A theory-guided combinatorial materials design of ductile Mg-based alloys utilizing ab initio and atomistic methods

Energy savings and CO2 reductions are among the most essential tasks in materials science and engineering. Any successful attempt to achieve both goals requires the development of novel light-weight metallic materials. New Mg alloys play a key role owing to their combination of relatively high strength and low density. However, their wider use is hindered by their low room temperature ductility. Recent mechanical testing of pure bulk Mg and of a single phase solid solution Mg-3-wt.-%-Y performed at the Max-Planck-Institut für Eisenforschung (MPIE) in Düsseldorf, Germany, showed that the addition of yttrium increases the room temperature ductility about 5 times and lowers theso-called I1 intrinsic stacking fault (I1 SF) energy. In order to obtain a deeper insight into the mechanisms responsible for this increased ductility, the above described experimental studies have been complemented at MPIE by a quantum-mechanical study. Using density functional theory (DFT) calculations, Y additions were found to significantly reduce the I1 SF energy in excellent qualitative agreement with experiments.This thesis contains a thorough analysis of the above mentioned scale-bridging connection between atomic-scale reduction of I1 SF energies and an increased macroscopic ductility in Mg alloys. DFT calculations of I1 SF energies in 20 different binary Mg alloys have been performed but only rare-earth elements have been found to reduce theI1 SF energy and increase the ductility. This prediction was experimentally confirmed at MPIE in case of Mg-Tb, Mg-Dy, Mg-Ho, Mg-Er alloys that all turned out to be ductile. When designing new ductile Mg alloys, rare earth elements do not represent an optimum choice due to their limited natural resources and environmental concerns in their mining and production. In order to identify alternative solutes ductilizing Mg alloys, we have searched for relations between materials properties of elemental solutes and theirimpact on the I1 SF energies in Mg alloys. The atomic volume, the electronegativity and bulk modulus of elemental solutes have been found related to the I1 SF energies and we propose a single numerical indicator based on these three inter-relations. Evaluating this new indicator for 76 binary Mg alloys, we hardly find any alternative to rare-earths. Therefore, we extended our search to ternary alloys and relatively high number of pairs of non-rare-earth solutes have been predicted to reduce the the I1 SF energies. Next, aiming at simulations involving a higher number of atoms, an embedded atom method (EAM) potential for Mg-Y alloys was developed. Both DFT and EAM simulations have shown changes of energy barriers within generalized stacking fault energy surfaces (so-called gamma surfaces) due to Y additions. Importantly, these changes sensitively depend on the dislocation slip plane and direction. Extending our DFT calculations to a broader set of 20 binary Mg alloys, we have evaluated changes in energy barriers and Peierls stresses. In order to reduce the number of input energies for gamma surfaces, a new computational approach was developed and the computed gamma surfaces were used within the Peiers-Nabarro model to determine the dislocation properties. All the above summarized findings have deepened our fundamental understanding of plasticity in Mg materials and have been used to propose ductile rare-earth-free alloys