Effect of calcium additions on the creep behavior of magnesium die-cast alloy ZA85
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I. INTRODUCTION
LIGHTWEIGHT magnesium alloys have a strong potential for weight saving in a wide range of technical applications because of the lowest density of all technologically relevant metallic materials. The most commonly used diecast magnesium alloys are AZ91 (9 wt pct Al, 1 wt pct Zn, 0.3 wt pct Mn) and alloys of the AM series (2 to 6 wt pct Al, 0.3 wt pct Mn). At room temperature, these alloys have outstanding mechanical properties, but their use is limited to temperatures of less than 120 °C.[1,2] For elevated temperatures, die-cast alloys AS21 (2 wt pct Al, 1 wt pct Si) and AE42 (4 wt pct Al, 2 wt pct rare earth element) have been developed. This is reflected in Figure 1, which shows the creep strain after 200 hours of several Mg die-cast alloys after stressing at 35 MPa at 150 °C. However, AS21 and AE42 are more difficult to cast and, in the case of AE42, much more expensive than AM60 or AZ91.[3] Alternatively, Mg die-cast alloys with zinc as the major alloy addition were developed, which show an acceptable die castability, corrosion resistance, and room-temperature properties.[4,5] The creep resistance of ZA85 (8 wt pct Zn, 5 wt pct Al) was found to be superior to AZ91, and more recent investigations revealed that the creep strength can be significantly improved by adding small amounts of alloying elements such as Ca or Sr[6,7] (Figure 1). In a previous study, we have shown that the good creep resistance of ZA85 in the regime of 150 °C to 200 °C is related M. VOGEL, Member, R&D Staff, is now with SiCrystal AG, Erlangen, Germany. O. KRAFT, Institute Director, is now with the Forschungszentrum Karlsruhe GmbH, Institute fur Materialforschung II, 76021 Karlsruhe, Germany. E. ARZT, Institute Director, is with the Max-Planck-Institut fur Metallforschung and Institut fur Metallkunde der Universitat, 70569 Stuttgart, Germany. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
to the formation of -phase precipitates (Mg32(Al,Zn)49).[8] At high stresses and short testing times, fine incoherent precipitates increase the creep resistance. However, at low stresses and long testing times, the precipitates overage during the test. From these observations, it has been concluded that deformation at elevated temperatures of Mg die-cast alloys is predominantly controlled by dislocation creep. Also, it has been argued that the improved creep resistance achieved by the addition of Ca is due to an enhancement of the thermal stability of -phase precipitates. This argument, however, has not been confirmed. A detailed microstructural investigation of respective alloys using electron microscopy and smallangle neutron diffraction revealed that Ca does not influence the volume fraction and aging behavior of the -phase precipitates.[9] It is the aim of this
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