Abstract
AZ91 alloy was deformed in tension at temperatures between room temperature and 300 °C. During deformation the machine was stopped and specimen was allowed to relax, i.e., the stress drop was estimated as a function of time of the relaxation. The stress relaxation curves were analysed as a function of the stress at the beginning of the relaxation, the test temperature and the time period of the relaxation. The activation volume was estimated from the stress relaxation curves. The activation volume decreases with the stress and at temperatures above 50 °C also with increasing temperature. Post relaxation effect, i.e., an increase of the flow stress at the beginning of plastic deformation after stress relaxation in comparison to the flow stress at the beginning of stress relaxation, is observed in some cases. The post-relaxation effect is sensitive to the relaxation time and temperature. Possible changes in the internal stresses and the mobile dislocation density as a consequence of the dynamic strain ageing are considered to be responsible for the post-relaxation effect.

Key words
stress relaxation, thermally activated process, activation volume, strain ageing

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