Hf_(x)Ta_(1-x)C体系力学性能及熔化曲线的第一性原理研究

First-principles Study on Mechanical Properties and Melting Curve of Hf_(x)Ta_(1-x)C System

Hf_(x)Ta_(1-x)C具有高熔化温度、高硬度、高强度,以及导电、导热性好等优异性能,是2000℃以上热防护领域极有潜力的候选材料,但其力学性质与熔化温度随组分变化规律尚不清晰。本研究基于特殊准随机结构(SQS)方法和第一性原理计算,从共价键强度、价电子浓度(VEC)的微观角度系统地探讨了Hf_(x)Ta_(1-x)C系固溶体力学性质随组分的变化机理。力学性质计算结果表明:5种组分(HfC、Hf_(0.75)Ta_(0.25)C、Hf_(0.5)Ta_(0.5)C、Hf_(0.25)Ta_(0.75)C与TaC)中,Hf_(0.25)Ta_(0.75)C固溶体具有最高的弹性模量和剪切模量,这主要归因于:(1)该组分具有最高的共价键强度;(2)来自C的p轨道和来自Hf或Ta的d轨道之间的特殊键合在VEC=8.75(Hf_(0.25)Ta_(0.75)C)附近被完全填充,它们强烈抑制形状变化。研究还使用基于从头算分子动力学(AIMD)的分子动力学Z方法计算了Hf_(x)Ta_(1-x)C系固溶体的熔化曲线。结果显示Hf_(x)Ta_(1-x)C系固溶体熔化温度反常增加的现象的确存在,且在Hf_(0.5)Ta_(0.5)C处熔化温度最高(4270 K),这主要归因于构型熵与共价键强度的协同作用。本研究结果为高熔化温度及高力学性能Hf_(x)Ta_(1-x)C系固溶体组分的实验选择及其耐高温涂层应用等提供了理论指导,也为其他过渡金属碳化物研究提供了参考。

Hf_(x)Ta_(1-x)C is a very promising candidate for thermal protection materials above 2000℃due to its excellent properties such as high melting point,high hardness,high strength,high electrical conductivity,and high thermal conductivity.However,the rules of its mechanical properties and melting temperature varying with the composition remain elusive.Firstly,the mechanism of the variation of mechanical properties of Hf_(x)Ta_(1-x)C system solid solutions with its components was systematically investigated from the microscopic point of view of covalent bond strength and valence electron concentration(VEC)based on the special quasirandom structures(SQS)method and first-principles calculations.It revealed that among the five components of solid solutions(i.e.,HfC,Hf_(0.75)Ta_(0.25)C,Hf_(0.5)Ta_(0.5)C,Hf_(0.25)Ta_(0.75)C and TaC),the Hf_(0.25)Ta_(0.75)C solid solution possessed the largest elastic modulus and shear modulus.It was mainly attributed to two reasons:(1)the component possessing the strongest covalent bonding strength among the above ternary compounds;(2)the special bonding states between the p-orbital from C and the d-orbital from Hf or Ta strongly resisting the deformation and being completely filled near VEC=8.75(for Hf_(0.25)Ta_(0.75)C).Secondly,the melting curves of the Hf_(x)Ta_(1-x)C system solid solutions were calculated using the ab initio molecular dynamics(AIMD)-based molecular dynamics Z method.It showed that there existed indeed the phenomenon for anomalous increase in the melting temperature of Hf_(x)Ta_(1-x)C system solid solutions,and the highest melting temperature of 4270 K was predicted on Hf_(0.5)Ta_(0.5)C,which was mainly attributed to the synergistic effect of the conformational entropy and the strength of the covalent bond.The results provide a theoretical guidance for the experimental selection of the optimal components of high melting temperature and high mechanical properties for Hf_(x)Ta_(1-x)C system solid solutions in the thermal barrier coating applications,as well as a reference for the study of other transition metal carbides.