Research ArticleMATERIALS SCIENCE

Bulk ultrafine grained/nanocrystalline metals via slow cooling

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Science Advances  23 Aug 2019:
Vol. 5, no. 8, eaaw2398
DOI: 10.1126/sciadv.aaw2398
  • Fig. 1 Microstructure of bulk UFG/nanocrystalline Cu-containing distributed WC nanoparticles.

    (A) SEM image of Cu-5vol%WC (by a cooling rate of 4 K/s) acquired at 52° showing well-dispersed WC nanoparticles in the Cu matrix. Inset is the image of a typical as-cast bulk Cu-5vol%WC ingot with a diameter of 50 mm. (B) Magnified SEM image of Cu-5vol%WC (4 K/s) showing the ultrafine and nanoscale Cu grains. (C) SEM image of the cross-section showing the UFG Cu matrix and the WC nanoparticles present beneath the surface of the sample. (D) Typical FIB image of Cu-5vol%WC (4 K/s) showing the UFG/nanocrystalline microstructure. (E) FIB image of pure Cu cast under the same condition showing coarse Cu grains. (F) EBSD image of Cu-5vol%WC (4 K/s) with grain size color code. Black phases are WC nanoparticles, red grains are smaller than 100 nm, and yellow and orange grains are smaller than 1 μm. (G) Summary of the average Cu grain sizes for different volume fractions of nanoparticles under different cooling rates. Error bars show the SD.

  • Fig. 2 Nucleation and grain growth control mechanisms by nanoparticles.

    (A) Typical DSC scanning result during the cooling of pure Cu and Cu-10vol%WC. (B) Typical TEM image of the Cu-WC interface showing the interface between the Cu matrix and the WC nanoparticle. (C) Fourier-filtered high-resolution TEM image of the marked red rectangle area in (B) showing a characteristic interface between the WC nanoparticle and the Cu matrix. Insets are the fast Fourier transformation of the Cu matrix (top right) and the WC nanoparticle (bottom left). (D) Undercooling requirement to overcome Gibbs-Thompson pinning effect for Cu, Al, and Zn. (E) Schematic illustration of the nanoparticle pinning effects. (F) SEM image of a Cu grain refined by WC nanoparticles. (G) Nanoparticles break the fundamental limit that existed in conventional grain refinement methods. (H and I) Schematic illustrations of phase evolution during solidification of pure metal (H) and metal with nanoparticles (I).

  • Fig. 3 Nanoparticle-enabled grain refinement in other materials systems.

    (A and B) FIB images of Al-10vol%TiB2 cast by furnace cooling (0.7 K/s) showing the distribution of TiB2 nanoparticles and ultrafine Al grains. (C) TEM image of Al-10vol%TiB2 (0.7 K/s) showing one ultrafine Al grain surrounded by TiB2 nanoparticles. (D) Al grain size distribution of Al-10vol%TiB2 (0.7 K/s). (E and F) SEM image of Zn-5vol%WC by air cooling (3.7 K/s). (G) FIB image of Zn-5vol%WC (3.7 K/s). (H) Zn grain size distribution of Zn-5vol%WC (3.7 K/s).

  • Fig. 4 Thermal stability of ultrafine/nanocrystalline Cu-containing WC nanoparticles.

    (A to D) STEM images of an area with a high percentage of WC nanoparticles at room temperature, 400°C, 600°C, and 850°C, respectively. (E to H) STEM image of an area with a relatively low percentage of WC nanoparticles at room temperature, 400°C, 600°C, and 850°C, respectively. (I) SEM image of Cu-34vol%WC after heat treatment (750°C for 2 hours). (J) EBSD image corresponds to the marked white rectangle in (I). (K) Cu grain size distribution of the heat-treated Cu-34vol%WC sample.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaaw2398/DC1

    Supplementary Text

    Fig. S1. Fabrication of Cu-containing WC nanoparticles.

    Fig. S2. Cooling curves for furnace cooling, air cooling, and water quenching of Cu-WC samples.

    Fig. S3. Size distribution of WC nanoparticles in the as-solidified Cu-WC sample.

    Fig. S4. Structure of bulk UFG/nanocrystalline Cu-containing WC nanoparticles.

    Fig. S5. STEM image of the WC nanoparticle-rich area showing that the Cu grain size is correlated with WC interparticle spacing.

    Fig. S6. Cooling curve during the DSC tests at a cooling rate of 5°C/min.

    Fig. S7. Mechanical properties of UFG/nanocrystalline Cu-containing WC nanoparticles.

    Fig. S8. Undercooling profile relative to solid fraction.

    References (3945)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Fabrication of Cu-containing WC nanoparticles.
    • Fig. S2. Cooling curves for furnace cooling, air cooling, and water quenching of Cu-WC samples.
    • Fig. S3. Size distribution of WC nanoparticles in the as-solidified Cu-WC sample.
    • Fig. S4. Structure of bulk UFG/nanocrystalline Cu-containing WC nanoparticles.
    • Fig. S5. STEM image of the WC nanoparticle-rich area showing that the Cu grain size is correlated with WC interparticle spacing.
    • Fig. S6. Cooling curve during the DSC tests at a cooling rate of 5°C/min.
    • Fig. S7. Mechanical properties of UFG/nanocrystalline Cu-containing WC nanoparticles.
    • Fig. S8. Undercooling profile relative to solid fraction.
    • References (3945)

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