Effect of characteristics and distribution of Mg₁₇Al₁₂ precipitates on tensile and bending properties of high-Al-containing Mg alloys

This study investigates the effect of characteristics and distribution of Mg₁₇Al₁₂ precipitates on the uniaxial tensile and three-point bending properties of extruded Mg alloys containing high Al contents. The extruded Mg–9Al–1Zn–0.3Mn (AZ91) alloy contains lamellar-structured Mg₁₇Al₁₂ discontinuous precipitates along the grain boundaries, which are formed via static precipitation during natural air cooling. The extruded Mg–11Al–1Zn–0.3Mn (AZ111) alloy contains spherical Mg₁₇Al₁₂ precipitates at the grain boundaries and inside the grains, which are formed via dynamic precipitation during extrusion. Due to inhomogeneous distribution of precipitates, the AZ111 alloy consists of two different precipitate regions: precipitate-rich region with numerous precipitates and finer grains and precipitate-scarce region with a few precipitates and coarser grains. The AZ111 alloy exhibits a higher tensile strength than the AZ91 alloy because its smaller grain size and more abundant precipitates result in stronger grain-boundary hardening and precipitation hardening effects, respectively. However, the tensile elongation of the AZ111 alloy is lower than that of the AZ91 alloy because the weak cohesion between the dynamic precipitates and the matrix facilitates the crack initiation and propagation. During bending, a macrocrack initiates on the outer surface of bending specimen in both alloys. The AZ111 alloy exhibits higher bending yield strength and lower failure bending strain than the AZ91 alloy. The bending specimens of the AZ91 alloy have similar bending formability, whereas those of the AZ111 alloy exhibit considerable differences in bending formability and crack propagation behavior, depending on the distribution and number density of precipitates in the specimen. In bending specimens of the AZ111 alloy, it is found that the failure bending strain (ε_{f, bending}) is inversely proportional to the area fraction of precipitates in the outer zone of bending specimen (A_ppt), with a relationship of ε_{f, bending} = –0.1A_ppt + 5.86.