Mechanical behavior of Al-Li/SiC composites: Part II. Cyclic deformation
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I.
INTRODUCTION
THE Al-Li alloys are excellent candidates for aerospace applications, owing to their elevated specific stiffness and strength. As maintenance and repair strategies in the aerospace industry are often dictated by mechanical performance under cyclic loads, the deformation and fracture mechanisms of Al-Li alloys under fatigue loading have been subjected to careful analyses.[1–8] These investigations indicated that the addition of Li is often detrimental from the point of view of fatigue crack initiation and early growth, although it increases the resistance to fatigue crack propagation. These effects were caused by the presence of the ordered d' (Al3Li) phase, whose resistance to dislocation motion decreases as it is sheared by dislocations, promoting planar slip and strain localization in slip bands. This results, successively, in an increased dislocation density in the glide plane, large slip offsets on the surface or stress concentrations at the grain boundaries, and early fatigue crack initiation.[1,2,3] Once a fatigue crack is nucleated, it propagates along the intense slip band at growth rates which were found to exceed those of long cracks by one to three orders of magnitude at the same nominal stress intensity–factor amplitude.[4] This fast growth is due to the particular stress distribution accompanying crack propagation along the slip band, which favors crack advance within the band,[5] and to the absence of any shielding mechanisms (i.e., crack closure) which would delay the crack propagation along one crystallographic plane within a grain.[4] The situation changes as the crack propagates into the bulk through grains with different crystallographic orientations. The d' precipitates encourage crystallographic crack advance during cyclic fracture, which gives rise to a faceted crack path P. POZA, Assistant Professor, and J. LLORCA, Professor, are with the Department of Materials Science, Polytechnic University of Madrid, 28040 Madrid, Spain. Manuscript submitted February 5, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
and improves the fatigue crack growth resistance through the shielding effects of fatigue crack closure and crack deflection.[4,6,7] These processes control the fatigue life of smooth specimens. Plastic deformation is very limited in the high-cycle fatigue (HCF) regime, where the stresses are predominantly elastic, and the adverse effects of slip localization on crack nucleation are minimized. Thus, the HCF performance of Al-Li alloys is excellent, owing to their resistance to fatigue crack propagation. On the contrary, the cyclic plastic strains in the low-cycle fatigue (LCF) regime lead to premature crack initiation, and the LCF resistance of binary Al-Li alloys is markedly inferior to that of pure Al. These pernicious effects increase with the volume fraction of d' precipitates and the plastic strain amplitude, although they are ameliorated by the presence of incoherent precipitates or other microstructural factors which promote slip homogeneity. As a result, the LCF re
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