Characteristics of morphology and crystal structure of α-phase in two Al-Mn-Mg alloys

The characteristics of morphology and crystal structure of α-Al12(Fe,Mn)3Si phase in two Al-Mn-Mg alloys in as-cast and homogenized states have been studied. The results show that the morphology of primary α-particles is the same as that of primary β-particles in as-cast ingots and ‘Al-spots’ is only the characteristic of α-phase transformed from β-phase. α-precipitates are sterically irregular plates. The α-phase precipitated from solid solution and transformed from β-phase has an ordered body-centered cubic crystal structure. There is no obvious difference in the morphology of α-phase between two Al-Mn-Mg alloys. But in the alloy with high Si content there are more Mg2Si needles, which then cause the α-precipitates in alignment along the direction [1 0 0] of Al matrix during heating at lower temperature.