On the Effects of Texture and Strain-Path Changes on the Ductile Fracture of Mg Alloys

Abstract

Increasing need for weight reduction to improve fuel efficiency and reduce emissions makes Magnesium alloys ideal candidates for structural applications, notably in transportation. However, limited formability at room temperature along with catastrophic failure after limited necking are among the drawbacks that limit their application. Plastic anisotropy is often invoked to rationalize low formability of strongly anisotropic materials. However, analysis based on homogenization theory suggests that certain forms of plastic anisotropy may hinder void growth under any triaxial stress state or loading orientation.

Here, two textures produced through Equi-Channel Angular Extrusion (ECAE), a severe plastic deformation technique were investigated and compared with the well-known rolling texture. Deformation anisotropy and damage accumulation were investigated at room temperature. A suite of analytical measurements and observations were carried out to characterize the microstructure in the as-rolled, post-processing and post-deformation states under multiaxial stress states. Their connection to macroscopic fracture strains and fracture mode (normal versus shear) was elucidated using postmortem fractography and microscopic analysis. A simple model was proposed to rationalize the trends.

The major findings suggest that anisotropy can be altered to aid ductility. The trends were well captured by the model and can help guide new processing routes. Any progress in fundamental understanding of limiting factors affecting ductility of Mg alloys must translate into tangible metrics in Mg sheets. For this reason, part of this work was aimed at addressing aspects of anisotropy and failure in Mg sheets through some punch stretching tests. In sheet metal forming, it is customary to use forming limit diagrams (FLDs) to determine formability limits. An important limitation of FLDs is that they are strictly valid under proportional loading conditions. However, metal forming operations involve various non-proportional loading paths. In addition, it is not clear whether the formability of Mg sheets is determined by plastic instabilities such as necking or by fracture. For these reasons, understanding the effect of strain path changes on the ductility, flow and ductile fracture of these alloys is important. In particular, a thorough experimental study of Mg sheet fracture under combined tension and shear was conducted during a five-month visit to Texas A& M Qatar in Doha.

Digital image correlation was extensively used to obtain whole field strain maps. An extensive set of finite element analyses were subsequently carried out to extract key information from the experiments regarding stress distributions and stress state indicators. Using this combined experimental/computational methodology, the fracture loci of the sheet were obtained according to various definitions. The designed program enables a comparison to be made between the fracture loci with and without load path change.