Christopher Booth-Morrison

Chromium and tantalum site substitution patterns in Ni3Al(L12) γ′-precipitates

The site substitution behavior of Cr and Ta in the Ni3Al(L12)-type γ′-precipitates of a Ni–Al–Cr–Ta alloy is investigated by atom-probe tomography (APT) and first-principles calculations. Measurements of the γ′-phase composition by APT suggest that Al, Cr, and Ta share the Al sublattice sites of the γ′-precipitates. The calculated substitutional energies of the solute atoms at the Ni and Al sublattice sites indicate that Ta has a strong preference for the Al sites, while Cr has a weak Al site preference. Furthermore, Ta is shown to replace Cr at the Al sublattice sites of the γ′-precipitates, altering the elemental phase partitioning behavior of the Ni–Al–Cr–Ta alloy.

The partitioning and site preference of rhenium or ruthenium in model nickel-based superalloys: An atom-probe tomographic and first-principles study

The partitioning behavior and sublattice site preference of Re or Ru in the Ni3Al (L12) γ′- precipitates of model Ni–Al–Cr alloys are investigated by atom-probe tomography (APT) and first-principles calculations. Rhenium and Ru are experimentally observed to partition to the γ(fcc)-phase, which is consistent with the smaller values of the γ-matrix Re and Ru substitutional formation energies determined by first-principles calculations. APT measurements of the γ′-precipitate composition indicate that Re and Ru occupy the Al sublattice sites of the Ni3Al (L12) phase. The preferential site substitution of Re and Ru at Al sublattice sites is confirmed by first-principles calculations.

On the interplay between tungsten and tantalum atoms in Ni-based superalloys: An atom-probe tomographic and first-principles study

The partitioning behavior of W in a multicomponent Ni-based superalloy and in a ternary Ni–Al–W alloy is investigated using atom-probe tomography (APT) and first-principles calculations. APT observations indicate that whereas W partitions preferentially to the γ′(L12)-precipitates in the ternary alloy, its partitioning behavior is reversed in favor of the γ(fcc)-matrix in the multicomponent alloy. First-principles calculations of the substitutional formation energies of W and Ta predict that Ta has a larger driving force for partitioning to the γ′ phase than W. This implies that Ta displaces W from the γ′-precipitates into the γ-matrix in multicomponent alloys.

On the field evaporation behavior of a model Ni-Al-Cr superalloy studied by picosecond pulsed-laser atom-probe tomography.

The effects of varying the pulse energy of a picosecond laser used in the pulsed-laser atom-probe (PLAP) tomography of an as-quenched Ni-6.5 Al-9.5 Cr at.% alloy are assessed based on the quality of the mass spectra and the compositional accuracy of the technique. Compared to pulsed-voltage atom-probe tomography, PLAP tomography improves mass resolving power, decreases noise levels, and improves compositional accuracy. Experimental evidence suggests that Ni2+, Al2+, and Cr2+ ions are formed primarily by a thermally activated evaporation process, and not by post-ionization of the ions in the 1+ charge state. An analysis of the detected noise levels reveals that for properly chosen instrument parameters, there is no significant steady-state heating of the Ni-6.5 Al-9.5 Cr at.% tips during PLAP tomography.

On the nanometer scale phase separation of a low-supersaturation Ni-Al-Cr alloy

The phase separation of a Ni–6.5 Al–9.5 Cr at. % alloy aged at 873 K was studied by atom-probe tomography and compared to the predictions of classical precipitation models. Phase separation in this alloy occurs in four distinct regimes: (i) quasi-stationary-state γ′(L12)-precipitate nucleation; (ii) concomitant precipitate nucleation, growth and coagulation and coalescence; (iii) concurrent growth and coarsening, wherein coarsening occurs via both γ′-precipitate coagulation and coalescence and by the classical evaporation–condensation mechanism; and (iv) quasi-stationary-state coarsening of γ′-precipitates, once the equilibrium volume fraction of precipitates is achieved. The predictions of classical nucleation and growth models are not validated experimentally, likely due to the complexity of the atomistic kinetic pathways involved in precipitation. During coarsening, the temporal evolution of the γ′-precipitate average radius, number density and the γ(fcc)-matrix and γ′-precipitate supersaturations follow the predictions of classical models.