Microstructural evolution and simultaneous strengthening and toughening of core-shell Mg₃Bi₂ reinforced Zn-1.2 wt% Mg zinc alloys

Microstructure evolution and simultaneous strengthening and toughening of core-shell Mg₃Bi₂ reinforced Zn-1.2 wt% Mg zinc alloys were investigated. The results show that the as-cast microstructure of Zn-1.2 wt% Mg zinc alloys with various Bi contents consists of dendritic Zn, eutectic Mg₂Zn₁₁, and a core-shell Mg₃Bi₂/Bi heterostructure. Transmission electron microscopy (TEM) results reveal that rod-like and spherical Mg₃Bi₂ intermetallics, identified as (Mg_1-xZn_x)₃Bi₂ variants with chemical compositions of Mg_0.99Zn_2.01Bi₂ and Mg_2.03Zn_0.97Bi₂, respectively, exhibit a crystallographic relationship between hexagonal structured Mg₃Bi₂ and hexagonal close-packed Zn matrix: (112¯0)_Mg3Bi2//(101¯0)_Zn and [112¯0]_Mg3Bi2//[0001]_Zn. In addition, a possible coexisting orientation relationship of (011¯)_Mg2Zn11//(020)_Bi is observed at Bi/Mg₃Bi₂ interface for Mg₂Zn₁₁ precipitates. Meanwhile, numerous stripes and subgrains in Mg₃Bi₂ are discovered to align at precise angles with dislocations, owing to lattice strains induced by substitution of Mg atoms with Zn. Moreover, the simultaneous improvement in strength and ductility of the Zn-1.2 wt% Mg zinc alloy containing 1.5 wt% Bi is predominantly governed by the core-shell morphology and sizes of the Mg₃Bi₂/Bi heterostructure. The strengthening-toughening mechanism of the core-shell Mg₃Bi₂ reinforced zinc alloy involves crack deflection and ligament toughening, while also impeding crack propagation through obstacles or crack-bridging behind the crack tip. This mechanism reveals a synergistic effect of soft-shell toughening and hard-core strengthening thereby enhancing cracking resistance of the zinc alloys.