Strengthening and toughening behaviors of a Mg-9Al alloy containing oxygen atoms

Oxygen atoms are dispersed in a Mg-9Al alloy (Mg-O-9Al alloy) with the decomposition of TiO₂ nanoparticles in melt. During the solidification process, the oxygen atoms are redistributed, affecting the development of the fine eutectic β-phase containing a higher concentration of oxygen atoms than that α-Mg since they have a limited solubility in α-Mg and assist the nucleation of α-Mg. The dissolved oxygen atoms expand the lattice structures of both α-Mg and β-phase, inducing the reduced lattice mismatch between α-Mg and β-phase. Therefore, yield stress of the Mg-O-9Al alloy is 143 MPa, much higher than 110 MPa observed in the Mg-9Al alloy. Fracture toughness values of 10.39 MPa m^1/2 and 12.86 MPa m^1/2 for both the Mg-9Al and Mg-O-9Al alloys are also respectively obtained. The fracture surface reveals that the crack propagates along the weak interface of β-phase in the Mg-9Al alloy. On the other hand, the β-phase disturbs the crack propagation route in the Mg-O-9Al alloy, showing many broken β-phases. Therefore, the improved interfacial feature with an addition of dissolved oxygen atoms in the Mg-O-9Al alloy results in enhanced mechanical properties.