Micromechanics of room and high temperature fracture in 6xxx Al alloys

Abstract

The micromechanics of ductile fracture has made enormous progress in recent years. This approach, which was mostly developed in the context of structural integrity analysis, is becoming a key tool for materials scientists to optimize materials fracture properties and forming operations. Micromechanical models allow quantitatively linking fracture properties, microstructure features at multiple lengths scales, and manufacturing conditions. After briefly reviewing the state of the art, this paper illustrates the application of the micromechanics-based methodology by presenting the results of an investigation on the damage resistance of 6xxx Al produced by extrusion. The presence of coarse, elongated, particles is the key microstructural feature affecting the fracture behaviour of 6xxx Al. The detrimental elongated β-type particles are transformed into rounded α-type particles by heat treatment. In situ tensile tests revealed that, at ambient temperature, the α particles and the β particles oriented with the long axis perpendicular to the main loading direction undergo interface decohesion, while the β particles oriented perpendicular to the loading direction break into several fragments. At high temperatures, only interface decohesion is observed. Uniaxial tensile tests on notched and smooth round bars were performed on two different alloys, at different temperatures ranging between 20 °C and 600 °C, under different loading rates, while systematically varying the content in β versus α particles. The ductility increases with decreasing amount of β particles, increasing temperature and strain rates, and decreasing stress triaxiality. A viscoplastic extension of the Gurson model has been developed for capturing the complex hierarchy of damage mechanisms, coupled with viscoplastic and stress state effects. Three populations of voids are modelled while accounting for the different void nucleation mechanisms leading to different initial void aspect ratio. Proper modelling of the initial void aspect ratio and of its evolution with void growth was the key to predict the effect of the β → α conversion on ductility. The void coalescence criterion takes into account the presence of secondary voids resulting from particle fragmentation. The characteristics of particles entering the model were all measured experimentally. The temperature and rate dependent flow properties of the matrix material have been obtained by inverse modelling. The only fitting parameters are the critical stresses for void nucleation. The model is validated by comparing the predictions to the experimental data involving different relative proportion of α and β particles, temperature, loading rate and stress triaxiality. This type of model opens the path for an “alloy by design” strategy which relates end-use properties to upstream manufacturing operations.