Study On External-field-induced Phase Transformation And Associated Functional Properties In NiMn-based Heusler Alloy Microwire

NiMn-based Heusler alloy is a new smart material which has attracted much attention in recent years.This kind of alloy is expected to play a key role as a new generation of sensor and actuator devices in automotive,energy,industrial automation and other fields due to its large magnetic-field-induced strain or output stress,quick response and precise control under magnetic field.In addition,the martensitic transformation temperature of NiMn-based Heusler alloy can exceed 773 K,which is expected to be used in nuclear reactor,aerospace and other high-temperature environments and fields.However,this kind of alloy is prone to intergranular fracture due to its high polycrystalline brittleness under applied stress,which seriously restricts its development and industrial application.Based on the above background,a series of NiMn-based Heusler alloy microwires were prepared by Taylor-Ulitovsky method,and the microstructure,external-field-induced phase transformation and associated functional properties of this kind of microwire were investigated.Firstly,the external-field-induced phase transition behavior was systematically studied in a series of Ni_45.2-xCu_xCo_5.1Mn_36.7In₁₃（x=0,0.5,1.0,1.5）metamagnetic shape memory microwires.It was found that the Ni_43.7Cu_1.5Co_5.1Mn_36.7In₁₃（x=1.5）microwire simultaneously exhibited huge tensile superelasticity and magnetic-field-induced first-order metamagnetic phase transition.This N143.7Cu1.5Co5.1Mn36.7In13 microwire showed oligocrystalline structure with bamboo grains,which remarkably reduced the strain incompatibility during deformation and martensitic transformation,and thus to improve the ductility.It was also found that a huge tensile superelasticity with a recoverable strain of 13%was achieved in the microwire.This huge tensile superelasticity was in agreement with the theoretical calculation based on the crystal structure and lattice correspondence of austenite and martensite and the crystallographic orientation of the grains.Owing to the large magnetization difference between austenite and martensite,pronounced magnetic-field-induced first-order phase transformation was achieved in the microwire,which could give rise to a variety of magnetically driven functional properties.For example,a large magnetocaloric effect with isothermal entropy change of 12.7 J kg^-1 K^-1 under 5 T was obtained in the microwire.The realization of magnetic-field-and tensile-stress-induced transformations in the microwire enables the use of magnetic field-stress coupling to significantly expand the temperature range of reversible magnetic-field-induced transformation.Secondly,the phase transition temperature,transformation entropy change,superelasticity,magnetically driven properties,shape memory effect and elastocaloric effect were investigated in a series of Ni₅₀Mn₃₄Fe_xIn_16-x（x=3,4,5,6）microwire.It was found that Ni₅₀Mn₃₄Fe₃In₁₃（x=3）microwire shows both giant tensile superelasticity and magnetic-field-induced first-order metamagnetic phase transition.In the temperature range of 233～283 K,the microwire exhibited a large recoverable strain up to more than 20%,which was the maximum value of Ni-Mn based shape memory alloy.Owing to the first-order phase transition induced by magnetic field,an isothermal reversible magnetic entropy change of up to 15.1 J kg^-1 K^-1 was obtained when the magnetic field changed from 0.2 T to 5 T.It was also found that the entropy change of martensitic transformation during cooling of the Ni50Mn34Fe5In11 microwire（x=5）was up to 43.6 J kg^-1 K^-1,and the high-energy X-ray diffraction experiment revealed that the large entropy change of the microwire was due to the large volume change during martensitic transformation.In theory,an isothermal entropy change of 34 J kg^-1 K^-1 and a maximum adiabatic temperature change of up to 27.7 K（at 343 K）could be achieved in the microwire with the strain of 15%in the temperature range of 318～343 K.In addition,it was found that the microwire showed a large shape memory effect with a recoverable strain of 16.2%,which was the highest value in the shape memory alloy reported up to now.Thirdly,the two-way shape memory effect（TWSME）in the Ni₅₀Mn_37.5Sn_12.5 metamagnetic shape memory microwire was investigated.An intrinsic TWSME with a fully recoverable strain of 1.0%was achieved in an as-prepared Ni₅₀Mn_37.5Sn_12.5 microwire.This intrinsic two-way shape memory effect was mainly owing to the internal stress caused by the retained martensite in austenite matrix,as revealed by transmission electron microscopy and high-energy X-ray diffraction experiments.After superelastic training for 30 loading/unloading cycles,the amount of retained martensite increased and the two-way shape memory strain increased significantly to 2.2%,which was the largest value in metamagnetic shape memory alloys.Finally,the properties of Ni₅₅Fe₄Mn₂₀Ga₂₁ microwire at high temperature,such as superelasticity,superelastic stability,shape memory effect and work output,were investigated.It was found that in the temperature range of 543～753 K,the alloy microwires showed excellent tensile superelasticity and the maximum recoverable strain was up to 15.8%.However,tensile superelasticity above 673 K has not been reported in shape memory alloy polycrystalline up to date.The excellent tensile superelasticity was possible due to the bamboo-like grain structure and favorable orientation of austenite at high temperature.It was also found that the Ni₅₅Fe₄Mn₂₀Ga₂₁ microwire showed excellent cycling stability at high temperature.The functional properties of the microwire did not change after 1200 times of loading and unloading at 633 K.This might be attributed to the almost single phase structure of the microwire.In addition,a large work output of up to 44.5 J cm^-3 was obtained in the microwire,which was much higher than the maximum value(30～35 J cm^-3)reported in the shape memory alloys.At last,an intrinsic two-way shape memory effect with a recoverable strain of 4.6%was also found in the microwire.After simple training,the two-way shape memory strain of the microwire increased to 6.4%.