This article examines the relationship between plane strain fracture toughness, KIc, the tensile properties, and the microstructure of the overaged 7449 aluminum-plate alloy, and compares them to the 7150 alloy. The 7449 alloy has a higher content of ??/? precipitates; and, the 7150 alloy contains a greater amount of coarse intermetallic particles, as it contains an appreciable amount of coarse S phase (Al2CuMg), which is largely absent in the 7449 alloy. The toughness of the alloys shows an increase on overaging, and the 7449 alloy shows a reasonably linear toughness—yield strength relation on extended overaging. Several mechanisms of failure occur: coarse voiding at intermetallics and a combined intergranular/transgranular shear fracture mode, with the former becoming more important as overaging progresses. Drawbacks of existing models for toughness are discussed, and a new model for plane strain fracture toughness, based on the microstructurally dependent work-hardening factor, KA, introduced in Ashby's theory of work hardening, is developed. This model predicts a linear relation between KIc and K0.85A/?0.35ys, where ?ys is the yield strength, which is consistent with the experimental data
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