Aging Hardening and Precipitation Characteristics of Extruded Mg-9Al-0.8Zn-0.2Mn-0.3Ca-0.2Y Alloy

In this study, an Mg-9Al-0.8Zn-0.2Mn-0.3Ca-0.2Y (AZXW9100) alloy having high corrosion and ignition resistance characteristics is recently developed. This study investigates the aging hardening and Mg17Al12 precipitation characteristics of the extruded AZXW9100 alloy by comparing those of a commercial AZ91 counterpart. Both alloys' microstructures consist of equiaxed recrystallized grains and they have an intense basal texture. Unlike the AZ91 alloy with a few Al-Mn particles, the AZXW9100 alloy contains numerous Al2Y, Al8Mn4Y, and Al2Ca particles. As the aging time increases from 0 to 8 h at 200 degrees C, the hardness increases from 63.6 to 95.4 Hv and 68.8 to 98.3 Hv for the AZ91 and AZXW9100 alloys, respectively. Although the hardness increments during aging and the peak aging times are almost the same for both alloys, their Mg17Al12 precipitation behavior considerably differs. During aging, the number of Mg17Al12 continuous precipitates formed in the AZXW9100 alloy is less than that in the AZ91 alloy because of the lower amount of Al solute atoms in the former. The nucleation and growth of Mg17Al12 discontinuous precipitates (DPs) are also suppressed in the AZXW9100 alloy because of the combined effect by the reduced Al solutes, finer grains, Ca and Y solutes, and second-phase particles. Consequently, the area fraction of DPs of the peak-aged AZXW9100 alloy is nearly half that of the peak-aged AZ91 alloy. This study demonstrates that adding small amounts of Ca and Y to AZ91 alloy causes drastic variations in the Mg17Al12 precipitation behavior during aging.