Stacking faults in sic particles and their effect on the fracture behavior of a 15 Vol Pct SiC/6061-AI matrix composite
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I.
INTRODUCTION
CERAMIC reinforcements, when incorporated into aluminum matrices, can introduce flaws into the matrix and cause premature failure of the compositeYm These flaws can manifest themselves in two ways: the first one is in the form of interracial debonding between the particles and the matrix, and the other is in the form of particle fracture. The occurrence of interracial particle/matrix cracking depends on boundary impurities, precipitates, and the wettability of the particles by the matrix. [~7} For aluminum matrices reinforced with SiC particles, particle fracture is of primary concernV.4.s] and can affect the fracture behavior of the composites. Fracture of reinforcing particles can occur either during processing or during subsequent deformation,r6] Crack initiation and propagation within the particles can occur either along grain boundaries inside large particles or transversely through single crystal particles. The latter phenomenon is more frequently observed because most reinforcing particles used are single crystals,tz,4m It is therefore of importance to understand and characterize the causes of crack initiation and propagation within particle reinforcements and to investigate the correlation between external cracks and inherent defects existing within the SiC particles themselves. Although extensive effort has gone into the study of SiC structures,t~~ less work has been conducted on the phase and defect structures of the particles used as reinforcements for metal matrix composites. Arsenault and Fisher observed a high density of growth faults in a-SiC particles in a SiCparticulate-reinforced Al-matrix composite, tx~ The detailed nature of the faults and their effects on composite properties, however, were not elaborated in their study. Fracture of SiC particles has been widely observed and reported in many cases and, as such, is believed to be the primary cause of failure initiation in such composite materials. In this work, we examine the nature of the stacking faults
that exist in the SiC reinforcing phase of an aluminum 6061 matrix composite. The correlation between the structural defects and particle fracture of the SiC reinforcement with the overall mechanical properties of the composite is reported. II.
EXPERIMENTAL
A SiC-particle-reinforced composite was obtained in the form of spray cast extruded rods, 1 cm in diameter. The aluminum alloy used as the matrix in the composites was commercial grade 6061-A1 (AI + 1.0 pct Mg + 0.6 pct Si + 0.28 pct Cu). The nominal volume fraction of the reinforcing SiC particles was 15 pct, while the average particle size of the SiC particles was determined by image analysis to be 15/zm. All of the composite samples were annealed at 450 ~ for 12 hours in air in a furnace prior to testing. The annealing treatment was primarily carried out to eliminate the effects of prior cold working and to leave the matrix in a low strength condition. Thus, the effect of the SiC particles on the composite strength could be characterized with minimal contributions fro
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