Martensite Transformation Of TiNi-based And TiZr-based Shape Memory Alloys Based On The First-principle Calculation

Environmental protection and sustainable development are irresistible trends in refrigeration technology.Elastocaloric refrigeration based on the thermoelastic martensite transformation of shape memory alloys is the most promising new refrigeration technology.The alloy composition is an important factor that affects the thermoelastic martensite transformation and its macro-functional characteristics.Therefore,the optimization of the composition is an important method to obtain an elastic thermal material with excellent performance.?-Ti alloy is a type of shape memory alloy widely studied at present,which has excellent superelastic properties.In this paper,the first principle plane wave pseudopotential method based on density functional theory is used to systematically study Ti Ni-based and Ti Zr-based alloys with different alloy compositions.The phase formation energy and electronic structure of the alloy illustrate the influence of alloy composition and applied stress on the martensitic transformation and macroscopic strain characteristics of the alloy.In this paper,the lattice constants,formation energies,and density of states of the B2 and B19 phases of Ti50Ni50-x Pdx alloy with different contents of Pd added to Ti Ni alloy B2 and B19' phases are calculated.With the increase of Ni content,The lattice constant of B2 phase in Ti Ni alloy is decreasing,and the difference in formation energy between B2 and B19' phases is increasing.At the Fermi level,Ti-50 at.%Ni alloy has the highest B2 phase density and the lowest B19' phase density.With the increase of Pd content,the lattice constants of B2 and B19 phases both increased nearly linearly,the formation energy of B2 and B19 phases both decreased,and the addition of Pd significantly improved the phase stability of B2 and B19 phases.The formation energy difference between the two phases of B19 increases and the phase transition temperature increases.As the stress increases,the free energy difference of the Ti Ni Pd alloy decreases.Under the same stress,the free energy difference of the alloy containing Pd 25 at.% changes the most,indicating that the Ti50Ni25Pd25 alloy stress has the highest sensitivity to phase transition temperature and d?/d T.The superelasticity of Ti Ni alloys originates from the stress-induced martensitic transformation,and its strain can be expressed by the martensitic transformation lattice distortion matrix.The thesis uses phenomenological theory to establish the lattice distortion matrix of different alloys.The phase transformation strain of Ti-50 at.%Ni alloy is larger than that of the other two alloys,and the phase transformation strain along the [001] direction is the smallest,which is 4.08%.The phase transformation strain along the [355 ] direction is the largest,18.87%;the Ti Ni Pd alloy has the maximum phase transformation strain in the [110]p crystal direction.As the Pd content increases,the maximum main lattice strain ?3 decreases,and the maximum phase transformation strain decrease.The lattice constants of ?,?' and ? " of Ti Zr and Ti Zr Nb alloys increase linearly with the increase of Zr content.The calculation results of phase formation energy and state density of Ti Zr alloy show that the formation energy of ? phase and the Fermi level The density of states at the location is higher than that of the ?' and ?" phases of martensite,and the total density of states at the Fermi level of ? phase decreases first and then increases with the increase of Zr content,of which Ti50Zr50 has the lowest,The difference between the formation energy of ? and ? " martensite phase is the smallest when the Zr content is 50 at.%,which explains the lowest phase transition temperature obtained in the Zr content 50 at.% alloy in the experiment;the formation energy and cost of the Ti Zr Nb alloy The total density of states at the meter level indicates that as the Nb content increases,the stability of the ?' phase decreases,the stability of the ? phase increases,and Nb is the stable element of the ? phase.And the freedom of the ?'and ?" martensite phases The total state density at the energy and Fermi level is lower than the ? phase,and the martensite phase is more stable in the ground state.The formation energy of the ?'and ? " phase formation energy and the Fermi level both change when the Nb content is 15 at.%.When the Nb content is less than 15 at.%,the formation energy of ?' is lower than ?",tending to Since ???' martensite occurs,when the Nb content is higher than 15 at.%,the formation energy of ? " is lower than ?',and ???" martensite tends to occur,which stabilizes the martensite phase from ?' phase It becomes the ? " phase.As the hydrostatic stress increases,the free energy difference of Ti Zr Nb alloys decreases.Under the same stress,the free energy difference of alloys Nb content of 12.5at.% changes the most,indicating stress is the most sensitive to the phase transition temperature in Ti-12.5Zr-12.5Nb alloy,and d?/d T is the largest.The main lattice strain of ???" phase transition of Ti Zr alloy hardly changes with the change of Zr content,and the maximum phase transition strain of Ti Zr Nb alloy is along [011]?.the phase transformation strain of this crystal direction decreases first and then increases with the increase of Nb content.The maximum value is obtained when the Nb content is 12.5 at.%.When Nb content is higher than 12.5 at.%,the phase transformation strains along [011]?,[001]? and [111]? are all greatly reduced.