Achieving Large Uniform Tensile Ductility in Nanocrystalline Metals

Synchrotron x-ray diffraction and high-resolution electron microscopy revealed the origin of different strain hardening behaviors (and dissimilar tensile ductility) in nanocrystalline Ni and nanocrystalline Co. Planar defect accumulations and texture evolution were observed in Co but not in Ni, suggesting that interfacial defects are an effective passage to promote strain hardening in truly nanograins. Twinning becomes less significant in Co when grain sizes reduce to below $\sim 15\mathrm{nm}$. This study offers insights into achieving excellent mechanical properties in nanocrystalline materials.

Figure 1

(a) SXRD spectra for NC-Co at 0% (red line) and 2.5% (blue line) strains (corresponding stress levels of 0 and 1365 MPa, respectively). The inset is a zoomed-in region of the $(200{)}_{\mathrm{fcc}}$ peak, showing a slight change of the peak intensity during straining. (b) Engineering stress-strain plots of NC-Ni and NC-Co, respectively. The inset is a summary of ${\sigma }_{0.2\%}$ vs ${\epsilon }_{\mathrm{unif}}$ for various NC metals with an average $D<30\mathrm{nm}$.

Figure 2

The $\Delta {\epsilon }^{hkl}$ (shown in the same scale range) as a function of the applied stress for (a) NC-Ni and (b) NC-Co.

Figure 3

Two-dimensional polar plots showing the change of (a) (101) and (b) (002) peak intensities for NC-Co, and (c) (200) for NC-Ni. The stress increment follows $\mathrm{\text{red}}(\mathrm{\text{inside}})\to \mathrm{\text{green}}(\mathrm{\text{middle}})\to \mathrm{\text{blue}}(\mathrm{\text{outside}})$ color. The solid black circles represent the initial intensity before loading. The blue circles represent a 5% increment in peak intensities.

Figure 4

(a) Statistical $D$ distribution for as-synthesized NC-Co. (b) HRTEM image of deformation twins in NC-Co after tensile failure. The inset Fourier-filtered image is transformed from the square area. (c) Statistical $D$ distribution of NC-Co and twinned grains after tensile deformation.