TaC与Ta_2C的第一性原理研究

First-Principle Study of TaC and Ta_2C

TaC和Ta_2C是钽在渗碳过程中形成的两种重要的过渡金属碳化物,它们的含量和分布对渗碳层性能有着重要的影响,对二者性质的了解是开展钽表面渗碳改性研究的基础。采用基于密度泛函理论(DFT)的第一性原理计算,讨论了TaC和Ta_2C的相稳定性,即结构稳定性、机械稳定性和动力学稳定性,使用Voigt-Reuss-Hill近似与Chen的半经验公式得到了二者的弹性模量与硬度,通过电子结构计算讨论了二者力学性能差异的原因,并基于准简谐近似理论获得了二者的热力学性能。研究表明:TaC的硬度为24.7 GPa,高于Ta_2C的硬度15.0 GPa,态密度与成键分析显示,Ta_2C与TaC相比具有较强的Ta-Ta键,导致其表现出较低的弹性常数与模量,同时具有较好的塑性。而另一方面,Ta_2C的热膨胀系数明显低于Ta与TaC,因此,预计Ta_2C可以起到改善渗碳层的塑性,提高抗冲击能力的作用,但Ta_2C的热膨胀能力与其他成分的差异可能会降低渗碳层的抗热循环性能。

TaC and Ta2C are two important transition metal carbides formed in carburization of tantalum. Their contents and distributions have significant effects on the performance of carburized tantalum layer. In this work, first-principle calculations based on density function theory （DFT） were performed to investigate the phase stability of TaC and Ta2C, i.e. , structural, mechanical and dynamical stability. Voigt-Reuss-Hill approximation and Chen＇s semi-empirical relation were used to work out the elastic modulus and hardness of them. Electron structure were calculated to discuss the cause of their differences in mechanical property. Further, based on the quasiharmonic approximation, their thermodynamic properties were also obtained. The results showed that TaC had the higher hardness of 24.7 GPa while TaEC had the lower of 15.0 GPa. Because of the stronger Ta -Ta bonding indicated by the calculated DOS and CO- HP, Ta2 C possessed the lower elastic constants, lower elastic modulus and better plasticity. On the other hand, the thermal expansion coefficient of Ta2C was obviously lower than that of Ta and TaC. Thus, these calculations suggested that the presence of Ta2C could improve the ductility and impact resistance of carburized tantalum layer, but might reduce the thermal cycling stability because of the difference of thermal expansion capability between Ta2 C and other phases.