H,He对Ti_3SiC_2材料力学性能影响的第一性原理研究

层状三元化合物Ti_3SiC_2兼具陶瓷与金属的优良性能而得到诸多研究领域的关注.本工作采用第一性原理密度泛函理论研究了氢、氦对该材料解理断裂行为的影响,以期探讨Ti_3SiC_2作为核应用材料的可行性.结果表明Si—Ti相对较弱的化学键使之相应的原子层间成为解理断裂面.氢与氦都易在此原子层间聚集.氦的聚集严重降低材料的解理断裂临界应力促使材料的断裂,而氢则对该临界应力影响不大.两者的差异源于这两类原子与材料中晶体原子相异的电子杂化行为.

Effect of H and He on the mechanical properties of Ti3SiC2: the first-principles calculation

Layered MAX phase ternary compounds (M = early transition metals, A = group A elements, and X = C or N) show promise of wide applications in many applied fields because these compounds have combined ceramic and metallic properties. As an exemple of the MAX phase family, Ti₃SiC₂ exhibits a high melting temperature, high electrical and thermal conductivities, and an excellent resistance to oxidation and thermal shock. Particularly, it possesses unusual mechanical properties, such as easy machinability, high Young's modulus, thus it is considered as a candidate in advanced nuclear reactors.In this work, we investigate the effect of hydrogen and helium on the cleavage fracture of Ti₃SiC₂ in order to evaluate the reliability of Ti₃SiC₂ used in nuclear industry. We have performed first-principles mechanical calculations by using the density functional theory as implemented in the Cambridge Serial Total Energy Package code. Uniaxial tensile simulations along c-axis have been done to calculate the stress-strain curve and the cleavage energy for each interlayer of Ti₃SiC₂. It is found that Ti₃SiC₂ has the cleavage characteristics, and the habit cleavage plane starts from Si-Ti interlayer because of relatively weak Si-Ti bond. Hydrogen and helium always accumulate in the Si layer. Helium decreases largely the critical stress of cleavage fracture of Ti₃SiC₂. In contrast, hydrogen does not efficiently affect the cleavage fracture in Ti₃SiC₂. The difference between helium and hydrogen behaviors in Ti₃SiC₂ originates primarily from the difference of electronic hybridization with lattice atoms of Ti₃SiC₂. For helium, the neighboring Si atoms will be ejected by helium atoms, and the Si-Ti bonds will be broken, thus resulting in the cleavage fracture. However, for hydrogen, it is primarily hybridized with the s states of neighboring Si atoms, which does not severely disturb the p-d hybridization between Si and Ti atoms. Thus, the cleavage fracture from Si-Ti interlayer is hardly aggravated in the presence of hydrogen. Fortunately, Ti₃SiC₂ has a self-repair ability at high temperatures. It will desorb helium atoms at high helium pressure through Si layers. This behavior will alleviate the cleavage fracture induced by helium. In summary, Ti₃SiC₂ may be a potential material applied in light water or other fission reactors in the future.