Effect of silicon particles on the fatigue crack growth characteristics of Al-12 Wt Pct Si-0.35 Wt Pct Mg-(0 to 0.02) Wt
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INTRODUCTION
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considerable amount of work has been reported on the mechanical, tensile, fatigue, and plastic deformation behavior of aluminum alloys. Among some of the works on fatigue behavior are those of McEvilyt~j and Beevers ~2j on the factors affecting fatigue mechanisms; of Mclntyret3j and Gerberich and Moodytnj on the fatigue fracture modes/crack propagation; and of Davidson~51 on the fracture behavior of aluminum-reinforced alloys. In the particular case of A1-Si alloys, relatively little is known, however, about the effects of silicon particles and the dendrite ann spacing (DAS) on the fatigue crack propagation behavior. Gurland and Plateau t6j and Inguantit7J have investigated fracture and crack nucleation in cast A357 alloy, while Hoskin e t al. tSj have studied the effect of the two-phase microstructure of as-cast A1-Si alloys and have shown that a relationship exists between the silicon phase in the aluminum matrix and the propagating fatigue crack. Couper e t al. t91 have investigated the effect of casting defects on the fatigue properties in CP601 (A356.2) aluminum alloy and shown their adverse influence upon the latter. Changes in the mode of crack extension are normally explained in terms of the plastic zone size v s the size or interparticle distance of the silicon particles, t~~ Various techniques have been employed to measure the plastic zone size, including electron microscopy,t~2,~3,~4] X-ray microbeam analysis, 1~5~ image distortion/ interferometry,I~6J the Moir6 technique, 1~71 and etching methods, t~8,~91The elastic stress-strain field near the crack tip and the stress components of silicon particles in the vicinity of the crack tip determine the critical stress for F.T. LEE, formerly Research Associate, D6partement des Sciences Appliqu6es, Universit6 du Qu6bec ~ Chicoutimi, is Executive, NatSteel Limited, Singapore 2262. J.F. MAJOR, Research Scientist, is with Kingston R & D Centre, Alcan International Limited, Kingston, ON, Canada K7L 5L9. F.H. SAMUEL, Professor, is with the D6partement des Sciences Appliqu6es, Universit6 du Qu6bec /l Chicoutimi, Chicoutimi, PQ, Canada G7H 2BI. Manuscript submitted June 15, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
Si particle cracking and consequently whether the Si particles will undergo cracking or decohesion from the aluminum matrix. Details of the theory and derivations involved are described elsewhere, t2~ The work reported in the present article covers a detailed, systematic study of the fatigue crack growth (FCG) characteristics and mechanisms in AI-Si-Mg and A356 casting alloys and the mechanistic role of the silicon particles in influencing the fracture paths through the microstructure during fatigue crack propagation, through the testing of compact tension (CT) specimens exhibiting different microstructures (modified v s unmodified) at different stress ratios and stress-intensity-factor ranges. Optical and scanning electron microscopy have been used to analyze the crack propagation path and fracture surfaces, respectively. Such
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