Precipitation Strengthening of a Mg-Al-Ca–Based AXJ530 Die-cast Alloy

Abstract

Precipitation of the Al 2 Ca phase in the a-Mg phase of the Mg-Al-Ca-based AXJ530 die-cast alloy and the effect of precipitation hardening on the creep resistance were studied. Precipitation of the Al 2 Ca phase is suppressed in the die-casting process due to its high cooling rate, but precipitation was observed during subsequent isothermal heat treatments above 448 K. The precipitate has a disk-shaped morphology parallel to the basal plane of the a-Mg matrix phase and increases hardness by aging in the temperature range of 448 to 573 K. The creep resistance of the die-cast material at 448 K was improved by a factor of 1.5~2, by a peak-age heat treatment at 523 K for 1 hour. Improvement in strength is limited by the low volume fraction of the precipitates; however, the precipitates are effective obstacles against dislocations gliding on nonbasal planes during creep deformation.

Evaluation of Magnesium Die-Casting Alloys for Elevated Temperature Applications: Microstructure, Tensile Properties, and Creep Resistance

“…The high coarsening rate, together with the high diffusion rate of Al in Mg, accounts for the poor creep resistance of AZ91, despite the relatively high yield strength. On the other hand, the main precipitate phase is Al 2 Ca for both MRI230D [23] and AXJ530 [24] and probably Mg 3 Nd for AM-HP2+. [43] With high melting points, Al 2 Ca and Mg 3 Nd are expected to have intrinsically high thermal stability and sustained precipitation strengthening at elevated temperatures, thus leading to the good creep resistance observed in MRI230D, AXJ530 and AM-HP2+.…”

“…[23] AXJ530, which was developed by General Motors for engine blocks and transmissions, is reported to have at least 40 pct greater tensile and 25 pct greater compressive creep resistance than AE42, [17] and corrosion resistance as good as AZ91D [19] and these properties have been shown to improve further with heat treatment. [24] D. Mg-Al-RE Alloys There are two important alloys in the Mg-Al-RE family, one is AE42 (Mg-4Al-2RE) and the other AE44 (Mg-4Al-4RE). AE42 has been used as a benchmark creep-resistant Mg die-casting alloy, which exhibits superior creep resistance to AZ91 at temperatures up to 423 K (150°C).…”

“…Obvious supersaturation of Al solute is seen in AZ91, AM60, AS31, AJ52, MRI153A, and MRI153M (Figure 1). In addition, there is some degree of superstation of solutes in MRI230D, AXJ530, AE42, and AM-HP2+ as well, which is inferred from the precipitation of fine particles, i.e., Al 2 Ca for MRI230D [23] and AXJ530, [24] Mg 17 Al 12 for AE42, [42] and Mg 3 Nd for AM-HP2+, [43] either after creep or artificial aging at elevated temperatures. On the other hand, there is probably no supersaturation of solute in AE44-2 and AE44-4, evidenced by the absence of precipitation in the Mg matrix after high temperature creep tests.…”

“…[5,11] Furthermore, a recent study [59] suggested that a highly interconnected and strong skeleton of intermetallic phase is able to effectively shield load from the softer a-Mg matrix and therefore alloys with a higher interconnectivity of intermetallic phase should have higher creep resistance than those with a lower interconnectivity. However, there are increasingly more studies [23,24,35,60,61] showing that intra-granular strengthening by solutes/precipitates also has a great influence on the creep resistance of Mg alloys. For example, Maruyama et al [60] reported that Y has a more significant effect in improving the creep resistance of Mg alloys than Al, which is largely due to a higher solid solution strengthening effect.…”

“…For example, Maruyama et al [60] reported that Y has a more significant effect in improving the creep resistance of Mg alloys than Al, which is largely due to a higher solid solution strengthening effect. Suzuki et al [24] studied the precipitation strengthening effect of Al 2 Ca in a die-cast AXJ530 alloy and reported that the creep resistance was improved by a factor of 1.5 to 2 after a peak aging treatment [1 hours at 523 K (250°C)]. More recently, Zhu et al [35] examined the microstructure and creep resistance of die-cast Mg-RE (RE = La, Ce or Nd) binary alloys with various RE contents in an effort to separate the relative contributions from grain boundary reinforcement by intermetallic phases and intra-granular strengthening by solutes/ precipitates.…”