急冷凝固法によるAl–6∼8%Mn–1∼3%Cr合金の組織と性質

In order to assess the optimum alloying composition, rapidly solidified P/M materials were prepared for Al–Mn–Cr ternary alloys containing 6∼8 mass%Mn and 1∼3 mass%Cr. Effects of alloying composition on precipitation hardening were studied with a purpose of obtaining precipitation-hardened materials with some acceptable ductility. The critical composition to observe precipitation hardening was identified as 8.8 mass%Mn + Cr, above which hardening increased with increasing Mn + Cr. The largest hardening was observed for 7Mn–3Cr alloy containing highest Mn + Cr. CrAl7 was formed in as rapidly solidified state, 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, thereby hardening occurred when amount of G phase formation is sufficiently large. For the as-extruded P/M materials, the highest tensile strength of 480 MPa was obtained at room temperature for 7Mn–3Cr with 7% elongation. Elongation of the as-extruded P/M materials kept unchanged up to 473 K but increased appreciably at 673 K. However, the heat-treated P/M materials showed poor ductility and hence tensile strength decreased with aging time regardless of occurrence of hardening. Increases in elastic modulus were observed after precipitation heat treatment.