Torben Boll

Investigation of the site occupation of atoms in pure and doped TiAl/Ti3Al intermetallic

Dual-phase TiAl/Ti3Al alloys consisting of a lamellar structure, comprising gamma-phase plus a small amount of alpha2 -phase, with addition of 1, 5 and 10 at.%Nb or 2 at.%Ag were prepared. The samples were investigated by means of the field ion microscopy (FIM), the tomographic atom probe (TAP) and supporting TEM, HRTEM analysis. The influence of doping elements on the variation of the field evaporation and microstructural parameters in the gamma-phase as studied by FIM and TAP will be reported in this contribution. A new algorithmic approach based on TAP results was developed to evaluate the site occupancies in such ordered structures. In addition, computer modelling and simulation of the field evaporation behaviour of the different species including the next neighbour interaction in a FIM specimen are performed for the first time

An APT investigation of an amorphous Cr–B–C thin film

A magnetron sputtered amorphous Cr-B-C thin film was investigated by means of atom probe tomography (APT). The film is constituted of two phases; a Cr-rich phase present as a few nanometer large regions embedded in a Cr-poor phase (tissue phase). The Cr-rich regions form columnar chains oriented parallel to the growth direction of the film. It was found that the Cr-rich regions have a higher B:C ratio than the Cr-poor regions. The composition of the phases was determined as approximately 35Cr-33B-30C and 15Cr-40B-42C (at%), respectively. The results suggest that this type of nanocomposite films has a more complex structure than previously anticipated, which may have an importance for the mechanical and electrical properties.

Characterization of Precipitation in Al-Li Alloy AA2195 by means of Atom Probe Tomography and Transmission Electron Microscopy

The microstructure of the commercial alloy AA2195 was investigated on the nanoscale after conducting T8 tempering. This particular thermomechanical treatment of the specimen resulted in the formation of platelet-shaped precipitates within the Al matrix. The electrochemically prepared samples were analyzed by scanning transmission electron microscopy and atom probe tomography for chemical mapping. The platelets, which are less than 2 nm thick, have the stoichiometric composition consistent with the expected Al2Cu equilibrium composition. Additionally, the Li distribution inside the platelets was found to equal the same value as in the matrix. The equally thin platelet deviates from the formula (Al2CuLi) in its stoichiometry and shows Mg enrichment inside the platelet without any indication of a higher segregation level at the precipitate/matrix interface. The deviation from the (Al2CuLi) stoichiometry cannot be simply interpreted as a consequence of artifacts when measuring the Cu and Li concentrations inside the platelet. The results show rather a strong hint for a true lower Li and Cu contents, hence supporting reasonably the hypothesis that the real chemical composition for the thin platelet in the T8 tempering condition differs from the equilibrium composition of the thermodynamic stable bulk phase.

Investigation of the site occupation of atoms in pure and doped intermetallic

Dual-phase TiAl/Ti3Al alloys consisting of a lamellar structure, comprising gamma-phase plus a small amount of alpha2 -phase, with addition of 1, 5 and 10 at.%Nb or 2 at.%Ag were prepared. The samples were investigated by means of the field ion microscopy (FIM), the tomographic atom probe (TAP) and supporting TEM, HRTEM analysis. The influence of doping elements on the variation of the field evaporation and microstructural parameters in the gamma-phase as studied by FIM and TAP will be reported in this contribution. A new algorithmic approach based on TAP results was developed to evaluate the site occupancies in such ordered structures. In addition, computer modelling and simulation of the field evaporation behaviour of the different species including the next neighbour interaction in a FIM specimen are performed for the first time

Grain Boundary Chemistry and Transport Through Alumina Scales on NiAl Alloys

It is widely accepted that the growth of protective α-Al2O3 scales on Ni-based alloys is governed by the inward diffusion of oxygen through the oxide grain boundaries (GB). However, there is also some outward diffusion of metal ions to the surface, but it is difficult to quantify. In this work we apply atomic force microscopy, scanning electron microscopy and transmission electron microscopy to investigate the outward flux of Al, which manifests as the growth of small ridges along the alumina GBs after the removal of the outermost oxide layer by mechanical polishing or focused ion beam techniques followed by additional oxidation. As a model alumina-former, NiAl with Hf and Zr additions was investigated. In comparison to Zr, Hf was found to reduce the outward Al diffusion. This outward diffusion was six orders of magnitude smaller than the O inward diffusion.

Atom probe tomography simulations and density functional theory calculations of bonding energies in Cu3Au.

In this article the Cu-Au binding energy in Cu3Au is determined by comparing experimental atom probe tomography (APT) results to simulations. The resulting bonding energy is supported by density functional theory calculations. The APT simulations are based on the Müller-Schottky equation, which is modified to include different atomic neighborhoods and their characteristic bonds. The local environment is considered up to the fifth next nearest neighbors. To compare the experimental with simulated APT data, the AtomVicinity algorithm, which provides statistical information about the positions of the neighboring atoms, is applied. The quality of this information is influenced by the field evaporation behavior of the different species, which is connected to the bonding energies.

Investigation of the Site Occupation of Atoms in Pure and Doped TiA1/Ti3Al Intermetallic

Dual-phase TiAl/Ti3Al alloys consisting of a lamellar structure, comprising gamma-phase plus a small amount of alpha2 -phase, with addition of 1, 5 and 10 at.%Nb or 2 at.%Ag were prepared. The samples were investigated by means of the field ion microscopy (FIM), the tomographic atom probe (TAP) and supporting TEM, HRTEM analysis. The influence of doping elements on the variation of the field evaporation and microstructural parameters in the gamma-phase as studied by FIM and TAP will be reported in this contribution. A new algorithmic approach based on TAP results was developed to evaluate the site occupancies in such ordered structures. In addition, computer modelling and simulation of the field evaporation behaviour of the different species including the next neighbour interaction in a FIM specimen are performed for the first time

L1 2 ordering and δ ′ precipitation in Al-Cu-Li

The precipitation mechanism of the δ′ (Al3Li) phase in Al-Li alloys has been controversially discussed in recent decades, specifically with respect to a conjectured congruent ordering process. However, kinetics in the Al-Li system does not allow to resolve the intermediate stages of precipitation and hence to experimentally clarify this issue. In this paper, we are revisiting the subject in ternary Al-Cu-Li alloys with pronouncedly slower kinetics, employing Transmission Electron Microscopy, High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy, Differential Scanning Calorimetry and Atom Probe Tomography. The results show clear evidence for congruent ordering in a selected compositional range, revealing an already strongly L12 ordered microstructure after natural aging with a chemically homogeneous Li distribution and a decomposition of the alloy upon annealing at elevated temperatures. The presented study of the δ′ precipitation evaluates the reaction pathway of this process and compares it to the predictions of the Bragg-Williams-Gorsky model with respect to decomposition and ordering in this alloy system.

Interfaces in Oxides Formed on NiAlCr Doped with Y, Hf, Ti, and B

This study applies atom probe tomography (APT) to analyze the oxide scales formed on model NiAlCr alloys doped with Hf, Y, Ti, and B. Due to its ability to measure small amounts of alloying elements in the oxide matrix and its ability to quantify segregation, the technique offers a possibility for detailed studies of the dopants fate during high-temperature oxidation. Three model NiAlCr alloys with different additions of Hf, Y, Ti, and B were prepared and oxidized in O2 at 1,100°C for 100 h. All specimens showed an outer region consisting of different spinel oxides with relatively small grains and the protective Al2O3-oxide layer below. APT analyses focused mainly on this protective oxide layer. In all the investigated samples segregation of both Hf and Y to the oxide grain boundaries was observed and quantified. Neither B nor Ti were observed in the alumina grains or at the analyzed interfaces. The processes of formation of oxide scales and segregation of the alloying elements are discussed. The experimental challenges of the oxide analyses by APT are also addressed.

Investigation of the site occupation of atoms in pure and doped TiAl/Ti3AlTiAl/Ti3Al intermetallic

Dual-phase TiAl/Ti3Al alloys consisting of a lamellar structure, comprising gamma-phase plus a small amount of alpha2 -phase, with addition of 1, 5 and 10 at.%Nb or 2 at.%Ag were prepared. The samples were investigated by means of the field ion microscopy (FIM), the tomographic atom probe (TAP) and supporting TEM, HRTEM analysis. The influence of doping elements on the variation of the field evaporation and microstructural parameters in the gamma-phase as studied by FIM and TAP will be reported in this contribution. A new algorithmic approach based on TAP results was developed to evaluate the site occupancies in such ordered structures. In addition, computer modelling and simulation of the field evaporation behaviour of the different species including the next neighbour interaction in a FIM specimen are performed for the first time