First Principles Study On Structural Stability And Mechanical Properties Of Ultra-high Temperature Diboride Ceramics

In this paper,the phase stability,mechanical properties and melting point of metal diboride solid solution(Ti/Zr)_1-x(Ta/Nb)_xB₂(0?x?1)and fifteen quaternary high entropy metal diborides(Me1_0.25Me2_0.25Me3_0.25Me4_0.25)B₂(where me represents IVB or VB refractory transition metals)are studied by the first principle calculation of supercell(SC)method and virtual crystal approximation(VCA)method.The study shows that the structure of metal diboride solid solution(Ti/Zr)_1-x(Ta/Nb)_xB₂ and quaternary high entropy metal diborides are stable,and the phasa that formed by the structural parameters?(lattice constant difference)and two thermodynamic parameters of?H_mix(mixing enthalpy)and?(the ratio of entropy term to enthalpy term),shows that the quaternary high entropy metal diborides meets the?-?Standard(i.e.?>1 and?<6.6%),and a stable structure can be formed.There are five of the quaternary high entropy metal diborides have been synthesized experimentally.The last ten quaternary high entropy metal diborides are predicted in this paper.The volume of Ti_1-x(Ta/Nb)_xB₂ in metal diboride solid solution(Ti/Zr)_1-x(Ta/Nb)_xB₂increases with the add of doping concentration,for the atomic radius of Ta and Nb is greater than that of Ti.The volume of Zr_1-x(Ta/Nb)_xB₂ decreases,because of the atomic radius of Ta and Nb as smaller than Zr.Diboride solid solution(Ti/Zr)_1-x(Ta/Nb)_xB₂ and quaternary high entropy metal diboride(Me1_0.25Me2_0.25Me3_0.25Me4_0.25)B₂ are brittle materials with stable mechanical properties.The dopants of Ta and Nb can significantly improve the brittleness,bulk modulus and fracture toughness of solid solution(Ti/Zr)_1-x(Ta/Nb)_xB₂,but the hardness will be reduced.The Quaternary high entropy metal diborides(Me1_0.25Me2_0.25Me3_0.25Me4_0.25)B₂ have high Vickers hardness,high fracture toughness and ultra-high melting point,which is significant in predicting and designing for the new high entropy ceramics.