Effect of cooling rates on as-cast microstructures of U-5.4Nb alloys

As-cast microstructural features of a U-5.4Nb alloy, including average grain size, dendrite structure, inclusion evolution behavior, solute (Nb) distribution and micromechanical properties were investigated with respect to a diverse number of calculated cooling rates in this paper. Various solidification techniques, including suction casting, arc remelting and induction melting followed by argon cooling, were employed to obtain wide ranges of cooling rates between 90 K/s and 9.9 × 105 K/s. Average grain sizes of suction cast samples decreased from 16.8 μm to 4.35 μm with increasing cooling rates. Similarly, inclusion evolution behavior was shown to refine in size with the increase of cooling rates. Fully developed dendrites inside the argon-cooled sample at a low cooling rate consisted of primary dendrite arms, secondary dendrite arms and tertiary dendrite arms that exhibited a fascinating dendrite structure with hexagonal symmetry. In addition, the segregation ratios of niobium decreased with decreasing cooling rates. Furthermore, the nanoindentation test indicated that the segregation of niobium had a significant influence on the mechanical properties of the dendrite arms and interdendritic regions. The elastic modulus and hardness for the primary dendrite arm, the secondary dendrite arm, and the interdendritic region were 61 GPa and 3.6 GPa, 62 GPa and 3.3 GPa, and 113 GPa and 6.5 GPa, respectively.