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Solute segregation and deviation from bulk thermodynamics at nanoscale crystalline defects

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Science Advances  21 Dec 2016:
Vol. 2, no. 12, e1601796
DOI: 10.1126/sciadv.1601796
  • Fig. 1 Overview of microstructures and shearing process.

    (A) SEM image of the precipitate (γ′) and matrix (γ) microstructure from the investigated alloy. Co3W laths are observed near the right of the image. (B) STEM image of the deformed Co-2Ta microstructure. A Co3W lath was observed to cross a γ′ precipitate. Numerous SISFs were observed to cross through the γ′ precipitates. (C) Schematic of the shearing sequence that is required to form SISFs in the γ′-(L12) blocks. The γ′-(L12) blocks must first shear, and segregation at the newly created SISF may occur, which enables the local SISF-(D019) composition to become more like the bulk Co3W-(D019) composition. Molecular drawings completed in VESTA (48).

  • Fig. 2 Composition analysis at the stacking fault.

    (A) High-resolution STEM HAADF image from the area denoted by the black box from Fig. 1B. A SISF is located adjacent to a Co3W lath. The vertically integrated EDS line scan across the STEM HAADF reveals differences in the local composition of the Co3W lath and the SISF. (B) The high-resolution STEM EDS data are in agreement with APT reconstructions, where a SISF intersects the APT tip on a {111} plane. Al atoms (50%) and W atoms (20%) are shown for clarity.

  • Fig. 3 First-principles calculations of the D019 and L12 phases.

    (A) Gibbs free energies of the D019 and L12 structures at 900°C with the common tangent construction. The L12 structure is predicted to be stable from 0 to 20 at % W, and the Co3W phase is predicted to be stable from 20 to 25 at % W. (B) Gibbs free energies of the D019 and L12 structures at 900°C with the Suzuki segregation criterion from Eq. 1 satisfied. It is represented by the two compositions: cL12 = Co-11Al-14W (at %) and cD019 = Co-1Al-24W (at %).

  • Fig. 4 Direct atomistic simulation of the equilibrium stacking fault composition.

    (A) L12 + D019 supercell (48). (B) (Inset) Average concentration for each sublattice site (colored) as a function of the average L12 composition determined from the L12 layers in the 32-atom supercell. For W-rich L12 compositions, the average composition within the SISF is higher, as shown by the dashed gray lines located at the γ′ composition of Co-11Al-14W (at %).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/12/e1601796/DC1

    Supplementary Materials and Methods

    Supplementary Text

    table S1. Compositions and heat treatment conditions.

    table S2. Creep testing conditions.

    fig. S1. HAADF STEM intensity profile.

    fig. S2. Gaussian distribution fit and model of the W + Ta composition at the SISF.

    fig. S3. Calculated and predicted ground-state energies for structures used in the cluster expansion.

    fig. S4. Effective cluster interaction parameters used in the cluster expansion.

    fig. S5. HAADF STEM image of specimen tested at a strain rate of 10−4 s−1.

    References (3947)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods
    • Supplementary Text
    • table S1. Compositions and heat treatment conditions.
    • table S2. Creep testing conditions.
    • fig. S1. HAADF STEM intensity profile.
    • fig. S2. Gaussian distribution fit and model of the W + Ta composition at the
      SISF.
    • fig. S3. Calculated and predicted ground-state energies for structures used in the
      cluster expansion.
    • fig. S4. Effective cluster interaction parameters used in the cluster expansion.
    • fig. S5. HAADF STEM image of specimen tested at a strain rate of 10−4 s−1.
    • References (39–47)

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