Effects of Different Microstructure on Strengthening Mechanism and Hardening Mechanism in Peak-Aged Mg–14Gd–0.2Sn Alloy

A bimodal-grained microstructure is formed in the Mg–14Gd–0.2Sn alloy extruded with a small extrusion ratio of 7 (E7 alloy), containing a large number of fine dynamic recrystallized (DRXed) grains with an average size of ~ 1.11 μm. In comparison, a much higher proportion of DRXed microstructure (~ 90%) with a coarser grain size of ~ 5.85 μm is formed in the alloy extruded with a large ratio of 16 (E16 alloy). The lower actual temperature during extrusion of the E7 alloy increases the supersaturation degree of α-Mg solid solution matrix, and induces a higher volume fraction of dynamic precipitates (β-Mg₅Gd) compared with the E16 alloy. After aging, the E7-A alloy exhibits a stronger precipitation strengthening effect with a peak-aged yield strength (YS) of 405 MPa. The YS of the E7-A alloy presents a remarkable improvement of 88 MPa compared with that of the E16-A alloy, which is mainly related to finer DRXed grains and stronger texture. In contrast, the hardness increment of the E16-A alloy is higher than that of the E7-A alloy, and the peak hardness of the E16-A alloy is comparable to that of the E7-A alloy. Although the high density of precipitates in the E16-A alloy contributes to a strong aging hardening response, the weaker precipitation strengthening effect of the E16-A alloy is mainly attributed to the activation of twinning in coarser grains with a weaker texture.