Study On Microstructure And Mechanical Properties Of Directionally Solidified Ni-Mn-Ga Alloys

The performance and structure control of Ni-Mn-Ga alloy has always been the research hotspots in the field of smart materials.In particular,theγphase in Ni-rich Ni-Mn-Ga alloys enhanced the ductility,showing that Ni-Mn-Ga alloys own potential in practical applications.However,there have been few studies on the morphology,orientation and composition distribution ofγphase and martensite phase in Ni-Mn-Ga alloys in previous researches.In addition,it is meaningful to study the crystallographic evolution and mechanical property under external fields.Therefore,in present study,we aim to obtain alloy samples with different structures by changing the alloy composition or adjusting the micro/macro segregation in the alloy,and further study its influence on the mechanical properties,as well as the microstructure evolution and performance responses of the Ni-Mn-Ga alloy under the external fields.The main conclusions of this study are as follow:The composition distribution and orientation growth of two nickel-rich Ni-Mn-Ga alloys with different compositions of Ni₅₈Mn₂₅Ga₁₇and Ni₆₀Mn₂₅Ga₁₅,as well as their mechanical properties and fracture characteristics were studied.It is found that the element Ni is mainly segregated in theγphase,and the elements Mn and Ga are mainly segregated in theβphase.In addition,in the Ni₅₈Mn₂₅Ga₁₇ alloy,the（001）_Aofβphase is along the growth direction,while in the Ni₆₀Mn₂₅Ga₁₅ alloy,theγPhase has（001）_A preferential growth direction.It is found that although there are both transgranular fracture and intergranular fracture in the Ni₅₈Mn₂₅Ga₁₇ alloy,the transgranular fracture is dominant.In Ni₆₀Mn₂₅Ga₁₅ alloy,theγphase owns good plasticity,which can inhibit the crack propagation of martensite transgranular fracture.Moreover,it is found that the variant reorientation during the stress-induced deformation is compatible with the external stress,and usually occurs in the variant with a higher Schmid factor.The influences of the transverse magnetic field on the segregation and orientation of the directionally solidified Ni-Mn-Ga alloys were studied,and the martensitic transformation evolution and mechanical behavior of the Ni-Mn-Ga alloy under the conditions of thermal field,stress field and magnetic field were also analyzed.It is found that the magnetic field can induce solute segregation on the macroscopic sample scale,and induce the formation of martensite clusters in the solute-rich area on one side of the austenite sample.In addition,the application of a magnetic field can reduce the segregation on the dendrite scale.Furthermore,the subsequent results show that the temperature-induced martensitic transformation grows in a"wedge-shaped"variant pair,which is mainly attributed to the self-accomodation characteristics of the crystal strain of the relevant structure before and after the transformation.In addition,the superelasticity of the sample significantly depends on its crystal orientation,which can be attributed to the different resolved shear stress factor（RSSF）in different directions.Furthermore,the magnetic field can promote the preferential formation of martensite variant with easy magnetization axis parallel to magnetic field during martensite transformation.The research reveals the influence of different structures on the properties of the alloy by adjusting the composition of the Ni-Mn-Ga alloy,explores the influence of the transverse magnetic field on the microstructure of the directionally solidified Ni-Mn-Ga alloys.The findings also deepen the understanding of external field on martensite transformation and mechanical behavior,and enrich the prepare technique study and martensite transformation theory in Ni-Mn-Ga alloys.