Studies On New Shape Memory Alloy In Co-based Heusler Alloys

In this thesis,the magnetic properties,electronic structure,atomic ordering and martensitic transformation in these Co-based Heusler alloys including Co₂V_1.5Z_0.5(Z=Al,Ga,In),Co₅₀V₃₂Y₂Ga₁₆(Y=Ti,V,Cr,Mn,Fe,Co,Ni,Cu),Co₂V_1.5Ga_0.5-xZn_x(x=0,0.25,0.5)are studied by theoretical calculations and experiments.The electronic structure,magnetic properties and martensitic transformation in Co₂V_1.5Z_0.5(Z=Al,Ga,In)alloys have been studied by first-principles calculations.It is found that the tetragonal martensitic phase has a higher stability compared with the cubic austenite phase and the martensite transformation possible in the alloys.This can be explained based on their electronic structures.In Co₂V_1.5Z_0.5 a ferromagnetic to paramagnetic transition occurs together with the martensitic transition.The spin polarization ratio also decreases drastically from close to 100%in austenite to almost zero in martensite.Among them,the energy difference?E_M between the martensite and austenite is large in Co₂V_1.5Al_0.5 and Co₂V_1.5Ga_0.5,suggesting that they are possible candidates for magnetic shape memory alloys(MSMAs).The existence of Co(A,C)-V(B)atomic disorder in the alloy will introduce weak ferromagnetism in the martensite phase and reduce the phase transition driving force.In order to test the possibility of synthesizing these alloys,single phase samples doped with different main group elements are prepared by melt-spun method.Single-phase Co₅₀V₃₂Y₂Ga₁₆(Y=Ti,V,Cr,Mn,Fe,Co,Ni,Cu)alloys with body-centered cubic structure are successfully prepared by melt-spun method.The magnetic measurement results show that in these alloys,the doping of Ti,Cr,Mn,and Cu can suppress the martensitic transformation.Simultaneously,the doping of Ti and Cu decreases the Curie temperature of the austenite,but the doping of Cr and Mn has opposite effect.However,the doping of Fe,Co and Ni is preferable for the martensite transformation and can increase the martensite transformation temperature.The martensite transformation temperature increases to near room temperature after doping with Co and Ni,which is consistent with the martensite diffraction peaks in the XRD pattern.The martensite transformation temperature after Fe doping is lower than room temperature and the martensite transition is further verified by?-T curve.The Ti-,Cr-,Mn-and Cu-doped samples are austenite at 5 K,which are ferromagnets,and Fe-,Co-and Ni-doped samples are martensite with paramagnetic character.First-principles calculations show that the Zn atoms tend to occupy the D site in the cubic lattice of Co₂V_1.5Ga_0.5-xZn_x(x=0,0.25,0.5)alloys.The charge density difference indicates that covalent hybridization is stronger and the structure is more stable in this kind of atomic ordering.The austenitic Co₂V_1.5Ga_0.5-xZn_x has both martensitic transformation and half-metallic properties.The doping of Zn can move the Fermi level to the middle of the energy gap,so the half-metallic property of the alloy is stabilized.Zn-doping can also increase the driving force of martensite transformation and raise the martensite transformation temperature of these alloys.A new all-d-metal Heusler alloy Co₂V_1.5Zn_0.5will be formed with Zn atoms replaceing Ga atoms completely.Finally,we study the change of unit cell volume during the phase transition of this material.