Microstructural effects of the cyclic and monotonic hardening of Al5Mg

The cyclic and monotonic hardening mechanisms in Al5Mg (where the composition is in approximate weight per cent) have been characterized by transmission electron microscopy and by differential scanning calorimetry (DSC). After constant-plastic-strain-amplitude tests to cyclic saturation, a high density of fine dislocation loops is generated. The corresponding DSC plots reveal a characteristic exotherm at 150°C (ΔH x in the range from −0.2 to −0.5 J g −1) which is shown to result from the annihilation of the fine dislocation loops. This behavior is not found after monotonic deformation by cold rolling or drawing to equivalent total plastic strains, for which dense dislocation tangles rather than loops develop. The 150°C exotherm occurs at saturation after fatigue of Al5Mg in both the annealed and the cold-worked states and also of Al5Mg0.5Ag aged to form homogeneous T precipitates. At lower temperatures in the range 50–110°C, an endotherm (probably due to Guinier-Preston (GP) zone dissolution) is also detected ( ΔH x ≈ 0.5 J g −1). The variation in the endothermic peak temperature is consistent with a GP zone size which depends on the aging or fatigue treatment.