Phase Stability Of Co₂CrZ(Z=Al,Ga,Ge,Si)Shape-Memory Alloys From First-Principles Study

Shape memory alloys,as smart materials,are one of the research hotspots in the field of materials.Studies have shown that Ni-Mn-Ga alloys have a low martensitic phase transition temperature,which limits their application in high temperature conditions,and the brittleness of the material makes it difficult to achieve large-scale applications in practical production.Therefore,it is necessary to find a high performance shape memory alloy material.Experiments have reported the existence of special martensitic phase transformation behaviour in tetrametallic alloys such as Co-Cr-Ga-Si and Co-Cr-Al-Si.However,relatively few theoretical calculations have been reported on Co₂Cr-based magnetic shape memory alloys.In this thesis,the physical relationship between phase stability and magnetic disorder and alloying effects of Co₂Cr Z(Z=Al,Ga,Ge,Si)alloys will be investigated by systematic theoretical calculations using the first-nature principle exact Muffin-Tin orbital combined with the coherent potential approximation method,in the hope of contributing to the martensitic phase transformation theory of Co₂Cr-base alloys and the experimental design of high-performance shape memory alloys.It is hoped that this will provide data to support the theory of martensitic phase transformation of Co₂Cr-based alloys and the design of high performance shape memory alloys experimentally.The ground state properties of the Co₂Cr Z(Z=Al,Ga,Ge,Si)stoichiometric ratio alloys obtained from theoretical calculations are compared with experimental and other theoretical calculations to confirm the accuracy of the EMTO-CPA calculation method.The results are also presented for the effects of tetragonal lattice deformation and magnetic disorder on the total electronic energy,magnetic moment,elastic constant and electronic structure of the Co₂Cr Z alloy.It is shown that all four Co₂Cr Z alloys have a stable structure of L2₁in the ferromagnetic state.As the magnetic disorder(y)increases,the energies of both L2₁and D0₂₂phases gradually increase.But the energy of D0₂₂phase relative to L2₁phase gradually decreases.Thus,the alloys with Z=Ge and Si have the D0₂₂stable structure when y?0.1,and the alloys with Z=Ga and Al have the D0₂₂stable structure when y?0.2 and 0.4.The tetragonal shear modulus of elasticity C'of the L2₁phase continues to soften as y increases.It is shown that magnetic disorder favours the L2₁-D0₂₂martensitic phase transformation of the four Co₂Cr Z alloys,both from an energetic and mechanical point of view.The electronic structure mechanism by which magnetic disorder affects the relative stability of the L2₁and D0₂₂phases is attributed to the Jahn-Teller instability effect.Further,the patterns of alloying effects on their ground state properties,magnetic moments and elastic constants were calculated for two groups of ternary alloys,Co₂Cr Ga_1-xCo_x,Co₂Cr Ga_1-xSi_xand Co₂Cr Ga_1-xGe_x,as examples,and the correlation between the composition-dependent patterns of these alloys and their L2₁and D0₂₂phase stability was analysed in order to find The study finds that the alloying effect favours the occurrence of tetragonal martensitic phase transformation.It was found that the alloying effect favours the L2₁-D0₂₂martensitic phase transformation of Co₂Cr Ga_1-xCo_xternary alloys.However,for both Co₂Cr Ga_1-xSi_xand Co₂Cr Ga_1-xGe_xternary alloys,both Si and Ge doping favoured the mechanical stability of the L2₁phase of the Co₂Cr Ga alloy,gradually enhancing the phase stability of the L2₁phase relative to the D0₂₂phase,thus helping to suppress the occurrence of tetragonal lattice deformation in this system.