Origin of acoustic emission in aged AlZnMg alloys I: The base ternary alloy

Abstract

The origin of acoustic emission during the deformation and fracture of Al-5.5wt.%Zn-2.5wt.%Mg has been studied as a function of aging. In quenched material, dislocation-solute interactions result in the generation of many weak signals both at yield and during dynamic strain aging. Aging at either 20 or 120 °C was found to suppress the dislocation-solute source but to cause very energetic signals to be emitted when precipitate shearing (resulting in coarse slip bands) occurs. This emission source is strongest in the peak-aged condition. Overaging at 180 °C, to produce unshearable precipitates, results in the disappearance of the energetic elastic waves and the reappearance of weak signals most probably generated by cooperative slip between precipitates. The results are interpreted in terms of a model for acoustic emission, which predicts that the strength of the acoustic emission generated by dislocation propagation is proportional to the product of dislocation velocity and propagation distance. Intergranular fracture in peak-aged material is likely to have generated energetic emission, but this could not be distinguished from emission due to deformation. In a companion paper the effect of microstructural changes as a result of alloying additions on the acoustic emission is discussed.