Microstructural and Mechanical Characteristics in Ni-Mn-Ga Alloys Under a Magnetic Field-Assisted Directional Solidification

In the present work, the effect of a transverse magnetic field-assisted directional solidification (MFADS) on the microstructures in Ni-Mn-Ga alloys has been investigated. The results show that the magnetic field is capable of inducing transversal macro segregation perpendicular to magnetic field, causing the emergence of martensite clusters in austenite matrix. Moreover, the magnetic field alleviates the micro segregation on a dendritic scale and promotes the preferred growth of austenite dendrites. On the basis of above investigation, several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress induced martensite transformation. The martensite cluster in austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles. The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized. In addition, the micro segregation on a dendritic scale can significantly influence the martensite variant distribution, corresponding to the performance during compressive circles based on the analysis about deformation gradient tensor. The stress-induced super elasticity is closely dependent on orientation, well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain. The crystallographic evolution has been characterized during in-situ stress-induced transformation. The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys, but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.