Anomalous Tensile Detwinning in Twinned Nanowires

In spite of numerous studies on mechanical behaviors of nanowires (NWs) focusing on the surface effect, there is still a general lack of understanding on how the internal microstructure of NWs influences their deformation mechanisms. Here, using quantitative in situ transmission electron microscopy based nanomechanical testing and molecular dynamics simulations, we report a transition of the deformation mechanism from localized dislocation slip to delocalized plasticity via an anomalous tensile detwinning mechanism in bitwinned metallic NWs with a single twin boundary (TB) running parallel to the NW length. The anomalous tensile detwinning starts with the detwinning of a segment of the preexisting TB under no resolved shear stress, followed by the propagation of a pair of newly formed TB and grain boundary leading to a large plastic deformation. An energy-based criterion is proposed to describe this transition of the deformation mechanism, which depends on the volume ratio between the two twin variants and the cross-sectional aspect ratio.

Figure 1

(a) Schematic drawing of a bitwinned NW. (b),(c) Cross-sectional TEM image and corresponding diffraction pattern of a bitwinned Ag NW, respectively. Scale bar, 20 nm. (d) A cross-sectional high-resolution STEM image of a bitwinned Ag NW. Scale bar, 1 nm.

Figure 2

(a),(b) Engineering stress-strain curves of bitwinned Ag NWs with a balanced and small volume ratio, respectively. Insets in (a) and (b) are the corresponding cross-sectional images of the tested NWs (each sectioned from the undeformed part after the test). The location of the TB is marked by a green arrow. Scale bar, 20 nm. (c),(d) Snapshots of microstructure evolutions corresponding to (a),(b), respectively. The five snapshots in each case correspond to the data points marked in (a),(b). Partial dislocations are marked by blue arrows and planar sliding by green arrows. The viewing directions are from the $⟨1\overline{1}0⟩$ zone axis of the large twin variant in (c),(d), which are marked by the orange arrows in the insets in (a),(b). The new inclined TBs in (d) are marked by orange arrows. Scale bar, 100 nm.

Figure 3

(a) Fracture morphologies of the bitwinned Ag NWs with balanced volume ratios from experiments and simulations, respectively. Scale bar, 100 nm for experiments and 5 nm for simulations. (b) Snapshots from MD simulations showing nucleation, propagation of partial dislocations, and interaction between the partials and the TB. A permanent slip step with one atomic layer is marked by an orange arrow in (b)vi. Scale bar, 2.5 nm. (c) Illustration of a double Thompson tetrahedron on the coherent $\{1\overline{1}1\}$ TB in bitwinned NWs. The front tetrahedron ($ABCD$) represents the matrix slip systems in the dominant twin variant, while the back one (${ABCD}^{\prime }$) represents twin slip systems in the small twin variant.

Figure 4

(a) Schematics showing two steps of the tensile detwinning mechanism. (i) The pristine bitwinned NW. (ii) Step-one detwinning showing the formation of a single-crystalline embryo (colored by blue) with a unique TB-GB-TB structure. (iii) Step-two detwinning showing the expansion of the single-crystalline phase. (b)–(e) Representative dislocation reactions during the step-one detwinning process. Only hexagonal close-packed atoms are made visible. (f),(h) Cross section of the bitwinned phase with an axial direction of $⟨110⟩$ and the single-crystalline phase with the axial direction reoriented to $⟨001⟩$, respectively. (g) Detailed structure of the newly formed TB-GB-TB structure during the step-one detwinning process. (i) MD snapshots in step-two detwinning showing the migration of the newly formed inclined TB and GB. Black arrows indicate the partials on the inclined TB that are responsible for the TB migration. (j),(k) Fracture morphology of the bitwinned Ag NWs with small volume ratios from experiments and simulations, respectively. The insets in (k) show the cross-sectional TEM image and corresponding diffraction pattern (zone axis, $⟨001⟩$) from the single-crystalline phase in the deformed part. Scale bar in (k) and in the inset, 100 and 20 nm, respectively.

Figure 5

(a) Energy change associated with the detwinning process as a function of the twin volume ratio for a sample with fixed $W$ and $H$. The inset in (a) shows the cross section of a typical bitwinned NW in MD simulations and its corresponding geometric parameters. (b) Contour plot of the energy change in the detwinning process for a fixed $H$ as a function of $H/W$ and twin volume ratio $r$, along with simulation and experimental data. For the simulation results, open black squares, open red rhombuses, and open blue triangles stand for the slip-dominated mode, the tensile detwinning mode, and the transitional deformation mode, respectively. For the experiment results, solid blue squares and solid red rhombuses stand for the slip-dominated mode and the tensile detwinning mode, respectively.