EFFECT OF PLASTIC WORKING ON AGE HARDENING BEHAVIOR OF RAPIDLY SOLIDIFIED AL-8-PERCENT-MN-2-PERCENT-CR ALLOY

Precipitation hardening of a supersaturated solid solution of extended solubility obtained by rapid soldification of an Al-% Mn-2% Cr alloy has been studied for the materials with or without plastic working prior to aging treatment. Mechanical alloying (MA) treatment was applied as cold working, and hot extrusion at 673 K was carried out as hot working. Specimens of four different thermal and mechanical histories; rapidly solidified flakes, extruded P/M material from the flakes, MA powder and extruded P/M material from MA powder were precipitation treated. The peak hardness in age hardening was increased by applying plastic working prior to aging treatment. The results indicate that cold-worked MA powders showed the highest peak hardness, followed by hot extruded P/M material from cold-worked MA powder and hot extruded P/M material from rapidly solidified flakes. The peak hardness of rapidly solidified flakes was the lowest. Precipitation hardening is caused by precipitation of the G phase, (Mn, Cr) Al12, and an increased peak hardness was obtained by introduction of heterogeneous sites by plastic working. The major constituent phases of this alloy were CrAl7 in as-rapidly solidified flakes, CrAl7 and the G phase in as-extruded flake-P/M materials, MnAl6 and the G phase in as-extruded MA-P/M materials, and the G phase in age hardened P/M materials.