Microstructure and mechanical properties of ZA104 (0.3-0.6Ca) die-casting magnesium alloys

In this paper, the microstructure of die-cast and slowly cooled specimens of two new experimental magnesium alloys (ZA104 + 0.3Ca and ZA104 + 0.6Ca) is examined. Microanalysis of specimens has revealed the presence of alpha-Mg phase (matrix) and two intermetallic compounds containing calcium with different Zn/Al ratios. These compounds, which have a crystal structure close to that of tau phase [Mg-32(Al,Zn)49], were also found in slowly cooled ZA104 + 0.3Ca specimens and identified as (tau(1)' and tau(2)'). The microhardness of these intermetallic compounds has been obtained from specimens having the intermetallic composition and slowly cooled from the melt. The tensile and creep resistance of the two die-cast alloys (ZA104 + 0.3Ca and ZA104 + 0.6Ca) are also presented. Tensile properties are equivalent to those of the AZ91D reference alloy, but their creep deformation rate is significantly reduced by a factor 3.5-3.8 at 150 degreesC and 4.0-7.2 at 175 degreesC. Besides, it was found that the ultimate tensile strength and elongation of ZA104 + 0.3Ca alloy are superior to those of ZA104 + 0.6Ca alloy. It is then established that the intermetallic phases tau(1)' and tau(2)' containing calcium are harder than T phase and improve substantially the creep resistance of these new ZA experimental alloys. (C) 2004 Elsevier B.V. All rights reserved.