An fcc solid solution of cobalt in gold was synthesized under high-pressure torsion (shear) in air at room temperature and in liquid nitrogen. The mixture of Au and Co powders of 99.99 and 99.60% purities, respectively, was preliminarily consolidated under a pressure of 1-2 GPa. The obtained compact was subjected to high-pressure torsion at a pressure of 8 GPa under different temperature conditions. It has been shown that synthesis in liquid nitrogen not only enhances the solubility of Co in Au compared to that at room temperature, but also results in virtually no contamination of the alloy with anvil material. The morphology of the fracture surfaces of the samples synthesized at cryogenic temperature has been described. The main part of the fractures are in a region of brittle intergranular fracturing, which corresponds to an ultra-fine grain structure. It has been shown that the fraction of brittle intergranular fracturing and the sizes of regions with another morphology of the fracture surface change as a function of the true strain. The size of elements of the ultra-fine grain structure determined on the surface of fracture is no more than 50 nm.