Vacancy-mediated transport and segregation tendencies of solutes in FCC nickel under diffusional creep: A density functional theory study
arxiv(2024)
摘要
The Nabarro-Herring (N-H) diffusional creep theory postulates the
vacancy-mediated transport of atoms under a stress gradient as the creep
mechanism under low-stress and high-temperature conditions. In multicomponent
alloys, we premise that this stress-assisted flow of vacancies to and from
grain boundaries will produce elemental segregation. An observation of such
segregation, validated with theoretical predictions, can provide the necessary
experimental evidence for the occurrence of N-H creep. Theoretical calculations
of the segregation tendencies via analyzing the dominant solute diffusion
mechanisms and the difference in diffusivities of the elements are therefore
essential. To this end, this study applies density functional theory
calculations of migration barriers and solute-vacancy binding energies as input
to the self-consistent mean field theory to assess the vacancy-mediated
diffusion mechanisms, transport coefficients, and segregation tendencies of Co,
Cr, Mo, Re, Ta, and W solutes in face-centered cubic Ni. We find Co, Re, and W
to be slow diffusers at high temperatures and Cr, Mo, and Ta to be fast
diffusers. Further analysis shows that the slow diffusers tend to always enrich
at vacancy sinks over a wide range of temperatures. In contrast, the fast
diffusers show a transition from depletion to enrichment as the temperature
lowers. Furthermore, our analysis of the segregation tendencies under tensile
hydrostatic strains shows that slow diffusers are largely unaffected by the
strain and favor enrichment. On the other hand, the fast diffusers exhibit high
sensitivity to strain and their segregation tendency can transition from
depletion to enrichment at a given temperature. The transport coefficients
calculated in this work are expected to serve as input to mesoscale
microstructure models to provide a more rigorous assessment of solute
segregation under N-H creep conditions.
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