Thermal stability of the microstructure of an aged Nb-Zr-C alloy

The effects of thermally aging with and without an applied stress on the microstructure of a Nb‐Zr‐C alloy containing 0.9 wt % Zr and 0.06 wt % C were studied. Chemical analysis, metallographic examination, energy dispersive x‐ray spectra of the bulk material, and chemical and x‐ray analyses of the phase‐extracted residue were used to characterize the microstructure. The samples examined were from a creep strength study involving hot and cold working, and various combinations of exposure to temperatures ranging from 1350 to 1755 K with and without applied load for times as long as 34 000 plus hours. The results showed that the initial microstructure consisted primarily of orthorhombic precipitates of Nb2C which were partially or completely transformed to face‐centered cubic carbides of Nb and Zr, (Zr, Nb)C, upon prolonged exposure to elevated temperatures. Furthermore, it was found that the microstructure of the alloy is extremely stable owing to the very finely distributed precipitates throughout its mat...