Research On Martensitic Transformation And Properties Of NiCoMnInAl Magnetic Memory Alloy

The Ni-Co-Mn-In magnetic memory alloy undergoes a structural transformation from high-temperature ferromagnetic austenite to low-temperature weakly magnetic martensite during temperature reduction,which can be driven by a magnetic field.As a class of Heusler alloys,this series of alloys are coupled with magnetic transitions in the process of structural transitions,so the phase transition course is also accompanied by sudden changes in various physical properties such as crystal structure,unit cell volume,and magnetization.Therefore,in this type of Heusler alloys,the phase transformation temperature zone of the alloy exhibits large magneto-induced strain effect,magnetocaloric effect and elastic-caloric effect,which makes the alloy have been abroad studied lately and has thriving application scenes.However,the inherent brittleness of this series of alloys and the high requirements for the critical driving magnetic field have also caused huge barrier and restriction to its potential application and development.In this dissertation,the element doping method by partially replacing In with Al improves the mechanical properties of the alloy while promoting the corrosion resistance of the alloy,and the magnetocaloric effect of the alloy is also well maintained.Ni₄₈Co₁Mn₃₇In_14-xAl_x（x=0,0.25,0.5,0.75,1,2at.%）alloy materials were smelted by WK-II non-consumable vacuum arc furnace.After cooling,the alloy was sealed and heat-treated with a vacuum quartz tube filled with argon,kept at 1173K for 24 hours,and then water-cooled and quenched.In this dissertation,metallographic microscope,scanning electron microscope（SEM）,energy dispersive analyzer（EDS）,X-ray diffractometer（XRD）,differential scanning calorimeter（DSC）,multi-parameter magnetic measurement system,physical properties measurement System（PPMS）,electronic mechanical testing device,electrochemical workstation and other experimental equipment are used.The microstructure characteristics,martensitic transformation behavior,magnetic properties and magnetocaloric characteristics,compressive properties and corrosion resistance of Ni-Co-Mn-In-Al alloys and their connected influencing elements were comprehensively studied.The function of the trace main group element Al-substituting In on the microstructure,martensitic transformation temperature and austenite Curie temperature of the alloy were clarified.The influence mechanism of Al doping on the magnetocaloric properties,compressive performance and corrosion resistance of the alloy were revealed.The results show that substituting Al for part of In in Ni-Co-Mn-In alloy can evidently reduce the grain size of the alloy,and the greater the amount of Al element,the more obvious the grain refinement effect is.The grain size of the Ni₄₈Co₁Mn₃₇In₁₂Al₂ alloy is reduced to about 10μm,which is about one-thirtieth of the grain size of the undoped alloy.With the addition of Al content,the solid solubility of Al in the alloy also increases.When the Al doping amount is 0.25at.%～2at.%,the contents of Ni,Mn,and Co in the matrix are basically stable,and the metal Al is well dissolved into the matrix,and no second phase is found in the scanning electron microscope observation.With the substitution of Al for In,the alloy transforms from a two-phase mixed structure of L2₁ cubic austenite and monoclinic 6M martensite to a single 6M modulated martensite phase structure at room temperature.The lattice constant b decreases and the cell volume has a decreasing trend.Ni₄₈Co₁Mn₃₇In_14-xAl_x（x=0,0.25,0.5,0.75,1,2at.%）alloy undergoes one-step martensitic transformation in which thermal and elastic effects are balanced in both heating and cooling periods.As the unit cell volume diminishs with the raise of Al content,the thermoelastic transformation temperature generally shows an upward trend.When x=0.25,0.5,1,the alloy transformation temperature is around room temperature.When x=2,the martensitic transformation temperature of the alloy increases from 270k without doping to 326k.The increase of transformation temperature is mainly due to the decrease of cell volume with Al doping.The phase transition temperature range of Ni₄₈Co₁Mn₃₇In₁₂Al₂ alloy reaches 18K,the enthalpy change value increases to 3.5J/g,and the latent heat of phase transition is three times that of the undoped alloy.Due to the increase of Al content,the thermal hysteresis of sample phase transformation slowly narrows,and the thermal hysteresis value of 2at.%Al alloy is 8K.When the Al doping amount is not more than 1 at.%,the Curie temperature of the alloy shows a decreasing trend as a whole,while the Curie temperature of the austenite phase of the 2at.%Al alloy is 380K,which is higher than that of the undoped alloy near 40K.The results of magnetocaloric analysis show that when 1at.%Al is used to replace In,the disparity of the two-phase saturation magnetization of the alloy under the 5T magnetic field reaches35emu/g,which is about 8 times that under the 0.1T magnetic field,and the magnetic transformation ability of the alloy are greatly enhanced under the magnetic field.In the isothermal magnetization curve,Ni₄₈Co₁Mn₃₇In₁₃Al₁ alloy has high magnetization in the high temperature region,showing the strong ferromagnetism of austenite phase.When the magnetic field reaches 5T,the magnetization of austenite phase is still in the unsaturated state.Under 5T magnetic field,the maximum magnetic entropy change of Ni₄₈Co₁Mn₃₇In₁₃Al₁ alloy near the phase transformation temperature is 4.5J/（kg·K）.By replacing In with Al,the compressive properties of the Ni-Co-Mn-In alloy are evidently improved,and the compressive strength of the alloy is continuously upwards.The compressive fracture strength of Ni₄₈Co₁Mn₃₇In₁₂Al₂ is 488MPa,which is 160%higher than that of Ni₄₈Co₁Mn₃₇In₁₄,and the compressive strain also increases from5.46%to 6.36%.The fracture morphology analysis shows that Al doping makes the fracture type of the alloy gradually evolve from a single intergranular fracture to a transgranular fracture,and the plasticity is greatly enhanced.Electrochemical corrosion experiments show that with the doping of Al element,the corrosion resistance of the alloy in artificial seawater solution is improved,and the self-corrosion current density of Ni₄₈Co₁Mn₃₇In₁₂Al₂ alloy decreases from 1.493μA/cm² to 1.038μA/cm²,and under the same conditions,its self-corrosion rate is slightly lower than that of 304 stainless steel.The above results show that when an appropriate amount of Al replaces in alloy,the increase of martensitic transformation temperature and Curie temperature can be realized,so as to broaden the working temperature window,improve the magnetothermal properties,compressive performance and anti-erosion properties of the alloy.It provides reference and ideas for optimizing the properties of Ni-Mn-In based magnetic memory alloys and developing new high-performance Ni-Mn-In based magnetic memory alloys.