473 Kで時効したMg–6%Al合金の時効硬化挙動に及ぼすMn濃度の影響

抄録

The effect of Mn contents on age-hardening in Mg–6 mass% Al alloy aged at 473 K has been investigated using samples having different Mn contents by hardness measurement and microstructure observation. The hardness of as-quenched condition decreased with increase of Mn contents, also, age-hardenability was decreased following further aging treatment at 473 K. Al–Mn intermetallic compounds were observed in Mn contained alloys. The area fraction of Al–Mn intermetallic compounds increased with increasing Mn contents, while the hardness of as-quenched condition was decreased. These compounds consumed Al by formation of Mg₁₇Al₁₂ precipitates in the matrix. Aged alloys showed the discontinuous precipitation (DP) at grain boundaries, and continuous precipitation (CP) in the matrix. The DP has a lamellar structure, and its area increased with increase of aging time. It was coarsened and broken up into particles with further aging, particularly in 0.5%Mn alloy. The calculated hardness of each alloy using hardness values of DP, CP and area fraction of DP through the rule of mixture was in good agreement with experimental result. These results suggest that not only discontinuous precipitates on grain boundaries but also continuous precipitates in the matrix contribute to hardness of these alloys.