Structure and properties of rapidly solidified P/M samples of Al-Mn-Cr alloys

Rapidly solidified P/M materials of Al-Mn-Cr ternary alloys were prepared containing 6∼8 mass% Mn and 1∼3 mass% Cr. The effect of alloy composition on precipitation hardening was studied with an aim of obtaining precipitation hardened materials having acceptable ductility. The critical composition for observing precipitation hardening was determined to be 8.8 mass% Mn+Cr; above this value hardening increased with increasing the Mn+Cr content. The greatest hardening was observed for Al-7Mn-3Cr alloy. In these alloys, CrAl7 was formed under rapid solidification and precipitation of MnAl6 occurred during hot extrusion in alloys of lower Cr content. Finally, G phase ((Mn,Cr)Al12) was formed'during heating of hot-extruded materials, causing hardening when the amount of G phase formation is sufficiently large. For the as-extruded P/M materials, the highest tensile strength at room temperature (480MPa) was obtained for Al-7Mn-3Cr, with 7% elongation. Elongation of the as-extruded P/M materials remained unchanged up to 473K but increased appreciably at 673K. However, precipitation hardened P/M materials showed poor ductility so that the tensile strength decreased with aging time. Increases in the elastic modulus were observed after precipitation heat treatment.