How grain boundary chemistry controls the fracture mode of molybdenum. (15th March 2018) - Leitner, K.
Abstract: The design of metallic alloys with superior properties requires a deep understanding of the atomistic processes governing the materials behavior. In this paper, we present results providing a close link between grain boundary (GB) chemistry and fracture behavior in a Mo-Hf alloy. By combining atom probe tomography and ab-initio simulations of GBs, we unravel the origin for the transition between intergranular and transgranular fracture in technological Mo-Hf alloys. The main agent affecting GB strength is not the primary alloying element Hf, but rather the impurities O, C and B. With larger Hf additions, an intricate interplay between segregation and precipitation leads to a strong GB enrichment of C and B and to a depletion of O and Hf resulting in a higher cohesion of the GBs and thus, leading to a change in fracture mode. Our investigation exemplarily demonstrates that smallest additions of solutes can be decisive for understanding fracture behavior on the macroscale. Graphical abstract: Unlabelled Image Highlights: For Mo-Hf alloys, a transition from intergranular to transgranular fracture is observed above 2 at.% Hf. Atom probe tomography reveals high fraction of B and C at grain boundaries in the alloys with transgranular fracture. Atomistic and thermokinetic modeling explains grain boundary chemistry as an interplay of segregation and Hf-precipitation. Segregated B and C with low O contents at grain boundaries increase cohesion and promote transgranular fracture in Mo.