Grain boundary and intragranular reactions during aging in Mg-Al system alloys poured into sand and iron molds

It is well-known that age hardening occurs in Mg-Al system alloys, when the alloy containing aluminum exceeds 6mass%. This precipitation reaction depends on the aluminum content and aging temperature. The aging behavior in AZ91 magnesium alloy was investigated and it is the subject of this paper. However, for the Mg-Al system alloys, the influence of aluminum content on age hardening characteristics has not been researched in detail so far. In this study, continuous and discontinuous precipitations during aging in Mg-Al system alloys cast into sand and iron molds was investigated by means of hardness measurement and microstructure observation with optical microscope and transmission electron microscope. The microstructure of AM60 magnesium alloy with 6mass% containing aluminum in the as cast condition was composed with a primary crystallized alpha magnesium phase and beta Mg₁₇Al₁₂ phase. The hardness of cellular precipitates existing near the grain boundary in sand mold castings decreased with increasing the aging time. On the other hand, the hardness of intragranular precipitates was constant against the aging time. Variation of hardness with aging was found to be caused mainly by the discontinuous precipitation along the grain boundaries from the composite rule in hardness. In iron mold castings, it was found that the variation of hardness with aging was found to be caused mainly by the continuous precipitation inside the crystal grain.