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: A1-2.9Li-0.11Sc at.% (Al-Li-Sc) and A1-6.3Li-0.07Sc-0.02Yb at.% (Al-Li-Sc-Yb). Heat treatment of these alloys produced nanometer-scale alpha'-Al-3(Li, Sc,Yb)(L1(2)) precipitates after isothermal aging at 325 degrees C. In some cases delta'-Al3Li(L1(2)) shells were formed on these precipitates after subsequent isothermal aging at 170 degrees 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 alpha'-Al-3(Li, Sc,Yb)(L1(2)) precipitates; (ii) alpha'-Al-3(Li, Sc, Yb)(L1(2)) precipitates plus delta'-Al3Li(L1(2)) 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. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.