Microstructure and creep properties of Mg-6Al-(Sr, Ca) alloys

The microstructure, tensile and creep properties of Mg-6Al-2Sr and two Mg-6Al-(12)Sr-1-Ca alloys have been investigated. The as-cast microstructure of Mg-6Al-2Sr alloy consists of the dendritic alpha-Mg and lamellar alpha-Mg+Al4Sr eutectic at interdendrites. Ca addition to Mg-6Al-2Sr alloy results in the formations of Mg2Ca compound and a Mg-Al-Sr ternary intermediate phase, and the significant improvement of creep properties. SEM observations for crept samples revealed that the intermetallics in the Mg-6Al alloys with Sr and Ca additions have high thermodynamic stability. Discontinuous precipitation of beta-Mg17Al12 Occurred in Mg-6Al-2Sr alloy during creeping, however, no obvious changes have been observed in the microstructure of the alloys with combined addition of Sr and Ca after creeping. The die-cast sample showed finer microstructure and higher tensile properties at room and elevated temperatures, but relatively lower creep resistance in comparison with the samples cast by water cooled mould.