Fracture toughness of discontinuously reinforced Al-4Cu-1.5Mg/TiB 2 composites
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INTRODUCTION
R E S E A R C H during the past two decades has examined discontinuously reinforced aluminum composites as potential materials for use in structural applications. These composites have superior stiffness and wear characteristics in comparison with conventional aluminum alloys. Compared with continuously reinforced composites, discontinuously reinforced aluminum matrix composites (DRAs) have the advantage of being amenable to conventional metalworking techniques while exhibiting isotropic properties. While many investigations t2'4'8-1~ have examined the tensile behavior of discontinuously reinforced composites, less attention has been paid to fracture properties. Studies have shown that the plane-strain fracture toughness (Klc) of aluminum alloy matrices reinforced with various ceramic reinforcements typically falls into the range of 10 to 40 MPaVm. t2'7,8'1~ Important factors influencing the toughness of DRA materials are reinforcement type, size, volume fraction and distribution, and matrix properties. It is difficult to make generalizations about the effects of these factors on DRA toughness because available information has been gathered on composites with different aluminum alloy matrices and tempers, different reinforcement types, different reinforcement sizes, and materials with different particle distribution. A recent review by Mortensen m has shown that it is inappropriate to compare the dependence of the toughness of DRA materials on a given parameter without holding the others constant. A few studies have been published in which the effects of various parameters on composite toughness are investigated systematically. Stephens et al.~23 compared the fracture toughness of cast A1-7 wt pet Si materials N.C. BECK TAN, Graduate Student, and R.M. BRIBER, Assistant Professor, are with the Department of Materials Engineering, University of Maryland, College Park, MD 20742-2115. R.M. AIKEN, Jr., Associate Professor, is with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 441067204. Manuscript submitted July 20, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
reinforced with either SiC or B4C and found an effect of reinforcement type; the latter material showed superior toughness. Several authors have found an inverse relationship between reinforcement volume fraction and fracture toughness. [3-61The relationship between particle size and fracture toughness is presently not clear. Several authors working on DRAs have reported increased toughness with increasing particle size, t2,4'51while others found toughness to be insensitive to particle size. tTl Indication that composite toughness is adversely affected by a nonuniform reinforcement distribution has been documented; i.e., propagating cracks showed an attraction toward particles in areas of high local volume fraction, and damage was accumulated in these areas, ts-m The effects of matrix properties on the fracture toughness of DRAs have been less widely studied. Two recent studies on 7XXC/Si
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