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

Abstract

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 orthorombic 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 matrix and along the grain boundaries. The lattice parameters of the cubic carbides were determined to vary from 0.458 to 0.465 nm as the Zr/Nb ratio varied from 38/62 to 75/25.

Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb

“…The conclusion from the STEM studies is that NbC may have resulted from residues of sheet processing, electropolishing and annealing, with distinct precipitates appearing in regions of Nb with added tensile strain. Such observations are consistent with studies of NbZrC alloys with low concentrations of Zr (~1%) and C (~0.4%), which show that with annealing only, NbC precipitates form throughout the Nb matrix that are very stable [25]. However, the results here show that the NbC near the surface is coherent with the host Nb.…”

“…Such observations are consistent with studies of NbZrC alloys with low concentrations of Zr (~1%) and C (~0.4%), which show that with annealing only, NbC precipitates form throughout the Nb matrix that are very stable [25]. However, the results here show that the NbC near the surface is coherent with the host Nb.…”

Processing and microstructure of Nb-1% Zr-0.1% C alloy sheet

“…As processing Ijecame more complex or the heat treatment temperature increased, the microstructure showed fewer of the coarser particles and more of (he finer ones, and the composidon analysis showed an increase in Zr/Nb rado of (he precipi(a(es. This is also coiisis(ent wKh the results of the study on a Nb-l%Zr-0.06%C alloy (Uz and Titran 1991) in whicli (he micros(ruc(ure of a sample (ransformed from one wKh predominandy NbjC (o one wKli (Zr,Nb)C precipi(tt(es in the earlier stages of high-temperature exposure with or without applied load as mentioned earlier.…”

“…Based on the results of the earlier study (Uz and Titran 1991) on a Nb-l%Zr-0.06%C alloy which showed excellent creep resistance and microstriictiiral stability at 1350 K, and an examinadon of (he micros(ruc(ure of the Nb-l%Zr-0.1%C alloy in this study, it would appear that the majority of the samples with die heattreated conditions from the alloy containing 0.1%C should have as good, if not be((er, higli-(enipera(Hre stability and creep resistance than those from the lower-carbon alloy even under more severe service conditions.…”

“…The primary candidates to provide such margins are the carbide-strengthened Nb-alloys, in particular, the so-called PWC-11 developed in the 1960s (DclGrosso et al 1967) with a nominal composition of Nb-l%Zr-0.1%C. An earlier study (Uz and Titran 1991) showed that a Nb-l%Zr-0.06%C alloy has excellcii( microsirucdiral s(al)il! (y a( (empera(ures of in(ciest with or without applied load.…”