Design And Preparation Of Ni-Mn-based All-d-metal Magnetic Shape Memory Alloys

This thesis mainly studied the effects of transition metal element doping on the structure,magnetic properties,and martensitic transformation in Heusler alloys Mn₂Ni Ga,Mn₂Ni_1.5In_0.5,Mn₂Ni_1.5Sn_0.5,and Mn₅₀Ni_30.5Co_9.5Ti₁₀both theoretically and experimentally.Possible factors and mechanisms which affecting the phase transition and magnetic properties of these alloys were investigated to explore new all-d-metal Heusler alloys in them.The atomic ordering,magnetism,phase transition of Mn₂Ni Ga_1-xZn_x(x=0,0.25,0.5,1)were studied based on first-principles calculations.The influence of cell volume change on the stability of martensitic type Mn₂Ni Ga_1-xZn_xwas also discussed.It is found that Zn is similar to Ga,plays the role of main group elements in Mn₂Ni Ga_1-xZn_xand prefers entering the D site of Heusler lattice.In addition,Zn doping can increase the martensitic transformation temperature of the alloys effectively without changing the original ferrimagnetic structure.The calculations on the cell volume change of the martensite show that the cell volume of martensite and austenite unit cells remains almost unchanged before and after the phase transition,which is consistent with the general theoretical approximation in normal Heusler alloys.The effect of Zn substitution for the main group elements In and Sn in magnetic shape memory alloys Mn-Ni-In and Mn-Ni-Sn were investigated.In Mn₂Ni_1.5In_0.25Zn_0.25,Mn₂Ni_1.5Sn_0.25Zn_0.25and Mn₂Ni_1.5Zn_0.5alloys,the cubic austenites have a ferromagnetic coupling between the spin moments of Mn(B)-Mn(D)atoms.But the magnetic moments of them in the tetragonal martensites turn to antiferromagnetic arrangement.So there exists a coupling between the structural phase transition and the magnetic phase transition.When Zn replaces In and Sn atoms,the direct hybridization between its d electrons and Mn,Ni atoms d electrons is weak.And the increase of Zn content has little influence on the magnetic structure and total magnetic moment of these alloys.But the addition of Zn helps to improve the stability of the martensite phase,and leads to the increase of the phase transition temperature.Zn doping can also increase the c/a ratio,which is beneficial to increase the theoretical phase transition strain.Therefore,the investigations on the Zn-based all-d-metal Heusler alloys can have great importance.The effects of the Ti,V,and Cr substitution for Ga in Mn₂Ni Ga on its physical properties were studied experimentally.Among them,the cubic structure of Mn₂Ni Ga is maintained after V and Cr doping,and the saturation magnetization of the alloys are increased after doping.The martensite transformation temperature of the Cr-doped sample decreases,and the phase transition disappears completely in the V-doped sample.The substitution of Ti for Ga introduces secondary phase in the sample,which reduces the saturation magnetization and suppresses the martensite transformation.Experimental studies on Mn₅₀Ni_30.9Co_9.5-xFe_xTi₁₀(x=0,1,2)and Mn₅₀Ni_30.5Co_9.5Ti_10-yHf_y(y=0,1,2)show that the samples remain cubic austenite after Fe-and Hf-doping.Fe-doping increases the phase transition temperature but decreases the saturation magnetization at 5 K;Hf-doping decreases the phase transition temperature but increases the saturation magnetization.The role of these transition metal elements in the all-d-metal Heusler alloys is worth investigations in our further works.