Research ArticleMATERIALS SCIENCE

Consecutive crystallographic reorientations and superplasticity in body-centered cubic niobium nanowires

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Science Advances  06 Jul 2018:
Vol. 4, no. 7, eaas8850
DOI: 10.1126/sciadv.aas8850
  • Fig. 1 Deformation-induced BCC-FCC-BCC phase transition in an Nb nanowire.

    (A) Pristine nanowire with a diameter of ~15 nm. The nanowire was aligned in the [100] zone axis and stretched along the Embedded Image direction. (B) Accumulation of elastic strain in the nanowire caused the formation of an FCC domain, as marked by the pink dashed line. Filtered TEM images in the insets show the atomic structures of the matrix (yellow) and the FCC domain (pink). (C and D) Reorientation from the [100] zone axis to the [111] zone axis finally occurred following a BCC-FCC-BCC pathway, and the reoriented region increased with the migration of phase boundaries. The pink and cyan dashed lines represent the [100]-BCC/[011]-FCC interface and the [011]-FCC/[111]-BCC interface, respectively. (E to G) Corresponding FFT patterns showing the structural evolutions during the BCC-FCC-BCC phase transformation. (H) Schematic showing the change of atomic configurations during the BCC-FCC-BCC phase transformation. (I and J) Lattice structure transition pathway in the Nb nanowire can be described by Bain’s model. The yellow, red, and blue spheres represent the initial BCC lattice, the new FCC lattice, and the final BCC lattice, respectively.

  • Fig. 2 Deformation twinning–mediated reorientation in Nb nanowires.

    (A to C) Deformation twinning in an Nb nanowire with a diameter of ~13.7 nm. Under Embedded Image tension, a twin band nucleated from the free surface, penetrated the whole nanowire, and then thickened gradually via the migration of its twin boundaries, resulting in a crystallographic rotation of the nanowire matrix by 21°. (D to G) Deformation twinning induced reorientation in another Nb nanowire with the same loading geometry. The Nb nanowire had a diameter of ~14.7 nm before deformation.

  • Fig. 3 Slip-induced crystal rotation in an Nb nanowire.

    (A to I) Continuous lattice rotation in a deformed Nb nanowire due to the occurrence of dense dislocation activities on adjacent Embedded Image slip planes. In this process, the zone axis of the nanowire remained nearly unchanged, but the slip planes rotated gradually with respect to the axial direction of the nanowire. Accompanying the dense dislocation activities was the generation of numerous surface steps, as pointed out by the red arrows in (B), (C), and (G). The inverse FFT images in the insets of (B) and (G) confirmed the existence of surface nucleated dislocations. The cyan lines in (A) to (I) represent the position of the initial Embedded Image plane for reference, while the red dashed lines in (B) and (D) present the formation of surface steps. (J) A four-atom-length atomic chain formed before the fracture of this nanowire. (K) A schematic showing the slip-induced lattice rotation in the Nb nanowire.

  • Fig. 4 Deformation-induced multiple reorientations and superplastic deformation in an Nb nanowire.

    (A) The pristine Nb nanowire with a diameter of ~13.7 nm and a length of ~13.9 nm. We applied tensile loading along the nanowire axis of the Embedded Image direction. (B) Phase transformation induced reorientation via a two-step BCC-FCC-BCC transition pathway, resulting in a change of the zone axis from [100] to [111] and the nanowire axis from Embedded Image to Embedded Image, as illustrated by the filtered TEM images in the insets. The yellow, pink, and cyan insets represent the lattice structures of the initial [100]-BCC, the metastable [011]-FCC, and the new [111]-BCC, respectively. (C) Twinning-mediated second reorientation of the Nb nanowire, resulting in a change of the nanowire axis from Embedded Image to Embedded Image. (D to F) Correlated reorientations via the slip-induced crystal rotations in the subsequent deformation, where the zone axis of the nanowire remained unchanged. Different reorientation processes can be distinguished by the inclined angles between the corresponding Embedded Image planes. For direct comparisons, the position of the Embedded Image plane after the third reorientation was marked by the cyan dashed lines in (D) to (F). (G) After multiple reorientations, the length of the nanowire elongated from 13.9 to 51.3 nm before necking localization, corresponding to a uniform elongation of ~269%.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaas8850/DC1

    Fig. S1. Postfracture characterization of an Nb nanowire.

    Fig. S2. Deformation-induced BCC-FCC-BCC phase transformation in an Nb nanowire.

    Fig. S3. Identification of the twin plane in the Nb nanowire presented in Fig. 2.

    Fig. S4. Surface nucleation of dislocation in an Nb nanowire.

    Fig. S5. Formation of an atomic chain before the fracture of the Nb nanowire shown in Fig. 3.

    Fig. S6. Multiple reorientations and superplastic deformation in an Nb nanowire.

    Fig. S7. An additional example showing the multiple reorientations and superplastic deformation in an Nb nanowire.

    Fig. S8. Statistical data showing the deformation modes and ductility of Nb nanowires with different diameters.

    Table S1. Schmid factors for dislocation slip and deformation twinning under different loading directions of Nb nanowires.

    Table S2. Activation energies of surface self-diffusion Ed on the close-packed surface of FCC and BCC metals and their corresponding N-body potentials.

    Movie S1. Deformation-induced phase transformation in an Nb nanowire with a diameter of ~15 nm.

    Movie S2. Twinning-dominated deformation in an Nb nanowire with a diameter of ~14.7 nm.

    Movie S3. Superplastic deformation of an Nb nanowire with a diameter of ~13.7 nm.

  • Supplementary Materials

  • The PDF file includes:
    • Fig. S1. Postfracture characterization of an Nb nanowire.
    • Fig. S2. Deformation-induced BCC-FCC-BCC phase transformation in an Nb nanowire.
    • Fig. S3. Identification of the twin plane in the Nb nanowire presented in Fig. 2.
    • Fig. S4. Surface nucleation of dislocation in an Nb nanowire.
    • Fig. S5. Formation of an atomic chain before the fracture of the Nb nanowire shown in Fig. 3.
    • Fig. S6. Multiple reorientations and superplastic deformation in an Nb nanowire.
    • Fig. S7. An additional example showing the multiple reorientations and superplastic deformation in an Nb nanowire.
    • Fig. S8. Statistical data showing the deformation modes and ductility of Nb nanowires with different diameters.
    • Table S1. Schmid factors for dislocation slip and deformation twinning under different loading directions of Nb nanowires.
    • Table S2. Activation energies of surface self-diffusion Ed on the close-packed surface of FCC and BCC metals and their corresponding N-body potentials.
    • Legends for movies S1 to S3

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  • Other Supplementary Material for this manuscript includes the following:
    • Movie S1 (.mp4 format). Deformation-induced phase transformation in an Nb nanowire with a diameter of ~15 nm.
    • Movie S2 (.mp4 format). Twinning-dominated deformation in an Nb nanowire with a diameter of ~14.7 nm.
    • Movie S3 (.mp4 format). Superplastic deformation of an Nb nanowire with a diameter of ~13.7 nm.

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