MaterialsScience and Engineering, A 107 (1989) 63-69 63 The Influence of Hydrostatic Pressure on the Ductility of AI-SiC Composites* A. K VASUDEVANand O. RICHMOND Alcoa Research ('entre, Pitt~sbur~h (U.S.A.) F.ZOK and J. D. EMBURY McMaster University, Hamilton (Canada) (ReceivedJune 1, 1988) Abstract Tensile tests with superimposed hydrostatic pressures were performed on two types of metal matrix composite." 2014 Al with 20% SiC particles and 2124 Al with 14% SiC whiskers. In the mater- ials with SiC particulate, the ductility increases rapidly with pressure and the mode of damage initiation is by particle fracture. Materials contain- ing SiC whiskers exhibit a different fracture mode involving whisker matrix decohesion, and strain localization which results in shear fracture. 1. Introduction The mechanical response of metal-matrix composites can be considered in terms of various microstructural features, such as the par- ticle-matrix interfaces and the work hardening characteristics of the matrix, and the process of damage accumulation which precedes fracture. Much of the work in the present conference relates to the detailed characterization of inter- facial structures and their role in the process of load transfer in metal-matrix composites. The present paper explores the pressure dependence of flow and fracture in both particulate- and whisker-reinforced Al-matrix composites. The results suggest that a combination of studies of the mechanical response under superimposed pressure, detailed metallographic and fracto- graphic studies, and micromechanical analyses can yield information which is complementary to the detailed structural studies of interfaces. The *Paper presented at the symposium on Interfacial Phe- nomena in Composites: Processing, Characterization, and MechanicalProperties,Newport, RI, June 1-3, 1988. work serves to delineate some of the critical damage processes which limit the ductility of metal-matrix composites. 2. Materials and experimental procedures Mechanical tests were performed on two types of SiC-reinforced Al-matrix composites and compared with the results of tests performed on the matrix materials without the reinforcing phase. One composite was a commercial 2014 AI alloy containing 20% SiC particles. This material was heat treated to the T6 condition (Fig. l(a)). The average particle size was 13 /zm and the average aspect ratio was 1.23 measured in the plane containing the tensile axis. Approximately 5% of the SiC particles were greater than 40/zm in size and had aspect ratios of the order of 4. The second material was a 2124 A1 alloy con- taining 14% SiC whiskers which were 1 /~m in diameter and 5 to 10 /~m in length (Fig. l(b)). Two ageing treatments were undertaken in order to examine the influence of precipitates located at the SiC-matrix interfaces on the ductility of the composite. The ageing times were selected to maintain constant matrix strength (as determined from microhardness measurements) as shown in Fig. 2. Transmission electron microscopy (TEM) observations indicated many more intermetallic compounds precipitated at the interfaces in the overaged condition than in the underaged condition. Tensile tests were conducted in a Harwood pressure unit with superimposed pressures up to 1000 MPa. The tensile samples had a gauge length of 15.2 mm and a gauge diameter of 3.2 0921-5093/89/$3,50 © ElsevierSequoia/Printedin The Netherlands