The goal of this project, a joint effort between the University of Virginia (UVA) and the Technical University Hamburg Harburg (TUHH) and the nearby national laboratory GKSS at Geestacht, Germany, is to develop a new wrought Mg alloy with optimized texture for promoting enhanced formability. Despite many attractive properties, the application of Mg alloy sheet is limited, in part, due to poor low temperature formability. Their forming behavior is strongly affected by crystallographic texture because of the intrinsic plastic anisotropy of the hexagonal close packed crystal structure. Conventional Mg alloy sheets exhibit strong textures (with basal planes parallel to the sheet plane) with little variation between alloys. It has only recently been reported that alloys containing significant additions of rare earth (RE) elements and yttrium (Y) develop much weaker textures during extrusion than conventional alloys, and preliminary results show that randomizing the texture of Mg alloy sheets results in improved formability. The mechanism by which RE/Y additions impart a change in the texture evolution is presently unknown, thus, an appropriate alloy design strategy is also unknown. It has been suggested that particle stimulated nucleation (PSN) of recrystallization (either static or dynamic) is responsible. However, unpublished results show that texture modification is also possible in dilute RE/Y alloys which are expected to have very little second phase. A critical step in the proposed research will be to discriminate between solid solution alloying effects and particle related effects. Computer modeling will be used to focus both the search for i) controlling mechanisms and ii) optimal compositions once the mechanism(s) of texture modification are identified. For example, it is possible that RE/Y solute atoms form nano-scale clusters which interact with dislocations in ways not presently accounted for by theories which assume ideal, random solutions. If the modeling results suggest as much, validation will be sought using diffuse scattering at a synchrotron or other experimental techniques. Regardless, phase stability and calculated phase diagram (CALPHAD) analyses will be used to identify promising compositions. Additionally, there is little experience upon which to recommend optimal textures. Forming limit diagrams (FLDs) provide an assessment of sheet metal response under various forming conditions. The texture dependence of the FLD will be predicted using a polycrystal plasticity model. This approach allows exploration of concepts which are not possible experimentally (e.g., to alter texture independent of grain size, alloy content, etc.) The predictions will be validated using experimental FLDs from TUHH/GKSS.
Students and young scientists will travel to the partner institutions to become involved in relevant aspects of the research, for instance students developing formability models at UVA will benefit strongly from exposure to formability testing facilities and expertise at TUHH/GKSS. The proposed research will serve a broader goal of enabling increased application of lightweight Mg alloys in automobiles, thereby promoting improved fuel efficiency and reduced emissions of harmful greenhouse gases. Further, there are a range of portable electronics goods (cell phones, cameras, laptops, etc.) which would benefit from a readily manufactured material with a density which is typical of polymeric materials and the durability, thermal conductivity and electronic interference shielding typical of a metal. The fundamental concepts explored and models developed, e.g., recrystallization, atomic clustering, and texture effects, have broad implications for many materials systems.
