Impurity-Dislocation Interaction and Repeated Yielding in a Commercial Al Alloy

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Abstract
Impurity‐dislocation interaction via a vacancy mechanism and the associated repeated yielding phenomenon in an aluminum alloy containing 2.8 at.% Mg, 2.4 at.% Zn, 0.65 at.% Cu, and 0.16 at.%Cr, have been studied. The repeated yielding is observed only in the temperature range of −40° to +90°C and strain rates of 10−5–10−4 sec−1. The onset of repeated yielding is thermally activated within this temperature range. From +20° to −40°C, the process is a normal Portevin‐LeChatelier effect observed by various investigators in dilute and concentrated alloys. The activation energy associated with this normal stage is found to be 0.66 eV and the exponent m of the semiempirical equation: Cν=Kεm, is 0.88, where Cν is the vacancy concentration, K is a constant, and ε is the plastic strain. From 40° to 90°C the corresponding values of m and the activation energy are 0.95 and 0.80 eV, respectively. An alternative method of analysis of the experimental data is proposed. Based on a simple random‐walk theory, this method can produce the activation energy without the ambiguity of the exponent m. Furthermore, this treatment is capable of yielding some information regarding the density of mobile dislocations. Contrary to certain recent results, the activation energy Em for vacancy migration in this alloy compares favorably with the vacancy migration energy in pure aluminum.