Creep and rupture properties of a squeeze-cast Mg-Al-Ca alloy
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I. INTRODUCTION
THERE are increasing demands from the automotive industry for low-cost magnesium casting alloys with sufficient strength and good creep resistance. Mg-Al–based alloys such as AZ91 (Mg-9Al-0.7Zn-0.2Mn; all compositions hereafter are in wt pct unless specified) and AM60 (Mg-6Al-0.3Mn) are the most commonly used magnesium alloys in automobiles because of a good combination of mechanical properties, corrosion resistance, and die castability. However, the use of Mg-Al alloys in automobiles has been restricted to components where creep resistance is of less concern, such as seat frames, steering wheels, instrument panels, valve covers, etc.[1] Since Mg-Al alloys exhibit inadequate creep resistance at temperatures above 125 °C, they are incapable of powertrain applications, where the operative temperatures can be as high as 175 °C for automatic transmission housings and 200 °C for engine blocks.[2] MRI153, which was developed by Dead Sea Magnesium and Volkswagen AG for use as automatic transmission housings with complex geometries,[3] can be regarded as a modified version of AZ91. MRI153 contains Al (9 pct) and Zn (0.7 pct) to ensure its yield strength and castability, while Ca (1 pct) and Sr (0.1 pct) are added to form intermetallic phases at grain boundaries and in the grain interior to improve the creep resistance. It has been shown that MRI153 possesses superior properties such as yield strength, creep resistance, castability, and corrosion performance to AZ91.[3] For instance, the minimum creep rate of MRI153 at 135 °C under a load of 85 MPa is about one order of magnitude lower than that of AZ91. However, there is a lack of a more detailed study of the creep deformation process and rupture properties of this alloy. Many studies have been conducted on the creep behavior of alloy AZ91 produced by various casting technologies, including die casting,[4–7] ingot casting,[8] squeeze casting,[9,10] and
thixoforming.[11,12] The creep tests were generally conducted in the temperature range 120 to 200 °C, since the maximum operating temperature for AZ91 seldom or never exceeds 200 °C. In most studies,[4,5,6,8–12] stress exponents in the range 5 to 11 and activation energies close to that for self-diffusion in magnesium were obtained, suggesting a climb-controlled dislocation creep mechanism. However, Dargusch and Dunlop[7] reported stress exponents of 2 at low stresses and 5 at high stresses. They suggested that the low-stress creep behavior was related to grain boundary migration and sliding, which was accompanied by discontinuous precipitation of -Mg17Al12 from the supersaturated -Mg matrix. A recent review paper[13] compared the creep behavior of AZ91 alloys produced by different technologies. The difference in creep response was rationalized qualitatively by microstructural factors such as grain size and intragranular precipitates. A threshold stress that depends on the volume fraction and size of precipitate particles was introduced to describe the strengthening effect of intragranular precipitates
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