Grain-size-dependent zero-strain mechanism for twinning in copper

It is generally accepted that deformation twinning in coarse-grained metals contributes the macroscopic strain, while most deformation twins in nanocrystalline (NC)metals, contrary to popular belief, yield zero net macroscopic strain via either the cooperative or random activation of all three Shockley partials. In the former, the three partials with a particular ($b_2$:$b_1$:$b_3$) triplet unit are successively emitted, while in the latter the three partials are randomly activated in equal numbers. Here we report that there exists a transition between the two zero-strain deformation twinning mechanisms, i.e., from cooperative activation of partials to random activation of partials in NC Cu with medium stacking-fault energy, that occurs with decreasing grain size at room temperature and different strain rates. This experimental finding provides insight into the understanding of deformation twinning.

Figure 1

(a), (f) Plan-view and (b) cross-sectional TEM micrographs showing the microstructure of the Cu film after tensile straining at 10${}^{-4}$/s. Inset in (a) is the corresponding selected area diffraction patterns, showing polycrystalline structure. (c) HRTEM image showing the DT tip of grain B in (a). Inset is the corresponding fast Fourier transform of the DTs and the inverse FFT HRTEM image of the DT tip containing stacking faults (SFs). (d) Magnified view of grain C in (b). (e) HRTEM image of the region containing the twins, and the 9$R$ structure [boxed area in (d)] and its FFT (inset). (g) Magnified view of grain D in (f). (h) Inverse FFT HRTEM image of one end of the deformation twin, showing a wider ITB composed of PB1, 9$R$, and PB2 in grain D. Inset is the corresponding FFT of 9$R$. Grain boundaries of grains B and D are marked by the asterisks.

Figure 2

(a) 275-nm-sized grain containing multiple twins deformed at strain rate 1 × 10${}^{-0}$/s. (b) HRTEM image of the front end of the twin indicated in (a). Inset is the corresponding FFT of the DTs. (c) Magnified view of twin ending in grain interior in (a). (d) HRTEM image of twin indicated in (c). Inset is the corresponding FFT of the 9$R$ structure. A periodic array of defects is observed with a repeat unit of three {111} planes in both (b) and (d).

Figure 3

Grain-size effect on the formation of DTs in Cu thin films tensile tested at two strain rates, (a) $1\times {10}^{-4}$/s and (b) $1\times {10}^0$/s. The solid line is a visual guide. (c) Schematic illustration of the $D$ effects on the DT propensity$P_{\mathrm{DT}}$, and its constituent $P_{{}_{\mathrm{partial}}}^{\mathrm{emission}}$ and $P_{\mathrm{partial}}^{\mathrm{promote}}$ contributions, which lead to $D_C$ and $D_{\mathrm{inv}}$.