Construction of thermal-stability maps for nanocrystalline metals and alloys with U-shaped dependence of stability on grain size

초록

Nanocrystalline metals and alloys are attractive for a wide range of applications, but their broader use is often limited by poor thermal stability. Classical curvature-driven grain-growth theory predicts progressively faster coarsening as grain size decreases. However, recent experiments across face-centered cubic (FCC) metals and alloys have revealed a non-monotonic stability trend: the thermal-stability-limit temperature, T st, defined as the temperature at which grain growth rapidly accelerates for a fixed exposure time—first decreases with grain refinement in the microscale regime but increases again in the nanoscale regime, producing a U-shaped stability response. In this work, we extend the classical grain-growth law by incorporating junction-mediated kinetic resistance, with coherent twin boundary (CTB)-mediated triple-junction drag identified as a key contribution. The dimensionless drag parameter is formulated from mechanistically motivated mobility contrasts, CTB density, and size-dependent junction statistics. A single calibration constant is determined from pure Cu and then held fixed across other materials to test transferability. Time-resolved mean-field simulations—using temperature-dependent parameters and purity-dependent diffusivities—reproduce the U-shaped stability behavior across FCC systems and capture the observed grain-size range of the crossover grain size, d c, while further enabling construction of time-temperature thermal-stability maps. These maps can be used to guide grain-size selection and define practical service windows for property-targeted microstructure design. © 2026 Acta Materialia Inc.

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