A study has been made of the role of aging treatment in influencing fatigue crack propagation and crack closure behavior in a high purity ingot metallurgy aluminum alloy 7150, with specific reference to crack growth at low and high load ratios in the near-threshold regime. A trend of increasing growth rates and decreasing threshold stress intensity ΔK
th values with increased aging was seen to be consistent with lower measured levels of crack closure and a decreasing tortuosity in crack path. On the basis of crack growth measurements in moist room air, where closure due to corrosion product formation was found to be negligible in this alloy, the superior fatigue resistance of underaged microstructured (compared with overaged structures of similar strength and peak-aged structures of higher strength) was attributed to greater slip reversibility and to enhanced roughness-induced crack closure and deflection from the more tortuous crack paths. Such factors are promoted in alloy systems hardened by coherent shearable precipitates where the mode of deformation is one of non-homogeneous planar slip.
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