Effect of aging state on low cycle fatigue performance of AA 2099 alloy

Abstract

The evaluation of the fatigue performance is a crucial subject in the research of structural materials utilized in the field of aerospace engineering. In order to examine the influence of aging state on fatigue performance, low cycle fatigue (LCF) experiments were conducted on specimens made from AA2099 alloy in the under-aged (UA), peak-aged (PA), and over-aged (OA) states, with varying strain amplitudes (Δεt/2=0.3 %-0.5 %). In the UA and PA states, the grain boundary precipitates exhibit a small size, thereby prompting the crack to propagate through the grains. Certain grains in this region exhibit a crystallographic feature of crack propagation along the (111) slip plane, which is associated with the formation of the precipitation free bands (T1-PFB) within the grains. In contrast, in the OA state, the size of the grain boundary precipitates increases significantly, accompanied by a minimum precipitation free zones (PFZs) width increase to 75 nm, exacerbating the stress concentration at grain boundaries, with cracks primarily extending along the grain boundaries. Furthermore, the presence of coarse grain boundary precipitates and the widened PFZs promote the initiation and propagation of fatigue cracks, leading to a pronounced reduction in fatigue life.