Variant Selection Of 7M Martensite In Ni-Mn-Ga Alloys Under External Stress Field

Giant magnetic-field-induced strain(MFIS)has been obtained in Ni-Mn-Ga alloys through variant reorientation induced by magnetic field.Numerous external field trainings have been performed on Ni-Mn-Ga Heusler type alloys with 5M,7M or NM martensite to eliminate the unfavorable variants.However,the variant selection and deformation mechanisms of martensite during training are not fully clarified.In this work,the deformation behaviors and mechanisms of stress field training and the transformation crystallography of thermo-stress field training were studied by analyzing the microstructure and crystallographic orientation evolution during these processes.(1)Microstructure and crystallographic characterizations were performed on the as-annealed Ni50Mn30Ga20 alloy.5 colonies transformed from one parent austenite grain were observed with each colony consisting of four variants with Type-?,Type-? and compound Transformation(TrF)-twin relations.By assuming an applied compressive load along the solidification direction(SD),5 colonies could be divided into two groups with respect of the Schmid factor(SF)of detwinning systems of Type-?/Type-? TrF-twin of the in-colony variants:three of them have high SFs and referred to as high SF colonies and the other two low SF colonies.(2)By characterizing the microstructure evolution and crystallographic orientation changes with in-situ neutron diffraction and EBSD measurement and crystallographic calculation,the deformation behaviors and the deformation accommodation mechanisms were further analyzed.In the early stage of deformation,the detwinning of Type-?/Type-? TrF-twin occurred primarily in high SF colonies,resulting in the thickening of the favorable 7M variants at the expense of the unfavorable variants.The twinning of Type-?/Type-?Deformation(DeF)-twin and intermartensitic transformation in low SF colonies were induced by the applied load,leading to the formation of 7M new variants and NM martensite.The corresponding strains in the low SF colonies were highly coordinated with those in the high SF colonies allowing not only the accommodation of the local strain but also the macroscopic strain.The small differences of deformation between twinning of Type-?/Type-? DeF-twin and intermartensitic transformation accumulated with the compression and caused local stress concentration in the late stage of deformation,resulting in reverse intermartensitic transformation to coordinate the local strain.The four original 7M martensite variants within one colony in the measurement area were replaced by two new 7M variants and NM martensite during unidirectional compression.(3)The transformation crystallography of martensitic transformation under thermo-stress field was analyzed by in-situ neutron diffraction experiments.Without external load,austenite transforms to 7M martensite with the decreasing temperature in the alloys,the orientation relationship(OR)is Pitsch OR({1 0 1}A//{1 2 10?7M,<1 0 1>A//<10 10 1>7M).Under a compressive load of 20 MPa,austenite transforms to 5M martensite with the decreasing temperature,the OR is Pitsch OR({1 0 1}A//{1 2 5?5M,<1 0 1>A//<5 5 1>5M)and {2 20}A//{1 2 5?5M,<2 2 0>A//<5 5 1>5M OR.Under a compressive load of 50 MPa,besides transforms to 5M martensite,austenite also transforms to 7M martensite with the decreasing temperature,the OR is {2 2 0}A//{1 2 10}7M,<2 2 0>A//<10 10 1>7M.The external load applied on martensitic transformation changes the path and the product of transformation.These differences come from the strong synergy between the external stress and the anisotropy of deformation produced by martensitic transformation.This work offers new insights into the deformation behaviors and the crystallography of martensitic transformation in Ni-Mn-Ga alloys,providing useful support for the performance optimization based on external field training.