Effects of Processing and Prolonged High Temperature Exposure on the Microstructure of Nb-1Zr-C Sheet

Abstract

AbstractHigh temperature stability of the microstructure of Nb-lZr sheet containing 0.1 and 0.06 wt.%C was studied as affected by processing and prolonged 1350-K exposure with and without applied stress. Sheets were fabricated by cold rolling bars that were single-, double- or triple-extruded at 1900 K. Creep samples were double-annealed (1 h @ 1755 K + 2 h @ 1475 K) prior to testing at 1350 K for 10,000 - 34,500 h. The microstructures of the as-cast, extruded, rolled, DA and crept samples were characterized using various metallographic and analytical methods. The precipitates were rather coarse Nb2C initially, but transformed to finer (≤1 µm) carbides of (Zr,Nb)C with each subsequent high temperature process. The grain size, and the relative amount and morphology of (Zr,Nb)C were found to be affected by the number of extrusions and to some extent by C-content. However, the microstructures of all the crept samples were similar with (Zr,Nb)C distributed throughout the matrix indicating that prolonged exposure to 1350 K gave rise to complete transformation of Nb2 C to (Zr,Nb)C regardless of the processing history. These and other observations are presented with the emphasis on the correlation between processing, microstructure and creep properties.