The Study Of Tailoring Ferromagnetic Transition And Magnetocaloric Effect In New All-d-metal Heusler Alloys

In recent years,people’s requirements for refrigeration technology have continued to increase,and traditional gas compression refrigeration technology cannot meet the needs of social progress.Environmentally friendly,easy to customize,and high-efficiency magnetic refrigeration technology was born.Magnetic refrigeration technology mainly uses magnetic materials as the refrigeration medium,and relies on its intrinsic properties to achieve refrigeration by the magnetocaloric effect（Magnetocaloric effect,MCE）.The new Heusler alloy（all-d-metal Heusler alloy）composed entirely of 3d transition group elements has attracted much attention in the field of condensed matter physics because of its giant magnetocaloric effect（MCE）.The magnetocaloric effect（MCE）of this type of material is still relatively large.Explore the space.In this paper,the fourth period 3d transition group elements V,Fe and rare earth element yttrium（Y）are used to replace Co and Mn elements in Ni-Co-Mn-Ti-based alloys,respectively.Ni₃₅Co_15-xFe_xMn₃₅Ti₁₅（x=2,4,6,8）,Ni₃₅Co_15-xV_xMn₃₅Ti₁₅（x=1,2,3,4）,Mn_50-xY_xNi_30.5Co_9.5Ti₁₀（x=0,0.3,0.5,0.7）alloy strip samples,and the microstructure,crystal structure,magnetism,ferromagnetic martensitic transformation and The magnetocaloric effect has been systematically studied.The main work and research results are as follows:（1）Ni₃₅Co_15-xFe_xMn₃₅Ti₁₅（x=2,4,6,8）alloy strip samples are all pure B2 cubic austenite phase at room temperature,and doped Fe atoms tend to occupy A（Co）sites.With the increase of doped Fe element,its ferromagnetic martensite phase transition temperature gradually rises from 181.9（6）K（x=2）to 229.9（6）K（x=8）,and both phases are maintained before and after the phase change Larger magnetization difference（ΔM）.The Fe-Ti bond generated during the doping of Fe element replaced the Co-Ti bond,and the weak covalent hybridization strength of the Fe-Ti bond resulted in the instability of austenite,so the sample ferromagnetic martensite transformation temperature gradually Elevated.In addition,the austenite Curie temperature（T_C^A）gradually decreased from 349.0（7）K（x=2）to 287.2（8）K（x=8）.As the Fe element increases,the Co element decreases correspondingly,and the magnetic exchange between Mn and Co at the B（D）site weakens,which affects the ferromagnetic austenite magnetic configuration,resulting in a gradual decrease in the austenite Curie temperature（T_C^A）.The maximum magnetic entropy change value of the sample can reach 9.5,23.4 J kg^-1K^-1（ΔH=20,50 k Oe）,and the refrigeration capacity（RC）can reach 79.4,206.8 J kg^-1（ΔH=20,50 k Oe）.（2）Ni₃₅Co_15-xV_xMn₃₅Ti₁₅（x=1,2,3,4）alloy strip samples are all pure B2 cubic austenite phase at room temperature（space group is pm3m）.With the increase of V doping,the ferromagnetic martensite transformation temperature gradually decreases from 235.96 K（x=1）to 81.99 K（x=3）.The alloy valence electron concentration（e/a）decreases with the increase of doped V elements,and a stronger covalent hybridization strength V-Ti bond is formed,which stabilizes the austenite phase and makes the sample ferromagnetic martensite The phase transition temperature moves to a low temperature.The content of Co in Ni₃₅Co₁₁V₄Mn₃₅Ti₁₅ alloy is relatively low.The combination of weakened Mn-Co-Mn magnetic configuration and lower valence electron concentration leads to a slight increase in the ferromagnetic martensite transformation temperature.The V element regulates the martensite transformation temperature while ensuring a large difference in magnetization（ΔM）between the two phases before and after the transformation.The maximum magnetic entropy change value of the sample can reach11.99,20.07 J kg^-1K^-1（ΔH=20,70 k Oe）,and the refrigeration capacity（RC）can reach73.00,356.68 J kg^-1（ΔH=20,70 k Oe）.（3）Mn₅₀Ni_30.5Co_9.5Ti₁₀ alloy strip samples coexist with 5M modulated martensite phase and B2-type cubic austenite phase at room temperature.With the rare earth element yttrium（Y）doping,the 5M modulated martensite phase gradually disappeared and became pure B2-type cubic austenite phase.From the analysis of SEM results,it can be seen that yttrium（Y）-rich phase appears with the doping of rare earth element yttrium（Y）,and grain boundary wetting occurs in all samples.The ferromagnetic martensitic transformation temperature of the sample gradually decreases from 280.00K（x=0）to 130.00 K（x=0.7）,and the two phases maintain a large difference in magnetization（ΔM）before and after transformation.The thermal hysteresis increases due to the grain boundary wetting transformation of the samples,which hinders the martensitic transformation.Due to the large internal stress and defects in the strip sample,the sample also has a large thermal hysteresis.The maximum magnetic entropy change of the sample can reach 6.16,23.70 J kg^-1K^-1（ΔH=20,70 k Oe）,refrigeration capacity（RC）can reach to 82.50,216.60 J kg^-1（x=0.7,ΔH=20,50 k Oe）.