Effects Of External Field Training On Microstructure And Output Strain In Ni-Mn-Ga Alloys

Ni-Mn-Ga magnetic shape memory alloys are expected to be widely used in actuator and sensor devices due to their outstanding advantages,such as high frequency and giant magneticfield-induced strain(MFIS).It is imperative to regulate the microstructure to obtain strongly oriented polycrystalline Ni-Mn-Ga alloys with excellent magnetic control properties.In this work,in order to obtain large output strain,the textured Ni50Mn28.5Ga21.5 and Ni50Mn30Ga20 polycrystalline alloys were prepared by directional solidification technique and the microstructure of the alloys were optimized by the external field training.Moreover,the effects of external field training on microstructure and output strain in textured Ni-Mn-Ga polycrystalline alloys were thoroughly investigated.The primary results are summarized as follows:The directionally solidified Ni50Mn28.5Ga2i.5 alloy consists of monoclinic five-layered modulated(5M)martensitic single phase at room temperature,which microstructural features can be characterized as alternatively distributed lamellae.The lamellae,which are arranged parallel to the solidification direction(SD),are composed of pairs of thin plates with thickness in the nanometer range.The directionally solidified Ni50Mn30Ga20 alloy is made up of monoclinic seven-layered modulated(7M)martensitic single phase which has the width of lamellae less than 5M martensite and also parallel to the solidified direction.There are four kinds of martensitic variants and three typical twin relationships between different variants in the 7M martensite of the directionally solidified Ni50Mn30Ga20 alloy.The variants alternate with each other and are arranged along the solidified direction,forming strong texture with[010]7M perpendicular to the solidified direction.Both of their original austenite are columnar grain along the solidified direction,which have a strong<001>A preferred orientation parallel to the solidified direction.Stress-coordinated martensite is formed by applying a compressive load along the direction of directional solidification during the martensitic transformation of the directionally solidified Ni50Mn28.5Ga21.5 alloy.With the increase of the number of cycles of thermal-mechanical training,the stress-coordinated martensite structure gradually increased,and the selfcoordinated martensite structure is gradually decreased.Finally only a small amount of selfcoordinated martensite remains.Thermal-mechanical training can reduce the number of martensitic variants and simplify the orientation relationship between the variants through the mechanism of twin boundary motion.After the thermal-mechanical training,the directionally solidified Ni50Mn30Ga20 alloy formed a stress-coordinated martensite structure,the 7M lamellae were straighter and at an angle of-45 with the applied the loading direction(LD).The number of martensite variants were decreased,which with the[010]7M//LD form preferentially,leading to the formation of a strong[010]7M preferred orientation along the LD.With the increase of the number of thermalmechanical training cycles,the stress-coordinated martensite structure gradually increases until the self-coordinated martensite structure is completely replaced,and the number of variants is gradually decreased.Eventually,that multi variant state can be transformed into a double variants(type I twin)of martensitic phase after ten cycles of thermal-mechanical training.The MFIS,when the measurement magnetic field is applied along the vertical loading direction,was increased to 0.58%in directionally solidified Ni50Mn28.5Ga21.5 alloy after ten cycles of thermal-mechanical training.When the measurement magnetic field along the parallel load direction,the MFIS was reduced and a reversible MFIS of 0.05%was obtained for the first time.The MFIS has also improved after two cycles of magnetic training.The phase transformation strain is only 0.1%in the directionally solidified Ni50Mn30Ga20 alloy.With a 1 T magnetic field is applied in the solidification direction,the phase transformation strain increases gradually with the increase of the number of cycles.After three cycles,the phase transformation strain value reaches 0.16%.We also measured the phase transformation strain after thermal-mechanical training.With an increasing cycle,the macroscopic strain increased gradually,i.e.,0.21%,and 0.32%for Cycle 5,and Cycle 10,respectively.