Niobium is the metal of choice for superconducting radio-frequency cavities for the future International Linear Collider. We present the results of atomic-scale characterization of the oxidation of niobium utilizing local-electrode atom-probe tomography employing picosecond laser pulsing. Laser pulsing is utilized to prevent a tip from fracturing as a buried niobium oxide/niobium interface is dissected on an atom-by-atom basis. The thickness of niobium oxide is about 15 nm, the root-mean-square chemical roughness is 0.4 nm, and the composition is close to Nb2O5, which is an insulator, with an interstitial oxygen concentration profile in Nb extending to a depth of 12 nm.

The temporal evolution of the nanostructure and chemistry of a model Ni–8.5at.% Cr–10at.% Al alloy with the addition of 2at.% Re was studied using transmission electron microscopy and atom-probe tomography in order to measure the number density and mean radius of the γ′ (L12) precipitates and the chemistry of the γ′-precipitates and the γ (fcc)-matrix. In this article, the coarsening behavior of the γ′-precipitates is discussed in detail and compared with the Umantsev–Olson model for multi-component alloys. In addition, the experimental results are evaluated with PrecipiCalc™ simulations. The results show that the diffusivities of the solute elements play a major role in the coarsening behavior of the γ′-precipitates and that the addition of Re retards the coarsening kinetics and stabilizes the spheroidal morphology of the precipitates by reducing the interfacial energy.

What determines the morphology of a decomposing alloy? Besides the well-established effect of the nucleation barrier, we demonstrate that, in a concentrated multicomponent Ni(Al,Cr) alloy, the details of the diffusion mechanism strongly affect the kinetic pathway of precipitation. Our argument is based on the combined use of atomic-scale observations, using three-dimensional atom-probe tomography (3D APT), lattice kinetic Monte Carlo simulations and the theory of diffusion. By an optimized choice of thermodynamic and kinetic parameters, we first reproduce the 3D APT observations, in particular the early-stage transient occurrence of coagulated precipitates. We then modify the kinetic correlations among the atomic fluxes in the simulation, without altering the thermodynamic driving force for phase separation, by changing the vacancy–solute interactions, resulting in a suppression of coagulation. Such changes can only be quantitatively accounted for with non-zero values for the off-diagonal terms of the Onsager matrix, at variance with classical models.

The key technology for the linear collider is the high gradient superconducting radio-frequency (SRF) cavity, approximately 20,000 of which will make up the accelerator. The preferred technology is to make the cavities from high-purity niobium-sheet. From the RF superconductivity point-of-view, the interface between the native niobium oxide on the surface of the cavity and near sub-surface region is the most important one. Superconducting properties of cavities depend on the chemistry and microstructure of the surface oxide and the concentration and location of impurity elements. Little is known, however, about this information and the effect of low-temperature baking on the surface region. Atom-probe tomography (APT) provides chemical information of the analysed materials on an atomic scale utilizing time-of-flight (TOF) mass spectrometry, with the field evaporation of materials permitting atom-by-atom dissection. We employ a 3-D local-electrode atom-probe (LEAP) tomography to analyse the chemistry of niobium tips, from the surface niobium oxide to underlying bulk niobium.

Recent technological advances in the design and fabrication of atom-probe tomographs and their commercialization are revolutionizing our ability to determine, on a sub-nanometer scale (atomic scale), the chemical identities of atoms in a nanostructure and to reconstruct this information in three dimensions. Thus, it is now possible to obtain data sets containing several hundred million atoms in a few hours, using either electrical or laser (femtosecond or picosecond) pulsing, and to reconstruct crystalline lattices using sophisticated software programs. Detailed quantitative results of the application of atom-probe tomography to study the kinetic pathways for precipitation in model nickel-based superalloys, Ni−Al−Cr and Ni−Al−Cr−Re, are presented as illustrative examples.

In a Ni-5.2 Al-14.2 Cr at.% alloy with moderate solute supersaturations and a very small γ/γ′ lattice parameter misfit, the nanostructural and compositional pathways during γ′(L12) precipitation at 873K are investigated using atom-probe tomography, conventional transmission electron microscopy, and hardness measurements. Nucleation of high number densities (Nv>1023m−3) of solute-rich precipitates (mean radius=〈R〉=0.75nm), with a critical nucleus composition of Ni-18.3±0.9 Al-9.3±0.7 Cr at.%, initiates between 0.0833 and 0.167h. With increasing aging time (a) the solute concentrations decay in spheroidal precipitates (〈R〉<10nm); (b) the observed early-stage coalescence peaks at maximum Nv in coincidence with the smallest interprecipitate spacing; and (c) the reaction enters a quasi-stationary regime where growth and coarsening operate concomitantly. During this quasi-stationary regime, the γ (face-centered cubic)-matrix solute supersaturations decay with a power-law dependence of about −1/3, while the dependencies of 〈R〉 and Nv are 0.29±0.05 and −0.64±0.06 at a coarsening rate slower than model predications. Coarsening models allow both equilibrium phase compositions to be determined from the compositional measurements. The observed early-stage coalescence is discussed in further detail.

The temporal evolution of the nanostructure of a model Ni-base superalloy (Ni-5.2 at.% Al-14.2 at.% Cr) is studied experimentally employing three-dimensional atom-probe (3DAP) microscopy in conjunction with kinetic Monte Carlo (KMC) simulations at 600 degreesC. It is emonstrated that not only can the mean compositions of individual gamma (Ni3Al with the LI2 structure) precipitates be measured but the Ni, Al, and Cr concentration profiles within the precipitates can also be determined for precipitates with a mean radius () as small as 0.85 nm. The three asymptotic time dependencies of the Lifshitz-Slyzov-Wagner (LSW) theory of coarsening (Ostwald ripening) are measured and found to deviate from its theoretical predictions; possible explanations for these discrepancies are discussed. At 0.25 hr. there is 3DAP microscope evidence for the presence of precipitates of another nickel-rich phase, approximate to"Ni3Cr" (Ni3Cr1-xAlx), which exhibits short-range order (SRO) and that is metastable with respect to Ni3Al. This metastable phase is also found by KMC simulations and has the composition Ni3Cr1-xAlx, which is Ni-2.91 at% Al-21.98 at% Cr at 16 hours. Our results demonstrate that the decomposition of the primary gamma (FCC) phase results in the concurrent formation of an ordered phase and a disordered phase by 0.25 hours.

Atom-probe field-ion microscopy (APFIM) is used to study partitioning of the alloying elements between the γ (FCC) and γ′ (L12) phases and their segregation behavior at γ/γ′ interfaces of a René N6 nickel-based superalloy. The atomic-scale resolution and real space reconstruction capability for elemental chemical mapping makes three-dimensional atom-probe microscopy especially suitable for subnanoscale investigations of complex multicomponent superalloys. Concentration profiles of this alloy, obtained from an atom probe analysis, reveal the partitioning behavior of the alloying elements in René N6. As anticipated, the matrix strengtheners, such as Mo and W, are partitioned to the γ (FCC) matrix, while Re segregates at the γ/γ′ interfaces; the Gibbsian interfacial excess of Re is determined by both one-dimensional (2.32 atoms nm−2) and three-dimensional atom-probe microscopies (3.92 atoms nm−2) and the values obtained are in reasonable agreement.