Martensitic Transformation And Elastocaloric Effect Of Co-V-Ga Heusler Alloy

With global warming,human demand for refrigeration is increasing day by day.However,the traditional gas compression refrigeration has low efficiency and the refrigerant used will cause environmental pollution.Therefore,it is urgent to find a new type of refrigeration technology that is environmentally friendly and energy efficient.In recent years,elastocaloric refrigeration based on the elastocaloric effect has become a research hotspot due to its large adiabatic temperature change and high cooling efficiency.Therefore,it is of great research significance to develop new elastocaloric materials with good elastocaloric effect.In this paper,Heusler-type Co-V-Ga alloy was selected as the research object,and texture-free polycrystalline Co-V-Ga alloy and textured polycrystalline Co（Fe）-V-Ga alloy were prepared by arc melting and directional solidification,respectively.The microstructure,magnetic properties,stress-induced martensitic transformation of the alloys were studied.And the elastocaloric effect of the alloys was characterized by an in-situ non-contact characterization method combining infrared thermal imaging（IR）technology and digital image correlation（DIC）technology.The main research contents of this paper are:（1）Texture-free polycrystals with composition Co_51.5+xV_31.5-xGa₁₇（x=0.1,0.2,0.3）were prepared by arc melting,and the martensitic transformation behavior and elastocaloric effect of annealed alloys were studied.It is found that the alloy undergoes a martensitic transformation from paramagnetic austenite to paramagnetic martensite,which is accompanied by a large latent heat(about 6～9 J g^-1).The stress-induced martensitic transformation and adiabatic temperature change of the samples were characterized in situ by DIC technique combined with IR.For the sample with x=0.2,at the test temperature of 301 K,an adiabatic temperature change of 13.4 K was obtained during loading and only 2.4 K during unloading.This irreversible adiabatic temperature change can be attributed to the self-cooling effect,heat dissipation,and valence bond structure.（2）The Co_51.6V_31.4Ga₁₇ polycrystalline alloy prepared by directional solidification has a strong<001>orientation and a large phase transition entropy change of 21.0 J kg^-1 K^-1.For the compression test near room temperature,it has smaller critical stress（109 MPa）and stress hysteresis（46 MPa）than texture-free polycrystalline,and still exhibits good superelasticity after multiple compressive cycles.Two-dimensional wide-angle X-ray diffractometer was used to observe the crystal structure change of the alloy during the martensitic transformation was induced by uniaxial stress.As the applied strain increased,the（220）main peak of the austenite phase began to transform into martensite phase（112）.The loading-unloading adiabatic temperature changes under different strain were obtained at 303 K,and the large adiabatic temperature change of 12.2 K was obtained when the strain was 6.0%,and the reversibility was significantly improved compared with that of texture-free polycrystalline.In addition,the samples were subjected to 10 rapid cycles of loading-unloading with remarkable training effects.（3）The doping of Fe element can effectively enhance the ferromagnetic ordering of the austenite phase of the alloy while reduce the martensitic transformation temperature.By composition optimization,the Co₄₈Fe₅V₃₀Ga₁₇composition(magnetization difference is 23 emu g^-1)with stronger coupling effect of magnetic transition and structural transition is selected.Utilizing directional solidification technology,the textured Co₄₈Fe₅V₃₀Ga₁₇ polycrystalline was prepared.This alloy undergoes a metamagnetic transition from ferromagnetic austenite to weakly magnetic martensite with a transformation entropy of 16.5 J kg^-1 K^-1.It exhibits good superelasticity at room temperature,and a large adiabatic temperature change of 11.5 K was obtained,indicating a good elastocaloric effect.