Microstructural, crystallographic, and mechanical characteristics in Ni-Mn-Ga alloys directionally solidified under a transverse magnetic field

Abstract

• Magnetic field-assisted directional solidification (MFADS) can control structure. • Macro-/micro-segregation and orientation can be adjusted by MFADS. • Microsegregation-induced microstructure influences mechanical properties. • Stress-induced crystallographic evolution is traced by in-situ EBSD. Abstract In this study, the effect of 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 macrosegregation perpendicular to the magnetic field, causing the emergence of martensite clusters in the austenite matrix. Moreover, the magnetic field alleviates the microsegregation on a dendritic scale and promotes the preferred growth of austenite dendrites. On the basis of the 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 the 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 microsegregation on a dendritic scale can significantly influence the martensite variant distribution, corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor. The stress-induced superelasticity 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.