Microstructure property relationships and hydrogen effects in a particulate-reinforced aluminum composite
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I.
INTRODUCTION
IT is well known
that metal matrix composites can offer significant improvements in certain physical and mechanical properties over their monolithic metallic counterparts. Typically, additions of moderate amounts of ceramic reinforcement to a metal can lead to improvements in specific modulus, specific strength, wear resistance, thermal expansion properties, and fatigue behavior, t1-41 These improvements can be optimized for specific applications by careful selection of the matrix and reinforcement. However, it has been found that the improvements are generally attained at the expense of fracture-related properties, e.g., tensile ductility and fracture toughness, t4-81 An examination of the literature shows that the failure mechanisms operating in composite materials are highly system specific and can be dominated by both matrix and reinforcement characteristics as well as by composite processing parameters. In this article, the strengthening and fracture behavior of a P / M 2124-20 pct SiCp composite was examined and related to the microstructure. The susceptibility of the composite to hydrogen embrittlement was also investigated through the use of straining electrode tests which involved simultaneous straining and cathodic hydrogen charging. These tests represented particularly severe conditions which, in materials with adherent protective oxide coatings like aluminum, c a n be useful in determining inherent susceptibility to embrittlement. The susceptibility of the composite was compared with that reported for monolithic 2124 aluminum tested under similar conditions, where significant effects of aging on this susceptibility have been demonstrated, t9j
C.P. YOU, formerly Postdoctoral Associate, Carnegie Mellon University, is Graduate Student with the School of Business, Stanford University, Palo Alto, CA. M. DOLLAR, formerly with Carnegie Mellon University, is Professor, Department of Metallurgical and Materials Engineering, Illinois Institute of Technology. A.W. THOMPSON, Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. I.M. BERNSTEIN, formerly with Carnegie Mellon University, is Chancellor, Illinois Institute of Technology, Chicago, IL 60610. Manuscript submitted May 31, 1990. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL DETAILS
The material studied was a 2124-20 pct SiCp composite supplied in the as-extruded condition by DWA Composite Specialties, Chatsworth, CA. The composition of the matrix alloy is shown in Table I. The composite was processed via a powder metallurgy route, involving blending of aluminum alloy powder and SiC reinforcement, compaction of the resulting blend, degasification, and elevated temperature consolidation and extrusion. The consolidation temperature used was in the supersolidus range. Typical microstructures of the material are shown in Figure 1. The backscattered image of the microstructure (Figure l(b)) clearly shows the presence of a secondary particle distribution in the
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