Atomic scale investigations of fracture in metal alloys

The demand for metal alloys with superior structural performance in industrial applications continues to increase, as does the need for more-energy efficient materials and methods. Many materials that are otherwise attractive for structural applications are limited by poor fracture properties at low and moderate temperatures, and these properties limit their formability and suitability for fracture-critical/energy-absorption applications. Ductility and fracture toughness are connected to intrinsic ductility and the competition between brittle cleavage and ductile dislocation emission mechanisms at an atomistic crack tip in crystalline metals. The competing crack tip mechanisms are studied here using atomistic simulations, but fracture of complex alloys is an especially challenging simulation environment for interatomic potentials. Pure magnesium (Mg) is an attractive metal for structural applications due to its low density, but also has low ductility and low fracture toughness. Dilute alloying of Mg with 3at.% yttrium (Y) improves the overall ductility of a crack as the overall fracture toughness is improved due to local solute-induced deformation phenomena at the crack tip. Local fluctuations of the solutes enable ductile rather than brittle behavior in crack orientations where difference in the critical load for cleavage and emission are small. Dilute alloying is unable to fundamentally change the brittle nature of the base Mg so basal-plane cleavage remains strongly preferred. Multicomponent, single-phase, polycrystalline High Entropy Alloys (HEAs) have recently emerged as a new class of metal alloys, and some refractory bcc HEAs show excellent strength retention up to very high temperatures but low ductility at room temperature (RT). A RT ductility criterion is proposed based on the elemental metals and is then applied to HEAs. Agreement with experimental trends in ductility vs. composition across a range of existing HEAs is demonstrated. The analysis is then extended across large composition spaces of the Mo-Nb-Ta-V-W and Mo-Nb-Ti alloy families, identifying new compositions with the potential for RT ductility. The cleavage and dislocation emission crack tip mechanisms are thermally-activated processes and are each associated with an energy barrier connected to the critical stress intensity. Ductilization at RT is possible only if the intrinsic cleavage and emission energy barriers are comparable, but many bcc HEAs have emission barriers that are very large and insurmountable on average compared to the cleavage barriers. Significantly reduced local emission energy in a random system is demonstrated with a model equiatomic MoNbTi random alloy, signifying the potential for local crossover from brittle to ductile behavior even if a material is brittle on average. The atomic misfit volume is a critical energetic contribution with the potential to introduce large variations in the emission energy. Ongoing work involves quantifying the connection between the stochastic compositional disorder with variations in the energy contributions for emission, which is necessary to develop an analytic theory. In the meantime, large average misfit volume is a potential supplementary criterion to improve broad screening for ductility in HEAs.