Torsional fatigue experiments were carried out with aluminium-magnesium alloys containing 0.52, 0.91, 3.46 and 5.15% magnesium at the quenched state, and △E-N and Tm-N curves were determined. Experimental results showed that, in the cases of the specimens containing 0.52% and 0.91% magnesium, △E drops with an increase of the stress cycle N when the torsion strain is small. However, when the torsion strain is higher, the △E-N curve keeps flat at the beginning, but △E increases subsequently and becomes stable after a fairly high value is reached. As the magnesium content in the specimen is 3.46%, the change of the △E-N curve is similiar to that of the Al-4% Cu alloy when the torsion strain is not too high. When the torsion strain is sufficiently large, △E increases at the beginning and decreases after it passed through a maximum value. The change of the △E-N curve of the specimen containing 5.15% magnesium is entirely similar to that of the Al-4% Cu alloy and no peak value was observed on the curve with the highest torsion strain used in the experiment. The Tm value shows an initial increase for specimens with all the magnesium contents studied.The results described above can all be explained with the assumption that "atmospheres" of solute atoms are formed because of the gradual migration of magnesium atoms to dislocations during fatigue loading. It is considered that, in the case of aluminium-magnesium alloys, the origin of the energy loss, △E, and the factor that controls the magnitude of △E in the initial stage of fatigue loading are all primarily due to the formation of atmospheres of solute atoms. When the magnesium content is relatively small, the atmospheres formed may be considered as "loose" for a sufficiently high torsion strain, so that a dislocation can drag its atmosphere along and work is done under fatigue loading. This leads to an increase of both △E and Tm. However, when the magnesium content is relatively high or the torsion strain is not sufficiently large, the atmospheres formed at the beginning of fatigue loading become already "dense" enough to pin the dislocations. With the continuation of faligue loading, the mobility of dislocations is further lowered by the migration of solute atoms to these dislocations. This leads to an initial decrease of △E and an initial increase of Tm. Fatigue experiments were also carried out on Al-0.52% Mg and Al-3.46% Mg alloys at various aging stages, and the phenomena of strain aging were observed. Such phenomena are in accord with the assumption of atmospheres of solute atoms described above.
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