Inherent strength of grain boundaries in tungsten

Grain boundary engineering represents a prospective tool for producing high performance structural and functional materials, including super-strong nanocrystalline materials. There remains a fundamental challenge to experimentally evaluate the inherent strength of grain boundaries providing a higher limit for the strength of polycrystals. Here we report that using a combination of atomic-scale and crystallographic characterizations and in situ hydrostatic tests in a field-ion microscope, the inherent strength of grain boundaries can be directly measured. It was found that mechanically annealed dislocation-free tungsten bicrystals having nanometer-sized dimensions are capable of withstanding extreme stresses close to the values of the theoretical strength of monocrystals. This finding provides a guideline for understanding the fundamental mechanical response of nanocrystals relevant to applications.