Interfacial microstructure and fracture characterization of Al₁₈B₄O₃₃/Al alloy composites

In order to investigate the relationship between the mechanical properties and the microstructure of aluminum borate (Al₁₈B₄O₃₃) whisker reinforced Al-1% Mg alloy composites fabricated by the squeeze casting process, the microstructure and fractured structure were observed by scanning and transmission electron microscopy (SEM and TEM). The interface between the matrix and whisker in an as-cast composite is smooth. By heat treatment at 550 °C for 2 h, the MgAl₂O₄ reaction layer with a spinel structure is produced at the whisker/matrix interface and appears trapezoidal. The interface between Al₁₈B₄O₃₃ and MgAl₂O₄ has a certain preferred orientation with very little mismatch. Cracks tend to propagate along the whisker/matrix and reaction product/matrix interfaces. The interface between the whisker and reaction product seems to have good bonding strength. There are many microcracks in the whiskers in fractured composites, which propagate along the (001) plane in the whisker. Many dislocations and strain fields were observed in the matrix around the reaction products. The bending strength decreases with increasing amount of reaction product. The degradation of the composite strength by heat treatment is caused by weakening of the strength of the interface between the reaction product and matrix or strain concentration at the convexity formed by the interfacial reaction.