Creep properties of conventional magnesium diecasting alloys

Conventional and creep properties of magnesium diecasting alloys, AZ91D, AM60 and AS21 were investigated. Conventional tensile test was conducted in a temperature range from room temperature to 250degreesC. Creep test was carried out at a temperature range from 125similar to175degreesC under a stress range of 25MPa to 75MPa. With increasing aluminum content ultimate tensile strength and proof stress increase, but minimum creep rate also increases. Stress exponent of minimum creep rate changes at a given threshold stress, and the threshold stresses decrease with increases in aluminum content and test temperatures. In AZ91D and AM60B alloys, Mg17Al12 phase precipitates at the grain boundaries even at the creep test condition of 125degreesC-25MPa. However, in the heat resistant magnesium alloy, AS21, such a precipitate is hardly observed at the entire creep test conditions. Such microstructural instability at the grain boundaries in alloys with high aluminum content may lead to acceleration of the creep rate.