Microstructure, microstructural stability and mechanical properties of sand-cast Mg–4Al–4RE alloy

This paper presents a methodology for assessing the phase composition and the results of structural stability tests of the sand-cast Mg–4Al–4RE alloy after annealing it at 175 and 250°C for 3000h. The microstructure was analyzed with optical, scanning electron, and transmission electron microscopy. The phase composition was determined with X-ray diffraction. The structure of the Mg–4Al–4RE (AE44) alloy is composed of large grains of α-Mg solid solution, needle-shaped precipitates of the Al11RE3phase, polyhedral precipitates of the Al2RE phase and Al10RE2Mn7 phase. After annealing at 175°C for 3000h, no changes in the alloy structure are observed, whereas after annealing at 250°C the precipitates of the Al11RE3 phase are found to be in the initial stages of spheroidization. The coarse-grained structure and unfavorable morphology of the intermetallic phases in the sand-cast AE44 alloy, which are caused by low solidification rates, result in low creep resistance up to 200°C and low mechanical properties at ambient temperature and at 175°C. •Complement the knowledge about the microstructure of Mg-Al-RE alloys.•Clarify the mechanism of formation of Mg17Al12 phase above 180°C.•Applying a chemical dissolution of the α-Mg in order to phase identification.•Applying a statistical test to assess the spheroidization of precipitates.•Quantitative description of microstructure of Mg-Al-RE alloys.