First-principles investigation of solution mechanism of C in TM-Si-C matrix as the potential high-temperature ceramics

Although the TM-Si-C ternary compounds are promising advanced ceramics, the solution mechanism of C element in TM-Si matrix is entirely unclear. In particular, the influence of C on the mechanical properties of TM-Si matrix is no clear. To solve the problem, we apply the first-principles calculations to study the occupied mechanism of C in TM5Si3 silicides. In addition, the influence of C element on the mechanical properties of TM5Si3 is further studied. Here, two typical TM5Si3 phases: D8(l)-Ta5Si3 and D8(m)-Mo5Si3 are considered. For C-doping, six C interstitial sites, Mo5Si3-C(1), Mo5Si3-C(2), Mo5Si3-C(3), Ta5Si3-C(1), Ta5Si3-C(2), and Ta5Si3-C(3), are designed. The calculated results show that six C-doped TM5Si3 matrices are thermodynamically stable. Among them, the C element prefers to occupy the Mo5Si3-C(2) and Ta5Si3-C(3) models. The calculated mechanical properties show that the additive C element weakens the bulk modulus, shear modulus, and Young's modulus of TM5Si3. However, the C element can improve the fracture toughness of Mo5Si3 and Ta5Si3, which are in good agreement with the experimental phenomenon. Naturally, the improvement of fracture toughness is related to the formation of TM-C bond and Si-C bond.

Automated summary

The study used first-principles calculations to investigate the occupation mechanism of C in TM5Si3 silicides, revealing that C element prefers to occupy Mo5Si3-C(2) and Ta5Si3-C(3) models. Additionally, the results show that the additive C element weakens the mechanical properties of TM5Si3, but can improve the fracture toughness of Mo5Si3 and Ta5Si3.