Influence of secondary phases on crack initiation and propagation during fracture process of as-cast Mg-Al-Zn-Nd alloy

The influence of secondary phases on the initiation and propagation of tensile crack during the fracture process is studied by using in-situ scanning electron microscopy (SEM) tensile testing in as-cast Mg-Al-Zn-Nd alloy. Results reveal that micro-cracks initiated in divorced eutectic Mg17Al12 phase, eutectic Mg17Al12 phase, twin boundaries and interface between alpha + beta eutectic structure and alpha-Mg matrix. The breaking of the divorced eutectic Mg17Al12 phase at the grain collapsing zone (GCZ) edge and the decreased effective force bearing area due to grain collapsing contributed to the propagation of transgranular cracks. Breaking of Mg17Al12 phase is accompanied by the formation of voids at the main crack tip, which provided a path for crack propagation and growth. Formation of voids can also blunt the main crack tip and divert the crack propagation direction. On the other hand, the voids difficultly form in Nd-containing secondary phases, i.e., Al2Nd and Al11Nd3, due to the superior mechanical properties and better interfacial bonding capability. Therefore, the Nd-containing compounds have an inhibitory effect on the propagation of crack of Mg alloys. Furthermore, the elastic modulus and electronic charge density difference of Mg17Al22, Al2Nd and Al11Nd3 phases have been calculated to verify the experimental results. The mechanical properties and the bonding mechanism of these phases are discussed briefly.