[SY-E8] Promotional effects of anisotropic strain on vacancy mobility in tungsten: the independence on the sign of strain
Tungsten (W) is one of the most promising candidates for the plasma facing materials (PFMs) in future fusion devices. Vacancy is the typical defect in W, and plays key role in the microstructure and mechanical properties of W. Vacancies can aggregate to form voids by migrating, and further lead to swelling, hardening and embrittlement of W. Therefore, the behaviors of vacancy in W have attracted many attentions. Generally, the presence of vacancy is accompanied with the disappearance of normal W atom, and induces local lattice distortion and forms stress field. Thus, one can expect that the external strain/stress field should have effect on vacancy behaviors in W, while little work focuses on this aspect so far.
Here, we have investigated the migration of vacancy in W under strain using a first-principles method in combination with the activation volume tensor and thermodynamic models. In general, vacancy inevitably induces local tensile stress field, tending to contract the lattice. Thus, it is considered that the mobility of vacancy responds to strain “monotonically”, i.e., the migration energy of vacancy will decrease (increase) with the increasing tensile (compressive) strain. The mobility of vacancy in W under triaxial strain follows this rule. Surprisingly, we have discovered that the vacancy mobility can always be promoted by anisotropic (biaxial) strain in W, independent of the sign of strain. In a wide range of strain values, the vacancy mobility is enhanced in the strained W. This anomalous behavior is found to be caused by an unusual variation of the vacancy activation volume tensor induced by anisotropic strain, which is originated from the Poisson effect. Further, it is found that the diffusivity of the vacancy in W with 5% tensile (compressive) biaxial strain will be increased by 3 (2) orders at 600K based on the Arrhenius equation. Meanwhile, the onset temperature for vacancy diffusion will also be markedly reduced by biaxial strain. Consequently, our finding suggests that anisotropic strain will significantly enhance vacancy mobility in W and promote the formation and growth of hydrogen/helium bubbles.
Here, we have investigated the migration of vacancy in W under strain using a first-principles method in combination with the activation volume tensor and thermodynamic models. In general, vacancy inevitably induces local tensile stress field, tending to contract the lattice. Thus, it is considered that the mobility of vacancy responds to strain “monotonically”, i.e., the migration energy of vacancy will decrease (increase) with the increasing tensile (compressive) strain. The mobility of vacancy in W under triaxial strain follows this rule. Surprisingly, we have discovered that the vacancy mobility can always be promoted by anisotropic (biaxial) strain in W, independent of the sign of strain. In a wide range of strain values, the vacancy mobility is enhanced in the strained W. This anomalous behavior is found to be caused by an unusual variation of the vacancy activation volume tensor induced by anisotropic strain, which is originated from the Poisson effect. Further, it is found that the diffusivity of the vacancy in W with 5% tensile (compressive) biaxial strain will be increased by 3 (2) orders at 600K based on the Arrhenius equation. Meanwhile, the onset temperature for vacancy diffusion will also be markedly reduced by biaxial strain. Consequently, our finding suggests that anisotropic strain will significantly enhance vacancy mobility in W and promote the formation and growth of hydrogen/helium bubbles.