Comparison between dislocation dynamics model predictions and experiments in precipitation-strengthened Al-Li-Sc alloys

Precipitate distributions were quantitatively measured by local electrode atom-probe (LEAP) tomography in several age-hardenable Al-based alloys to provide input to both analytical models and dislocation dynamics simulations of critical resolved shear strength, for comparison with experimentally measured Vickers microhardness values. The method of reconstructing precipitate- containing volumes from LEAP tomography, then importing these data to dislocation dynamics simulations, is explained in detail in the supplementary material available in the online version of this paper. Two alloys were studied: Al-2.9Li-0.11Sc at.% (Al-Li-Sc) and Al-6.3Li-0.07Sc-0.02Yb at.% (Al-Li-Sc-Yb). Heat treatment of these alloys produced nanometer-scale α′-Al ₃(Li, Sc, Yb)(L1₂) precipitates after isothermal aging at 325 °C. In some cases δ′-Al₃Li(L1₂) shells were formed on these precipitates after subsequent isothermal aging at 170°C. Dislocation dynamics results and experimental measurements were combined to define empirical strengthening superposition rules for the cases of contributions from: (i) Li in solid-solution plus α′-Al ₃(Li, Sc, Yb)(L1₂) precipitates; (ii) α′- Al₃(Li, Sc, Yb)(L1₂) precipitates plus δ′-Al₃Li(L1₂) shells in doubly aged Al-Li-Sc-Yb. Simulations of aged Al-Li-Sc overpredict the strength if a single dislocation is used, and underpredict the strength if instead a cooperative dislocation pair is considered. For simulations of dislocation pairs in Al-Li-Sc (single-phase precipitates), the precipitate bypass mechanism depends on the aging condition of the alloy. At peak age, precipitate shearing occurs mainly by pairs of closely spaced dislocations moving cooperatively. As overaging progresses, Orowan looping increasingly dominates and the distance between the leading and trailing dislocations increases. For dislocation pairs in doubly aged Al-Li-Sc-Yb with some core/shell precipitates, the measured and simulated strength values agree to within their uncertainties.