THE EFFECT OF GRAIN SIZE, STRAIN AND TEMPERATURE ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR OF POLYCRYSTALLINE ALUMINIUM AND AL ALLOYS

Abstract The tensile yield and flow stresses of aluminium, A1-2.63Mg alloy and A1-2.07Li alloy at room temperature are shown to depend on the inverse square root of the polycrystal grain size and are described empirically by the Hall-Petch relation. The same relation describes the flow stress-grain size dependence for A1-2.07Li alloy at temperatures ranging from - 196°C to 400°C. The strain hardening in the friction stress of each material at 20°C is independent of the grain size, is approximately parabolic and is greatest for the precipitation strengthened A1-2.07Li alloy. The grain size contribution to the tensile flow stress is dependent on both the tensile strain and composition. The friction stress, σ₀, and slip band stress intensity parameter, k_ε, at yield, k_y, are both dependent on temperature. Low temperature suppresses dislocation annihilation and recovery processes, leading to planar pile-ups at grain boundaries and a hardening that is linear with strain. Weak hardening is observed at 250°C and 400°C due to extensive annihilation and recovery. The value of k_ε, at all temperatures falls following initial yielding with the generation of freshly unlocked sources.