direction when viewed along the<001>A direc-tion.Further PTMC analysis indicates that 14 types of twinning relations are theoretically possible between 7M martensite variants.The experimentally observed Type Ⅰ,Type Ⅱ,and compound twins can all be categorized as conventional twin types.The analysis method used in this work,based on the mar-tensitic transformation orientation relationship,shows good applicability in explaining the martensitic micro-structural morphology and variant distribution in Ni-Mn-based alloys.However,the phenomenological theory of martensite transformation is primarily used to predict all theoretically possible twin types between martensite variants and cannot directly determine the specific twinning relations observed in experiments.