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Vol.34, No.9, 1163 ~ 1171, 1996
Title
Research Paper - Mechanical Behavior - : Effect of Microstructure on the Fatigue Crack Propagation of Al - Li 8090 Alloy at Room and Cryogenic Temperatures
박기종Ki Jong Park, 이종수Chong Soo Lee
Abstract
A study has been made to investigate microstructural influence on the fatigue crack propagation(FCP) of an Al-Li 8090 alloy in relation to intrinsic and extrinsic nature at room (300K) and cryogenic temperatures (77K). By applying different heat treatment paths, two types of microstructures were produced : one is the δ`(Al₃Li) phase dominant microstructure exhibiting planar slip characteristics and the other is the S`-δ` microstructure showing homogeneous slip behavior. Crack closure loads measured by the crack-opening displacement method have been used to obtain intrinsic fatigue resistance. At cryogenic temperature of 77K. crack growth rate curves of both microstructures shifted toward higher ΔK region as compared to those at room temperature. The morphology of fatigue fracture surface was significantly changed with decreasing temperature in two microstructures. In a microstructural aspect, such variation of fracture surface was attributed to the transition of slip behavior with decreasing temperature. In the δ` microstructure, planar deformation mode inducing faceted cracks at room temperature changed into rather homogeneous deformation mode at cryogenic temperature. Whereas in the S`+δ` microstructure, intensity of planar slip mode increased at cryogenic temperature resulting in higher degree of surface roughness comparable to that of δ` microstructure.
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