Due to the increasing demand of weight reduction of transportation vehicles for higher energy efficiency and reduction of CO₂ emission, the development of magnesium alloys for structural applications receives significant interest recently. Currently, 85 - 95% magnesium is being used for cast products. This is because the magnesium lacks formability because of its hcp structure. Lack of slip system makes the wought applications of magnesium alloys limited. Also the typical yeild strength of magnesium alloys are below 200 MPa, which is too low for many structural applicaiton. If one can develop economically viable high strength wrought magnesium alloys, the usage of magnesium alloys for structural components will be dramatically expanded. For wrought applications, alloys must show excellent extrudability or formability. Mg-Zn and Mg-Al-Zn alloys are most widely used commercial wrought magnesium alloys, but their strength is not high enough. Although both alloys are precipitation hardenable, precipitation hardening has not been used in these commerical wrougt alloys, because their age hardening responses are too low. After wrought process, solution heat treatment or aging treatment cause recovery and recrystallization of extruded or rolled alloys, resulting in softening of the material. Therefore, there is no heat treatable wrought magnesium alloys. A few exceptions are rare earth containing alloys, but rare earth elements are too expensive for general structural applications. Thus, the purpose of this study is to develp commercially viable high strength, low cost wrought magnesium alloys without rare earth elements in order to expand the number of possible applications of magnesium alloys in various structural applicaitons.

Unlike wrought aluminum alloys, the majority of wrought magnesium alloys attain their strength through grain size refinement, not through precipitation hardening.The Hall-Petch coefficient for magnesium was reported to be approximately 0.7MPam^-1/2 for coarse grains (30-87mm) and 0.13 MPam^-1/2 for fine grains (17-30mm), which are significantly higher than the Hall-Petch coefficient for aluminum alloys, 0.07MPam-1/2. Thus, the strengthening via grain size control is more effective in magnesium alloys than precipitation hardening. If the precipitation hardening response is significantly enhanced, it will be possible to strengthen wrought magnesium alloys by heat treatments such as T5 (aging without solution treatment) or T6 (solution treatment and aging). The age hardening response of magnesium alloys may be enhanced by promoting precipitates on the most effective planes for strengthening by addition of trace elements. Only a few studies have been made on the effectiveness of trace additives in Mg-Zn alloys. Recently we reported that trace additions of 0.1at% of both Ag and Ca to Mg-2.4Zn leads to an increase in the peak hardness of approximately 2.5 times that of the binary alloy, which is attributed to the refinement of MgZn2 precipitates ( phase). The combined addition of Ag and Ca used in this investigation was chosen based on the recently proposed guidelines for the choice of trace additives as shown in Fig. 1.


Figure 1 Age hardening responses of Al-Zn alloys with trace additions of Ag, Ca, and Zr.

Fig. 2 Stress-strain curves and TEM images of as-extruded and T6 treated Mg-2.3Zn-0.1Ag-0.1Ca-0.1Zr alloy. For comparison, the stress strain curve of as-extruded Zr-free alloy is also shown. The precipitates observed in extruded and T6 treated alloys are totally different, the former was confirmed to be Mg(Zn,Zr) and the latter was confirmed to be Mg2Zn by 3DAP and TEM. Ca was confirmed to be enriched in the Mg2Zn precipitates. The mechanical properties of the developed alloy are compared with those of existing wrought alloys.

Figure 2 shows mechanical properties of the developed Mg-Zn-Ag-Ca-Zr alloy in the as-cast, extruded and extruded and T6 treated states. Compared to the as-cast alloy, both yield strength and ultimage tensile strength are substantially improved in the as-extruded alloy because of the grain size refinement effet. The fine grain size was due to the dynamical precipitation and recrystallization during the extrusion process at 350C as shown in the TEM image. The fine precipitates dispersed in the extruded alloy were identified as Mg(Zn,Zr) by TEM and 3DAP.

For the development of high strength wrought magnesium alloy with superior heat resistance, one promising system is a precipitation hardenable Mg-Sn alloy. Since the Mg2Sn phase has the high melting temperature of 1043 K, which is comparable with the precipitates in Mg-RE system (RE stands for rare earth elements), there is a possibility to develop cost effective “RE-free” Mg-based alloy with superior heat resistance. In addition, the growth of the dynamically recrystallized grains may be suppressed by the Mg2Sn particles that precipitate during extrusion as reported in previous investigations in other systems. Research on the Mg-Sn based alloys is, however, mainly limited to the cast alloys. This is probably because the wrought processing was expected to be difficult in the Mg-Sn alloy due to the reduced ductility in the binary Mg-Sn alloy. Pekgüleryüz et al. indicated the possibility of the ductility enhancement by the addition of Zn to the Mg-Sn alloy. The addition of the Zn is also known to enhance the precipitation response of the Mg-Sn alloy by the refinement and homogeneous dispersion of the Mg2Sn phase. Hence, the Mg-Sn-Zn alloy may be suitable for the extrusion process, which can open up a possibility for higher strength wrought magnesium alloy with superior heat resistance. In the present study, we have developed a new extrudable high strength alloy, Mg-2.2Sn-0.5Zn-1.0Al (at.%), for enhancing yield strength of Mg-Sn binary alloys in the as-extruded condition using the dynamic recyrstallization and precipitation phenomena together with solid solution hardening by Al. The newly designed alloy was found to show excellent extrudability at low temperature of 523 K exhibiting superior yield strength in the as-extruded condition as shown in Fig. 3.


Figure 3 Tensile and compression stress strain curves of extruded Mg-2.2Sn-0.5Zn-1.0Al alloy its microstructure observed by EBSD and TEM.
Recent review paper on precipitation of Mg alloys PFAM18

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Related Publications

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The building block of long-period structures in Mg-RE-Zn alloys
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Precipitation-hardenable Mg-2.4Zn-0.1Ag-0.1Ca-0.16Zr (at%) wrought magnesium alloy
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