Synthesis and Characterization of Nonbanded U-Nb Plate Material

摘要

This report describes the synthesis and characterization of four plates, two each of U-5.5Nb and U-7.5Nb (nominal wt%) for aging studies described elsewhere. The plates were induction melted and cast into graphite molds that were unheated and approx. 0.5 inches thick to maximize the cooling rate and minimize large length-scale Nb segregation (banding). Microstructural images and electron microprobe traces observed after various processing stages, including casting, hot rolling, and homogenizing are documented. The as-cast microsegregation assumed the form of an isotropic cellular structure, with an amplitude of 315 wt% Nb and 4050 micron-length scales. Subsequent thermomechanical processing was shown to be sufficient to attain Nb compositional homogeneity on local scales of hundreds of microns. The results of chemical analysis and other characterization methods are given. The principal impurity elements (of the 40+ elements measured) were carbon, boron, oxygen, tantalum, and iron. In all four plates, after homogenization, the Nb distribution across the entire plate cross-section showed minima at the plate faces and a broad maximum in the center, the differential being 0.50.7 wt% in U-7.5Nb and 0.20.5 wt% in U-5.5Nb. None of the impurity elements showed statistically significant variations between the center 50% of the plate volume vs the outer 25%. These plates were considered nonbanded and compositionally homogeneous for their proposed use because the required tensile, metallographic, and dilatometer specimens could be extracted from the fairly homogeneous center portion of the plate cross-section. Characterization of the phases and their transition temperatures by x-ray diffraction and dilatometry in rapidly quenched specimens from the final product confirmed that the microstructure of this plate material was suitable for the intended aging studies. The as-quenched tensile response from multiple specimens taken from each plate showed some variability, especially in the ultimate tensile strength and elongation to failure.