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 3000 h. 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 Al₁₀RE ₂Mn₇ phase. After annealing at 175°C for 3000 h, 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.