High Temperature Strength and Stress Relaxation Behavior of Dilute Binary Mg Alloys
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INTRODUCTION
THE importance of solid solution hardening to the creep strength of Mg alloys has often been stressed in the literature.[1–10] This assertion, however, calls for some qualification considering that AZ91, the commercial alloy with the highest solid solubility of its main solute, is also a textbook example of poor creep performance, even in the solution heat treated state in which as much as ~9 at. pct of Al is in solution at the start of the test. That is, solid solution effects on the creep behavior of Mg alloys are more related to the nature of the solute itself than to just its mere presence. Though this may seem obvious, its practical value, with reference to cold chamber high-pressure die cast (HPDC) alloys in particular, is made evident next. HPDC Mg alloys are characterized by a fine grain size microstructure near the casting surface, or casting skin, while large dendritic grains which are externally solidified in the shot-sleeve accumulate in the center, or core.[11] The incipient solidification in the shot sleeve enriches the remaining liquid, and as a result the skin region also contains a very large volume fraction of eutectic forming a spatially interconnected network SAEIDEH ABASPOUR, Research Assistant, and CARLOS H. CA´CERES, Reader in Casting Technology, are with the ARC Centre of Excellence Design in Light Metals, Materials Engineering, School of Engineering, The University of Queensland, Brisbane QLD, 4072, Australia. Contact e-mail: [email protected] Manuscript submitted December 5, 2014. Article published online January 5, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
that contributes a measurable amount to the alloy strength.[12] The fine grain size accounts for the largest component of the casting strength through the Hall– Petch effect, whereas solid solution hardening adds a third contribution to the strength. The latter is very dependent of the solidification conditions due to coring effects. Pronounced coring due to the fast solidification leaves the center of the a-Mg grains largely denuded of solute.[10,13] Solute profiles of HPDC Mg-Al alloys created using the Scheil–Gulliver equation (e.g. Figure 2(a) of Reference 14) or obtained through microprobe profiles (Figure 4 of Reference 1), show that even for a solute like Al with very high terminal solubility, the core of the grains retains only a minute fraction in solution. Consequently solid solution hardening is necessarily limited, and unless heat treated, an option not available for HPDC alloys, the core of the a-Mg grains is little more than pure Mg, constituting an inherent weak link in the alloy microstructure. It follows that the relative efficiency of the solid solution strengthening at dilute (~1 at. pct) concentrations is crucial when it comes to optimizing the microstructural design of all cast (i.e. not heat treated) Mg alloys, and particularly so HPDC alloys. A case study illustrating the two strengthening mechanisms described above, i.e. the interconnected intermetallics and the residual solid solution, was recently des
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