Effect of Strain Rate and Temperature of Tensile Test on Lüders Strain in Al-Mg Alloys

Liiders strain observed in Al-high Mg alloys such as commercial AA5182 alloy is in many respects different from that reported in the case of Al-low Mg alloys such as AA5052 alloy. In order to elucidate such differences more clearly, and in order to clarify the mechanism inducing Liiders strain in Al-high Mg alloys, the effects of strain rate and temperature of the tensile test on Liiders strain were studied in detail in AA5182 alloy. Also, observations were made of the morphologies of the yielded zones developed on the specimen surface during the tensile test. It was found that Liiders strain is much larger and more complex in Alhigh Mg alloys than in Al-low Mg alloys. In Al-high Mg alloys strained with low strain rate, serrated yielding due to the Portevin-Le Chatelier effect occurred concurrently with Liiders strain, so that serrated Liiders strain was observed on the stress-strain curve. In such cases, serrated yielding initiated in the region where the Liiders front had just swept, i.e. just behind the Liiders front. Due to such contribution of the concurrent Portevin-Le Chatelier effect, the apparent Liiders strain increased at such strain rates, giving a local maximum in the Liiders strain versus strain rate curve. In Al-high Mg alloys, pinning points inducing Liiders strain are, for the most part, formed during furnace cooling at temperatures between 350 and 450 C. They are thermally quite stable and can be eliminated by reheating in the same temperature range. © 1999 Carl Hanser Verlag. All rights reserved.