Characterisation of nano-sized Al-Mn-(Mg) particles in AZ91 and their effect on Mg17Al12 precipitation

Al-Mn-(Mg) particles in the size range of 20-200 nm have been found in a sand cast AZ91 magnesium alloy. Their morphologies, chemical composition and structures were investigated using transmission electron microscopy. It was found that these particles have a chemical composition of (Al6.52Mn)1−xMgx (x < 0.13) and a decagonal quasi-crystalline structure. These particles were stable during a solution treatment at 420 °C for 72 h and acted as preferential nucleation sites for Mg17Al12 precipitates during the subsequent ageing. The potential effect of these Al-Mn-(Mg) particles on the strengthening of AZ91 magnesium alloys is discussed.