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To elucidate the relationship between aged microstructure and fatigue crack initiation, tensile fatigue tests and analyses on crack initiation sites were performed on T4- and T6-tempered specimens of an Al-Cu-Mg alloy containing the same amount of Cu and Mg as 2024 alloy but with almost no Mn, Fe or Si. Despite lower statistic strength, T4 specimen had higher fatigue strength than T6 specimen. Crack initiation took place on the surface at slip bands inside the grain and at grain boundaries in T4 and T6 specimens, respectively. The aged microstructure in T4 specimen was presumed to consist of GPB(1) zones with almost no precipitate free zones (PFZs) along grain boundaries, while that in T6 specimen was composed of coarse S phase particles on grain boundaries, S’ phase laths and probably GPB(2) zones inside the grain with PFZs of about 600nm width. Hence, it was deduced that the fatigue cracks occurred by the intrusion and extrusion mechanism in T4 and T6 specimens along persistent slip bands and along PFZs, respectively. The latter was caused by softer PFZs in T6 specimen than the matrix of T4 specimen, resulting in an earlier crack initiation and consequently lower fatigue strength.
Research papers (academic journals)
