Kevin E. Yoon

Nanoscale studies of the chemistry of a René N6 superalloy

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.

The use of 3-D atom-probe tomography to study nickel-based superalloys

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.

Atomic-scale chemical analyses of niobium oxide/niobium interfaces via atom-probe tomography

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.

Atomic-Scale Chemical-Analyses of Niobium for Superconducting Radio-Frequency Cavities

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.