Precipitation phenomena in B4C-reinforced magnesium-based composite

The artificial aging characteristics at 177 °C of MgAlZn (AZ80A) alloy and its composite reinforced by B4C particles were compared. The morphology and the different distributions of the hardening precipitate (Mg17Al12) were observed by optical microscopy in these two materials. The aging sequence was studied by differential scanning calorimetry. This technique coupled with X-ray diffraction analyses allowed us to evaluate both the precipitation kinetics and the amount of Mg17Al12 obtained as a function of the aging time. The mechanical characteristics were evaluated by hardness measurements. The ceramic reinforcement was found to increase the aging rate of the composite with respect to that of the unreinforced alloy. Furthermore B4C particles affect the precipitation mechanism and the precipitate-strengthening distribution. In fact the heterogenous nucleation of Mg17Al12 close to the interface between ceramic and metal matrix is greatly enhanced in the composite. As a consequence the hardness increase is slightly lower for composite than for the alloy, because of the different precipitate distributions in the matrix.