Development of microstructure and properties during extrusion and rolling of Al-7Mg-1 Mn gas atomised powder

Cold compacted, non-degassed billets were consolidated by hot extrusion, the product of which was subsequently rolled at both elevated and room temperatures. By employing a combination of X-ray diffraction and transmission electron microscopy, a detailed picture of the micro structural transformations that occur during consolidation and rolling was obtained. Tensile and fracture properties were evaluated for the alloy in both the consolidated and rolled conditions. The effect of consolidation temperature and rolling reduction on both microstructure and properties is discussed. Rapid solidification successfully retarded the formation of Mn containing intermetallics in this alloy, permitting the precipitation of a fine distribution of MnAl 6 particles during consolidation. At the lower consolidation temperatures employed (350 °C) an additional coarse ternary Al-Mg-Mn phase was formed, while at the highest consolidation temperature (550°C) rapid diffusion of the Mn species led to coarsening of the MnAl 6 dispersoids. The optimum dispersoid distribution of MnAl 6 was observed to occur in material consolidated in the temperature range 450-500°C. The primary effect of the presence of the MnAl 6 dispersoids was that of substructural refinement, with resulting subgrain sizes in the range 0.8-1.7 μm. The strength of the alloy decreased with increasing consolidation temperature, as a direct result of subgrain coarsening, while fracture toughness appeared to be directly related to the dispersoid distribution, the optimum toughness being observed at the consolidation temperature which provided a homogeneous distribution of fine MnAl 6 dispersoids. Owing to the high work hardening rate of the alloy at room temperature, cold rolling was limited to very low rolling reductions (<20%), and while such low reductions led to a rapid increase in strength, this was accompanied by an equally dramatic decrease in both ductility and toughness. Rolling at 300°C permitted much larger reductions to be attempted, and this had the effect of increasing strength with no accompanying decrease in toughness. Coarsening of the Mn containing dispersoids was observed to occur during hot rolling; however, it is proposed that any consequent weakening of the material is counteracted by an improvement in the integrity of the alloy produced by the mechanical deformation imparted to it.