Al-Mn-based decagonal quasicrystal in AZ magnesium alloys and its nucleation on Al8Mn5 during solidification

Abstract

Manganese is added to many magnesium alloys to control impurity iron, but its effects on phase transformations and microstructure formation remain incompletely understood. Here we show that an Al-Mn-based decagonal quasicrystal (d-QC) forms in the late stages of solidification in AZ31 and AZ91 magnesium alloys at relatively slow cooling rates, here down to 0.1 K/s. The d-QC has a periodicity of ∼ 12 Å, grew as decagonal rods and commonly shared interfaces with Al8Mn5 and eutectic Mg17Al12. A reproducible orientation relationship (OR) was measured with Al8Mn5 only, indicating that the d-QC nucleated on Al8Mn5. The OR is consistent with the structural relationship between quasicrystals and gamma brasses and gives the d-QC nucleation advantages over LT-Al11Mn4 during solidification. A subsequent heat treatment at 410 °C caused both Al8Mn5 and the d-QC to transform into LT-Al11Mn4.