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

The tensile yield and flow stresses of aluminium, Al-2.63Mg alloy and Al-2.07Li alloy at room temperature are shown to depend on the inverse square root of the polycrystal grain size and are described empirical ly by the Hall-Fetch relation. The same relation describes the flow st ress-grain size dependence for Al-2.07Li alloy at temperatures ranging from - 196 degrees C to 400 degrees C. The strain hardening in the fr iction stress of each material at 20 degrees C is independent of the g rain size, is approximately parabolic and is greatest for the precipit ation strengthened Al-2.07Li alloy. The grain size contribution to the tensile flow stress is dependent on both the tensile strain and compo sition. The friction stress, sigma(0), and slip band stress intensity parameter, k(epsilon) at yield, k(y), are both dependent on temperatur e. Low temperature suppresses dislocation annihilation and recovery pr ocesses, leading to planar pile-ups at grain boundaries and a hardenin g that is linear with strain. Weak hardening is observed at 250 degree s C and 400 degrees C due to extensive annihilation and recovery. The value of k epsilon at all temperatures falls following initial yieldin g with the generation of freshly unlocked sources.