Effect of whisker surface treatment on the strength of Al₁₈B₄O₃₃/Al alloy composites

In order to increase the whisker preform strength and decrease the interfacial reactivity between Al₁₈B₄O₃₃ whisker and aluminum alloy, three kinds of whisker surface treatments have been carried out. They were SiO₂ binder addition, magnesium deposition coating and spinel (MgAl₂O₄) coating. By a low vacuum evaporation technique (under vacuum 10 Pa at 650°C for 1 h), magnesium was deposited on the whiskers uniformly. Magnesium deposition reacted with the whisker at 800°C for 0.5 h in air to create a MgAl₂O₄ layer (8-10 nm in thickness) on the whisker. Three kinds of surface-treated whiskers and the untreated whisker were used to reinforce AC4CH aluminum alloy by squeeze casting method. Those surface treatments could increase the strength of whisker preform and hence decreased the preform deformation during squeeze casting. However, SiO₂ binder did not prevent the interfacial reaction. Being similar to untreated whisker composite, SiO₂ added whisker composite had a deterioration of strength after T6 treatment. Magnesium coating just avoided the interfacial reaction of Al₁₈B₄O₃₃ whisker with the matrix but did not prevent that with the magnesium coating itself. After T6 treatment, the strength decrease due to whisker damage and the strength increase from the matrix age strengthening did not change the total strength of composites. MgAl₂O₄ coating existed as an effective barrier to prevent the interfacial reaction. During T6 treating, no further whisker damage occurred and the matrix was strengthened. This produced considerable enhancement of the composite strength.