The aim of the present paper is to monitor the precipitation hardening potentials during aging at 150°C in Al–Mn–Mg alloys containing a small amount of Cu. The age-hardening response and phase transformation are significantly affected by the Si contents of the order of 0.1%. The hardening response increases during aging with increasing Si content. The hardening potential is strongly deteriorated by tensile deformation prior to the heat treatment. The pre-strain causes an inhomogeneous nucleation of precipitation on dislocations and cell walls and restricts the transgranular precipitation during aging. In the 10% pre-strained alloys, a short-term aging leads to a dislocation recovery, resulting in increase in n-value. The further decrease of dislocation density and increase in n-value occur due to a small degree of precipitation hardening in a 0.15% Si added alloy, whereas the dislocation recovery is suppressed by the competitive precipitation in a 0.36% Si added alloy during the prolonged aging.

The effect of Mn contents on age-hardening in Mg–6 mass% Al alloy aged at 473 K has been investigated using samples having different Mn contents by hardness measurement and microstructure observation. The hardness of as-quenched condition decreased with increase of Mn contents, also, age-hardenability was decreased following further aging treatment at 473 K. Al–Mn intermetallic compounds were observed in Mn contained alloys. The area fraction of Al–Mn intermetallic compounds increased with increasing Mn contents, while the hardness of as-quenched condition was decreased. These compounds consumed Al by formation of Mg₁₇Al₁₂ precipitates in the matrix. Aged alloys showed the discontinuous precipitation (DP) at grain boundaries, and continuous precipitation (CP) in the matrix. The DP has a lamellar structure, and its area increased with increase of aging time. It was coarsened and broken up into particles with further aging, particularly in 0.5%Mn alloy. The calculated hardness of each alloy using hardness values of DP, CP and area fraction of DP through the rule of mixture was in good agreement with experimental result. These results suggest that not only discontinuous precipitates on grain boundaries but also continuous precipitates in the matrix contribute to hardness of these alloys.