Influence of Heat Treatment on Near-Threshold Fatigue Crack Growth Behavior of High Strength Aluminum Alloy 7010

In this study, aluminum alloy 7010 was subjected to three different ageing treatments i.e., peak ageing (T6), over ageing (T7451) and retrogression and re-ageing (RRA) to study the influence of precipitate microstructure on the fatigue crack growth rate (FCGR) behavior. The microstructural modifications were studied by using TEM to examine the change in size and morphology of the precipitates. The size of the precipitates in the matrix range from 16–20 nm in T7451, 5–6 nm in RRA and 2–3 nm in T6 alloys, respectively. The FCGR tests were performed on standard compact tension (CT) specimens as per ASTM E647 standard in a computer controlled servo-hydraulic test machine with applied stress ratio, R = 0.1 and loading frequency of 10 Hz. The crack growth was measured by adopting compliance technique using a CMOD gauge attached to the CT specimen. The fatigue crack growth rate was higher in T7451 and lowest in RRA treated alloy. The RRA treated alloy showed higher ∆K_th compared to T7451 and T6 treated alloys. The measured ∆K_th was 11.1, 10.3 and 5.7 MPam^½ in RRA, T6 and T7451 alloys, respectively. In the near-threshold regime, the RRA treated alloy exhibited nearly 2–3 times reduction in the crack growth rate compared to the T6 alloy. The growth rate in the RRA alloy was one order lower than that of the T7451 condition. The surface roughness of RRA treated alloy was more pronounced. The reduction in FCGR observed in RRA alloy was correlated to partial crack closure due to tortuous crack path and partially due to increased spacing between the matrix precipitates. The reduction in near-threshold FCGR and increase in ∆K_th is expected to benefit the damage tolerant capability of the aircraft structural components under service loads.