High-temperature behaviour of as die-cast and heat treated Mg-Al-Si AS21X magnesium alloy

The mechanical behaviour of the magnesium alloy AS21X (Mg–2% Al–1% Si alloy AS21 modified by addition of rare-earth elements)was experimentally investigated in the as die-cast and in the solution treated and artificially aged conditions in the temperature range from 70 to 210 ◦C. The as die-cast microstructure was characterized by coring effects and by the presence of Mg2Si and Mg17Al12 particles. Holding at temperatures below 180 ◦C caused massive formation of fine-scale precipitates at the grain boundaries. The homogeneous distribution of Al in the Mg-rich -phase of heat treated material caused at these temperatures the formation of further amounts of Mg17Al12 particles, also within the grains. At 210 ◦C, Mg17Al12 particles dissolved in both material conditions. At low temperatures, longer times to creep rupture and better creep resistance were displayed by the as cast (AC) material. The properties in the heat treated (HT) condition approached those of the as cast material at the highest test temperature. The suggested deformation mechanism of the alloy up to 180 ◦C is climb-controlled dislocation creep, for which core diffusion could play a significant role at 70 ◦C in the as cast material.