Abstract
An investigation into the effect of indentation on the fatigue behaviour of an aluminium alloy (A1 - 2.5wt% Cu - 1.2wt% Mg) has been carried out. Room-temperature aged (RTA) and high-temperature aged (HTA) specimens were cycled in tension-compression under conditions of constant plastic strain in the range epsilonp = 1.0 x 10 -4 to 1.4x10-3 in a 20 kN Mand servohydrolic machine. The fatigue behaviour was cyclic hardening in the RTA state and cyclic softening in the HTA state. During fatigue, the RTA specimen surfaces were quickly covered with coarse slip lines, which after prolonged cycling developed into PSBs. Unlike the slip lines, the PSBs did not always traverse the entire grain, but started off in patches which steadily grew larger and more intense. Dislocation structures were studied using TEM and were found to develop from a structure consisting of quenched-in loops and helices into slip bands concentrated on groups of {111} planes, often dominated by dislocations of a single Burgers' vector. Further cycling resulted in more concentrated slip in localized bands leading to a misorientation of these bands relative to the matrix. The development of surface relief in HTA specimens was much slower than in the case of the RTA specimens and took place over thousands of cycles. The slip lines were shorter and broader, and in late stages more wavy in appearance, both features typical of cross-slip. The dislocation structure consisted of some loop formation but mostly dislocations tangled with the S precipitates and were densely packed at the precipitate-matrix boundary. There was no evidence for the break-up of S precipitates during cyclic deformation. Surface damage was introduced in the form of Vickers micro-hardness indentations. The indentations were made in the middle of the grains in order to obtain a single-crystal response. Optical microscope studies of the surface were carried out using Nomarski contrast and scanning laser microscopy, and the dislocation configurations were studied by TEM. Analytical models were used to see whether a correlation could be found between the observations of slip and calculations of resolved shear stresses. A reasonable correlation was found, which prompted the extension of the modelling to fatigue slip near indentations. The development of fatigue damage was studied both before and after various stages of fatigue, and it was found that the presence of indentations had a significant effect on the fatigue behaviour of the indented grain. During the initial fatigue cycles, slip lines were observed near indentations in the RTA specimens, and modelling indicated that the effect could be due to the residual stresses surrounding the indentation. In all cases the slip appeared in trace directions of planes containing slip systems with high Schmidt factors, but in many cases on planes not containing the active slip system in fatigue. In the HTA specimens a similar effect was noted at the stage when surface slip started to appear, but only associated with slip on the slip plane containing the active slip system. Cracking was initiated near most indentations, but none were found to lead to the ultimate failure of the specimens. This could be mainly due to stress concentrations at the specimen shoulders. Cracks initiated along the ridge of the indentations oriented approximately perpendicular to the stress axis and appeared at the corresponding corners of the indentations approximately half-way into the fatigue life. In the case of the RTA specimens, the cracks propagated along traces of {111} slip planes, whereas in the HTA specimens the cracks were more irregular and sometimes followed the traces of {100} planes. Modelling suggested that the preferred path for crack propagation was associated with a concentration of shear stress on highly stressed slip planes. The cracks were never observed to stop propagating, and some cracks propagated into neighbouring grains, and it is thought that the condition for damage to lead to failure is the size of the indentation in combination with the magnitude of the applied stress/strain.