Yielding of Aluminum-Magnesium Alloys under Dynamic Loading

The lower yield stress of high purity aluminum-magnesium alloys with Mg contents of 0.86, 2.05 and 3.67% and a commercial alloy of 4.75% Mg, which have been well-known as alloys showing a sharp yield elongation and exhibiting the propagation of Lüders bands during yielding, was investigated under dynamic loading at room temperature and compared with that in the static test with special attention to its grain size dependence, following the Cottrell-Petch equation σy=σi+kd−1⁄2. The observed increase of the lower yield stress in the dynamic test from that in the static test was interpreted to be due to the increase of the factor, k, relating to the stress necessary to unpin the dislocations from their locking atmospheres and due to the increase of the frictional stress, σi. These increases are mainly due to a smaller contribution of thermal fluctuation to unpinning of dislocations and dislocation movement in the lattice under dynamic loading owing to the lack of time for the dynamic test. Further, the effect of Mg contents on k was discussed and some dislocation locking mechanisms were considered. The effect of Mg contents on σi was also discussed by taking into account the degree of precipitating or solute clustering and dislocation intersection to produce point defects. As a result, clusters or precipitates are pointed out to be an effective factor to determine the value of σi. The multiple yielding, as seen in the static test, was not observed under dynamic loading.