Orientation-dependent ductile-to-brittle transitions in nanostructured materials

Strengths of materials close to the upper theoretical limit have been observed in a number of recent experimental studies. The dimensions of the samples have been on the micron length scale and the strength is conferred by the absence of flaws. Below a critical length scale crystalline materials can sustain stresses near the theoretical strength despite being imperfect. We develop the concept of a ductile-to-brittle transition in orientation space based on the intersection of two surfaces in orientation space: a theoretical tensile strength (TS) and a projected shear strength (SS) surface. The intersection of the two surfaces generates isolines which separate ductile from brittle regions in orientation space. The concept is first demonstrated using the simplest possible analytical potential models and further advanced using the embedded atom method for a range of body-centered cubic and face-centered cubic materials. The results define an orientation-dependent critical length scale below which material strength is determined by the theoretical strength and is independent of the presence of flaws.