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Several mechanisms are proposed, but none predicts the exponent and its variation between metals.","posed_since":"2004","precise":"Flow stress of FCC micropillars scales as $\\tau \\sim D^{-n}$ with $n \\sim 0.6 \\text{ to } 0.7$ for diameters D from about $10^{-5} \\text{ to } 10^{-3}\\,\\mathrm{cm}$ (Uchic et al. 2004), while BCC metals show smaller n correlated with the temperature dependence of screw dislocation mobility. Determine which mechanism (truncation of single-arm sources, dislocation starvation, exhaustion hardening) controls n, and whether n is universal for FCC, via 3D dislocation dynamics compared with experimental statistics. An answer is the mechanism with predicted n versus material, D and initial dislocation density.","problem_ref":"f0a3391ddf8d6b7ac8867d34f4e8345dcfe09664b7883c94e22bf2c21c1bfb9b","references":"","settled_by":"Dislocation dynamics simulations with measured initial microstructures reproducing n for FCC and BCC pillars, tested on pillars of controlled initial dislocation density.","status_note":"Source-truncation models reproduce $n \\sim 0.6$ for FCC; the BCC trend and the role of initial dislocation density remain unresolved.","title":"Origin of the size dependence of micropillar 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