[SY-G1] Defect Energetics in W-Based Transition-Metal Ternary Systems
W has been considered as a potential core element for plasma facing material of fusion reactors for ITER thanks to its outstanding high-temperature properties. However, studies so far on W and W-based binary alloys show precipitation and defect clustering under neutron irradiation. In this talk, we report the first principles calculations of the defect energetics in ternary W alloys. W-Ta-Re systems are chosen for extensive calculations on interstitial defect structures around solute atoms.
Overall, as in pure W or W binary alloys such as W-Re, the most preferable defect type is found to be the bridge interstitial, followed by <111> interstitials. With negative solute-solute binding energy, Ta and Re atoms prefer to make a solute pair in the system and the solute pair strongly attracts W self-interstitial atom, forming a solute-interstitial complex. The binding between a Ta-Re solute pair and a W self-interstitial atom can be stronger in this ternary alloy than in binary systems, and plays a role in slowing down the W interstitial diffusion as a primary dissociation barrier. The defect energetics in the alloys can be understood as the combined results of both strain-relief effect and electronic effect, but the former, which is related with atomic size of elements and local pressure felt by each atom in lattice, seems dominant.
To summarize, the defect complexes in W-Ta-Re alloys are expected to trap W self-interstitial atoms, preventing self-interstitial atoms from escaping to the surface or grain boundary. The reduction in the number of disappearing self-interstitial atoms increases the odds for Frenkel defects to annihilate, reducing residual void concentration and hence void formation. It is also expected that, at elevated temperature, the rise in configuration entropy increases the interaction energy between solute pair and self-interstitial atom, and hence further slows interstitial migration.
Overall, as in pure W or W binary alloys such as W-Re, the most preferable defect type is found to be the bridge interstitial, followed by <111> interstitials. With negative solute-solute binding energy, Ta and Re atoms prefer to make a solute pair in the system and the solute pair strongly attracts W self-interstitial atom, forming a solute-interstitial complex. The binding between a Ta-Re solute pair and a W self-interstitial atom can be stronger in this ternary alloy than in binary systems, and plays a role in slowing down the W interstitial diffusion as a primary dissociation barrier. The defect energetics in the alloys can be understood as the combined results of both strain-relief effect and electronic effect, but the former, which is related with atomic size of elements and local pressure felt by each atom in lattice, seems dominant.
To summarize, the defect complexes in W-Ta-Re alloys are expected to trap W self-interstitial atoms, preventing self-interstitial atoms from escaping to the surface or grain boundary. The reduction in the number of disappearing self-interstitial atoms increases the odds for Frenkel defects to annihilate, reducing residual void concentration and hence void formation. It is also expected that, at elevated temperature, the rise in configuration entropy increases the interaction energy between solute pair and self-interstitial atom, and hence further slows interstitial migration.