Heat treatment-microstructure-mechanical property relationships in U-Cr castings

The microstructures of cast near-eutectic uranium-chromium alloys consist of dendrites of proeutectic uranium or chromium, plus eutectic colonies of parallel chromium fibers in a uranium matrix. Provided the post-solidification cooling rates are <10C/s the uranium phase undergoes the equilibrium gamma to beta to alpha transformation sequence on cooling to room temperature. Despite the inherent ductility of alpha-uranium, the parallel eutectic chromium fibers provide a continuous path for easy crack propagation, resulting in low ductility in the as-cast conditions. The eutectic chromium fibers can be spheroidized at 8000C, thus eliminating the continuous path for easy crack growth. This results in a substantial increase in the ductilities of hypoeutectic and eutectic alloys, unless >10C/s cooling from the spheroidizing temperature results in retention of brittle beta-uranium. The much larger proeutectic chromium dendrites are more resistant to spheroidization than the eutectic chromium fibres, and restrict the ductilities which can be obtained in hypereutectic alloys. (orig.)