DEVELOPMENT OF MICROSTRUCTURE AND PROPERTIES DURING EXTRUSION AND ROLLING OF AL-7MG-1MN GAS-ATOMIZED POWDER

Cold compacted, non-degassed billets were consolidated by hot extrusio n, the product of which was subsequently rolled at both elevated and r oom temperatures. By employing a combination of X-ray diffraction and transmission electron microscopy, a detailed picture of the microstruc tural 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 consoli dation temperature and rolling reduction on both microstructure and pr operties is discussed. Rapid solidification successfully retarded the formation of Mn containing intermetallics in this alloy, permitting th e precipitation of a fine distribution of MnAl6 particles during conso lidation. At the lower consolidation temperatures employed (350-degree s-C) an additional coarse ternary Al-Mg-Mn phase was formed, while at the highest consolidation temperature (550-degrees-C) rapid diffusion of the Mn species ted to coarsening of the MnAl6 dispersoids. The opti mum dispersoid distribution of MnAl6 was observed to occur in material consolidated in the temperature range 450-500-degrees-C. The primary effect of the presence of the MnAl6 dispersoids was that of substructu ral refinement, with resulting subgrain sizes in the range 0.8-1.7 mum . The strength of the alloy decreased with increasing consolidation te mperature, as a direct result of subgrain coarsening, while fracture t oughness appeared to be directly related to the dispersoid distributio n, the optimum toughness being observed at the consolidation temperatu re which provided a homogeneous distribution of fine MnAl6 dispersoids . Owing to the high work hardening rate of the alloy at room temperatu re, cold rolling was limited to very low rolling reductions (< 20%), a nd 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-degrees-C permitted much larger reductions to be attempted, and this had the effect of increasing strength with n o accompanying decrease in toughness. Coarsening of the Mn containing dispersoids was observed to occur during hot rolling; however, it is p roposed that any consequent weakening of the material is counteracted by an improvement in the integrity of the alloy produced by the mechan ical deformation imparted to it.