| The current study was initiated to develop an understanding of the low-toughness fracture in Al-SiC metal matrix composites (MMCs) with respect to the role of the various elements of the microstructure and their probable contribution to degradation. The constitutive response and fracture in these composites were studied experimentally, in terms of tensile and fracture toughness testing, and numerically, utilizing the finite element method. It was found that the matrix alloy controls both flow properties and fracture in the materials investigated. As with the monolithic Al alloy, Al 7191-SiC composites are strain rate insensitive with microvoid coalescence as the predominant fracture micromechanism. Moreover, the onset of fracture followed the stress-modified critical strain criterion developed for ductile fracture by void coalescence. The fracture path in these composites is preferentially in the matrix adjacent to Al/SiC interfaces with moderate particle cracking.The low-toughness fracture is believed to be an inherent property of these MMCs and is caused, mainly, by the differential elastic and thermal properties of the two constituents. These differentials degrade the matrix alloy near the interface by consumption of a substantial part of its strain hardening capacity, and by stress intensification introduced by the SiC particle geometry and incompatible deformations at the interface. Consequently, the matrix near the interface is subjected to high triaxiality and localized damage leading to premature fracture, which was reflected from the decreased true fracture stress and low values of strain to fracture. It is concluded that a higher toughness composite requires a proper choice of constituent properties which dominate the stress state at the interface. |
