Prominent role of high-volume fraction Mg17Al12 dynamic precipitations on multimodal microstructure formation and strength-ductility synergy of Mg–Al–Zn alloys processed by hard-plate rolling (HPR)

Microstructure evolutions and mechanical properties of Mg–Al–Zn alloys processed by large-reduction hard-plate rolling (HPR) were investigated in the present work. During the HPR process, a high volume fraction of submicron-sized Mg17Al12 particles (~23% and ~224 nm) precipitated in the Mg–9Al–1Zn (AZ91) alloy, in comparison to the dilute Mg–3Al–1Zn (AZ31) and Mg–6Al–1Zn (AZ61) alloys. Moreover, this work highlights the effect of high-volume fraction Mg17Al12 precipitates on the favorable formation of a multimodal microstructure consisting of coarse grains and ultrafine/fine grains with heterogeneous texture distribution by detailed microstructural characterizations. In contrast, both the HPRed AZ31 and AZ61 alloys exhibit a uniform fine-grained structure. Notably, the multimodal-grained AZ91 possesses a much higher yield strength (YS) of ~246 MPa and ultimate tensile strength (UTS) of ~370 MPa than the uniform-grained AZ31 (YS of ~182 MPa and UTS of ~260 MPa) and AZ61 alloys (YS of ~221 MPa and UTS of ~312) while exhibiting a comparable high uniform elongation ~14%. It breaks the conventional viewpoint that an increase in Al content usually leads to an enhanced strength but decreased ductility. The reasons for the superior combination of strength and ductility of the HPRed AZ91 alloy are discussed in terms of the synergy effect of multimodal grain structure, heterogeneous texture distribution, and high-volume fraction precipitates.