Quench sensitivity of the extruded Mg-6Zn-0.2Zr/Mg2Si composite: Position dependence and role of dislocation density

Abstract

The quench sensitivity of the Mg-6Zn-0.2Zr/Mg2Si composite is investigated by microstructure and mechanical properties analysis. It is position-dependent, which is high in the center and low on the surface of the composite. In the center, the mismatch in the CTE between the Mg2Si and Mg matrix leads to high dislocation density in the matrix upon fast quenching, which promotes age hardening. The dislocation density decreases for a lower quenching rate, and leads to lower hardness. Such origin of quench sensitivity is specific to the magnesium composite in addition to the well-recognized solute and vacancy loss. On the surface, low concentration of Zn element leads to low hardness under both cooling conditions. The dislocation and precipitates act as a buffer zone to mitigate the strain incompatibility, which alleviate stress concentration near the Mg/Mg2Si interface. So, the interface debonding is delayed, and twin growth is suppressed. The lower quenching rate negates such a beneficial effect.