Thermodynamics-Based Selection and Design of Creep-Resistant Cast Mg Alloys
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INTRODUCTION
WEAK aging response and relatively low creep resistance are still stumbling blocks on the way to advanced structural applications of Mg alloys. Since there are no evident structural reasons for either one (e.g., Reference 1, the melting point of Mg is similar to that of Al), it has often been pointed out that these issues have more to do with the nature of the solutes than with the host itself.[1–4] The solubility of many solutes decreases with the temperature in the same fashion as in age-hardenable Al-based alloys, and at first sight most Mg-based alloys appear as suitable candidates for precipitation hardening through controlled aging.[2,5–13] Following the Al alloys model, aging of Mg alloys is carried out by a combination of solution heat-treatment within the single phase field in the relevant phase diagram, followed by quenching to create a supersaturated solid solution. Subsequent aging at an intermediate temperature is thus expected to harden the alloy through a mixture of coherent and non-coherent precipitates finely distributed in the a-Mg matrix. As shown by Figure 1, SAEIDEH ABASPOUR, Postdoctoral Fellow, and CARLOS H. CA´CERES, Reader in Casting Technology, are with the ARC Centre of Excellence for 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 September 14, 2015 5972—VOLUME 46A, DECEMBER 2015
more often than not the results fail to meet the expectations: whereas aged Al-Cu alloys can reach in excess of 150HV and Al-Zn alloys 200HV, most aged Mg alloys remain well below 100HV. Among them, the binary Mg-Al and Mg-Zn alloys are textbook examples of weak aging response.[3,4] The overall behavior in Figure 1 is in line with the classical experiments by DeLuca and Byrne,[14] who concluded that while age hardening can make Al alloys 200 times harder than the pure metal, a mere increase of a factor ~30 is normally achieved with Mg-based alloys. At ~130HV, recently developed quaternary alloys, such as Mg-Gd-Y-Zr and Mg-Y-Nd-Zr, can be quoted as acceptable examples of age hardening,[13] although still well below Al-Zn alloys. A fairly similar situation exists regarding creep strength, with many Mg alloys softening at about 393 K to 422 K (120 C to 150 C),[1,8,12,13,15–17] as shown in Figure 2. Mg-Al is again a case in point: despite the high solid solubility of Al and its measurable strengthening effects at room temperature,[18] solid solution hardening does not extend to high temperatures. On the positive side, as an example of feasibility, Figure 2 also shows that Mg-Th alloys, which are currently being phased out due to the radioactivity of Th, remain stable up to 573 K to 623 K (300 C to 350 C).[12,17] Likewise, some of the recently developed RE-based alloys, WE43, WE54, and EZ43 approach a service temperature of 523 K (250 C)[11,17,19,20] and new alloys are under consideration,[11] whereas new developments in hi
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