The effect of thermal exposure on the interface and mechanical properties of Al₁₈B₄O₃₃w/AZ91 magnesium matrix composite

The effect of high temperature exposure on the interface and tensile properties of Al₁₈B₄O₃₃w/AZ91 magnesium matrix composite fabricated by squeeze casting was studied. The interfaces of the composites were examined using transmission electron microscopy, and the mechanical properties were evaluated by tensile tests. The interfacial region was composed of MgO which was formed by the reaction between the Al₁₈B₄O₃₃ whisker and the Mg matrix. An thin and uniform interfacial reaction layer with a thickness of 10-20 nm formed during squeeze casting could act as a barrier to prevent the further reaction between Al₁₈B₄O₃₃ whisker and Mg alloys. The interfacial reaction rate was very low during thermal exposure below the liquidus temperature of AZ91 matrix alloy. Only after exposure above the liquidus temperature of the AZ91 matrix alloy for a long period, the thickness of the interfacial layer increased obviously. However, the interfacial reaction rate was much slower than that in the Al₁₈B₄O₃₃w/Al-Mg composites. The interfacial reaction kinetics was controlled by the seepage of Mg and release of Al and B along the narrow diffusion channels between the fine MgO particle. The composite did not show a degradation in the tensile properties after exposure below the solidus temperature of AZ91 for long periods, suggesting that the Al₁₈B₄O₃₃w/AZ91 magnesium matrix composite could be subjected to long-term service at elevated temperature.