Enhancement of fatigue properties of Al-Mg and Al-Mg-Si alloys at elevated temperatures by addition of small amount of Scandium

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The effects of small amount of Sc addition on the fatigue properties of Al-Mg and Al-Mg-Si alloys at elevated temperatures have been examined. Cyclic softening after initial hardening is found in the Al-Mg alloys with small Al_3Sc particles, but no cyclic softening takes place in the alloys with larger particles. The cyclic softening is explained by a loss of particle strengthening through particle shearing within strongly strained slip bands. The larger Al_3Sc particles have a stronger retardation effect on the formation of fatigue-induced stable dislocation structure than small particles even at a high temperature. After peak-aging treatments, the Al-Mg-Si alloys has fine lath-shaped β' precipitates. In the Sc-containing Al-Mg-Si alloy, spherical Al_3Sc precipitates of about 11 nm in diameter are co-existed. The Al-Mg-Si alloy with Sc exhibits cyclic hardening to saturation, while the alloy without Sc shows clear cyclic softening after initial hardening. In the Sc-bearing Al-Mg-Si alloy, owing to the existence of non-shearable Al_3Sc precipitates, dislocations were uniformly distributed, resulting in the improvement of fatigue life.