Effect of manganese on the hot strength of aluminum alloys

Conclusions

• 1. The importance of X-ray shadow microscopy for defining the structural condition of complex alloys after heat treatment and after longtime hot testing has been established.

• 2. The structure of the cast alloy AL19 consists of α-solid solution depleted in copper but enriched in manganese; a CuAl₂ phase which crystallizes mainly as large plates along the α-solution grains and a phase T(Al₁₂Mn₂Cu) appearing as fine particles largely distributed throughout the volume of the α-solution grains but also as relatively large platelets at grain boundaries and finally, an Al₃Ti phase also of the platelet type.

• 3. The structure of solution-treated AL19 consists of an α-solid solution enriched in copper but with reduced manganese compared with the as-cast condition and a T(Al₁₂Mn₂Cu) phase in the form of a multitude of minute particles distributed within the α-grains. The primary Al₃Ti and T-phase plate-like particles remain practically unchanged.

• 4. After long-time tests at 300°C the alloy structure consists of a non-uniform α-solid solution. A multitude of T particles exists within the grains. This structural condition is very clearly revealed by X-ray shadow microscopy.

• 5. The complex microheterogenous structure of the α-solid solution grains in alloy AL19 with a multitude of small (several μ in size) T-phase particles apparently strongly hampers the movement of dislocations on slip planes and along boundaries thereby inproving the hot strength of the alloy.

• 6. The low diffusivity of manganese compared with zinc, magnesium, copper, silicon and other elements and the weak tendency to coalesce of the manganese phase particles both inside the α-grains and along their boundaries slow down the softening of the alloy. This could account for the increased hot strength of AL19 compared with alloy AL7.