Damage mechanisms in a cast ductile iron and a Al 2 O 3p /Al composite
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INTRODUCTION
Particulate-reinforced metal matrix composites (MMCs) have received renewed interest recently, due to the difficulties in developing continuous fiber-reinforced MMCs. The prime problems associated with manufacturing continuous fiber-reinforced MMCs are high cost, anisotropic properties, and complex fabrication methods.[1–9] Particulate-reinforced MMCs, especially Al matrix composites, have low density, high specific stiffness and wear resistance, isotropic properties, reduced coefficients of thermal expansion, and increased fatigue resistance. Furthermore, low-cost, high-volume production methods are available now, including powder metallurgy, stir casting, pressure or pressureless infiltration, spray deposition, etc.[10,11,12] Of the various processing routines, pressure infiltration is a promising process in which the molten aluminum is injected into the interstices left between particulates packed in a selfsupporting body (called a preform). This process makes the economical, large-scale, net- or near-net-shape production of Al-matrix composites possible. Recently, pressure infiltration was used to fabricate alumina particulate (Al2O3p)-reinforced Al matrix composites,[13,14] and a program is in progress to demonstrate the suitability of this material for use in automobile parts with the goal of reducing the weight and increasing the vehicle performance. Specifically, a pressure infiltration–processed Al2O3p-reinforced Al matrix composite is under consideration as a candidate material for a brake caliper, in replacement of the currently used cast ductile iron. So far, only limited work has been done on the microstructures and fracture toughness behavior of cast ductile J.H. ZHU, Graduate Student, and P.K. LIAW, Professor and Ivan Racheff Chair of Excellence, are with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 379962200. J.M. CORUM, Corporate Fellow, and J.G.R. HANSEN, Project Manager, are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-8051. J.A. CORNIE, President, is with Metal Matrix Cast Composite, Inc., Waltham, MA 02154. Manuscript submitted October 14, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
iron.[15,16] A comparison of the basic mechanical properties between the ductile iron and Al2O3p/Al composite and elucidation of the underlying damage mechanisms are needed to determine the feasibility of replacing the ductile iron with the Al2O3p/Al composite. In this article, four-point bending tests of un-notched and single edge-notched beam specimens were conducted to determine the Young’s modulus, modulus of rupture (MOR), and fracture toughness of both the ductile iron and Al2O3p/Al composite. Microstructural characterizations and damage assessments were undertaken using optical microscopy (OM) and scanning electron microscopy (SEM) to gain an understanding of the damage mechanisms in these two materials. In particular, great efforts were devoted to developing and studying the micrographs of matching fracture surfaces.[17]
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