Direct Observation of the Deformation Mechanism of Twin-Structured Ni NWs under Bending Strain

Abstract

In situ atomic-scale bending tests of twin-structured Ni nanowires were realised using a homemade deformation device. The results showed that the plastic deformation mechanism in twin-structured Ni nanowires depended on the deformation stage. At the early stages of bending deformation, the plasticity of twin-structured Ni nanowires was controlled by dislocations interacting with the twin boundaries or parallel to them. With increasing bending strain, both dislocation and face-centred cubic–body-centred tetragonal phase transition occurred. At very high bending strain, grain boundaries resulting from the lattice distortion/collapse were formed. This study details the deformation mechanisms of the twin-structured Ni nanowires under bending deformation, which advances the basic understanding of the plasticity mechanisms in metals.

1. Introduction

The mechanical properties of nanomaterials, including strength, hardness, and ductility, are directly related to their atomic-scale plastic deformation mechanisms [1,2]. Therefore, revealing the atomic-scale plastic deformation mechanism of nanoscale metals can guide the design of nanomaterials with desired mechanical properties [1,2,3,4,5]. Twin-structured nanomaterials have recently attracted considerable attention because of their ultrahigh strength [3,4,5,6]. Particularly, a large number of studies have shown that twin-structured metallic nanowires (NWs) can approach the ideal strength [7,8,9,10,11,12,13] because the twin boundary (TB) can prevent the dislocation motion and nucleation, thereby causing a significant strengthening effect [7,8,9,10,11,12,13]. In the past decades, many studies have been conducted on the mechanical properties of twin-structured metallic NWs because of the importance of their deformation mechanism; however, most of these studies are based on conventional tensile or compressive experiments [5,7,8,9,10,11,12,13,14,15]. At the same time, the mechanism of accommodation of the bending strain in twin-structured NWs has rarely been investigated experimentally because direct atomic-scale experimental techniques are usually difficult to apply [16,17,18,19,20,21,22]. For this reason, our current understanding of the atomic-scale deformation mechanisms of twin-structured metallic NWs under bending strain heavily relies on molecular dynamics (MD) simulations [7,8,9,11,12,13,15]. However, the deformation mechanisms proposed by MD simulations must ultimately be validated experimentally, which has not yet been done. Therefore, it is important to obtain direct atomic-scale data on the mechanism of accommodation of the bending strain in the twin-structured metallic NWs.

To address this issue, in this study, the twin-structured Ni NWs were subjected to bending strains from 1.9 to 36.5% using our previously developed experimental method [23]. Our in situ atomic-scale observations show that the dislocation interaction with TBs, face-centred cubic (fcc)–body-centred tetragonal (bct) phase transitions, and grain boundary (GB) generation via lattice collapse may all occur during bending deformation. Notably, we reveal that these plastic behaviours appear in the above order at different deformation stages.

2. Experimental Section

The atomic-scale in situ bending tests of the metallic NWs were realised using a homemade in situ deformation device [23]. As illustrated in Figure 1a, the NWs were randomly spread to bridge on a prebroken colloidal thin film on a transmission electron microscopy (TEM) specimen grid. The scattered NWs were then bent or stretched controllably through expansion or contraction of the colloidal thin film under electron beam irradiation. Figure 1b shows typical TEM images of the bending of Ni NWs with diameters of ~40 nm at the effective average strain rate of ~10⁻⁴ s⁻¹. We monitored the continuous bending deformation of the Ni NWs by observing their bending angle. Using our specially designed device, the sample could be tilted in two orthogonal directions at angles of ±20°, which allowed us to deform NWs without the need for other sample holder or mechanical tensile attachment. The NWs could also be rotated to a low-index crystallographic direction, allowing in situ recording of the atomic-scale bending or tensile deformation of individual metallic NWs. With this in situ deformation device, the bending process of the Ni NWs was recorded at the atomic scale, as shown in Figure 1c. The experiments were performed at strain rates of ~10⁻⁴–10⁻³ s⁻¹, and observations were made using transmission electron microscope (TEM) JEOL-2010F (JEOL, Tokyo, Japan) with a point resolution of 0.12 nm.

3. Results

3.1. TEM Characterisation of the Twin-Structured Ni NWs

Ni NWs for the in situ bending tests were ~50 μm in length and ~40 nm in diameter. Figure 2a shows a typical TEM image of twin-structured Ni NWs consisting of parallel {111} twins. Figure 2b provides the corresponding selected area electron diffraction patterns captured along the $\left[\overline{1}10\right]$ axis of NWs. This is a typical twin-structured diffraction pattern, showing that the NWs grew along the [112] direction (as shown in Figure 2c) and have nanoscale growth twins. The growth of twins with thicknesses of 1–14 nm was observed in many high-resolution TEM (HTEM) images. In particular, Figure 2c presents a typical HTEM image revealing the atomic-scale structure of TB: the NWs are well crystalline with no pre-existing dislocations on TBs.

3.2. In Situ TEM Observation of the Bending of Twin-Structured Ni NWs

Figure 3a–e present low-magnification TEM images. These images outline the nano-scale structure evolution of a Ni NW with growth twins. According to a previously reported theoretical formula ε_bent=r/(r+R)% [24], where ε_bent is the bending strain, R is the bending curvature, and r is the NW diameter, the maximum bending strain in the Ni NWs increases from 1.9 (Figure 3a) to ~36.5% (Figure 3e). During bending deformation, the atomic and nanoscale structural evolution and the plastic behaviours of the bent Ni NWs were recorded in situ. Figure 3f–i display several high-magnification TEM images showing arc-shaped TBs. Two GBs (indicated with the red arrows in Figure 3i) formed during the severe bending deformation of the twin-structured Ni NWs. This is direct evidence that the deformation of twin-structured Ni NWs under high bending strain involves GB generation.

3.3. Dislocation Governs Plasticity at the Low Strain Stage

To demonstrate how the twin-structured Ni NWs accommodated the plasticity under bending deformation, their atomic-scale deformation mechanisms at different straining stages were investigated. At very low strain values, partial dislocation activities frequently occurred. In twins with a thickness of ~2.4 nm, partial dislocation glide on the {111} plane, which was parallel to the TBs, was the dominant plastic activity. Figure 4a shows the HTEM image captured along the $\left[\overline{1}10\right]$ zone axis during bending deformation. In Figure 4a, many steps on the original flat TBs in the thinner twins are observed. These steps resulted from the nucleation and gliding of the partial dislocations on the TBs, which was the dominant plastic deformation event in twins with thicknesses below ~6 nm. In the thicker twins, partial dislocations resulting in stacking faults (SFs) were frequently observed. These partial dislocations intersected with the TBs during the bending of the NWs. Figure 4b shows a typical HTEM image captured after the occurrence of plastic deformation. As shown in Figure 4b, most of the partial dislocations intersect with the TBs. These dislocation activities were frequently observed in twins with thicknesses greater than ~6 nm, indicating a considerable effect of twin thickness on the deformation mechanism of the metallic NWs. Similar observations have been reported for twin-structured nanocrystalline Cu [3,4,25,26,27,28,29,30,31,32,33,34,35].

3.4. In Situ Atomic-Scale Observation of Reversible fcc–bct Phase Transition

At relatively large bending strains, the complex stress distribution in the bent NWs enabled the direct observation of the transition of the original fcc structure into the bct structure occurring with increasing bending strain. In particular, Figure 5 shows the in situ atomic-scale observation of bending inducing an fcc–bct phase transition in a severely bent Ni NW with growth twins. This phase transition was frequently detected when the maximum bending strain was increased to approximately 10.3%. Figure 5 shows that the lattice angle of the two {111} planes increased from 70.5° (projected along the $\left[\overline{1}10\right]$ direction) to ~90°. As shown in the HRTEM image captured along the $\left[\overline{1}10\right]$ zone axis (Figure 5a), the lattices exhibit a parallelogram shape at relatively low strain values. Figure 5b shows that with increasing bending strain, the original parallelogram-shaped lattices transformed into square-shaped lattices.

Interestingly, the square-shaped lattices were also reversed by localised strain relaxation, such as dislocation nucleation in the bct-structured region. Figure 5c,d show a complete reversal of the fcc–bct transition after the bct lattice region is interrupted by dislocation nucleation. As shown in Figure 5c, the lattices are square-shaped, corresponding to the bct lattice. Based on the values reported in previous studies, the measured lattice distances are very close to those of the bct-structured Ni viewed along the [001] zone axis [36,37,38,39,40]. This result directly confirms the occurrence of a phase transformation in the Ni NWs under bending strain and the dependence of the deformation mechanism on the deformation stage. With increasing bending strain, as shown in Figure 5d, three full dislocation nucleations made the square lattices revert to the parallelogram shape and caused the lattice angle of the two {111} planes to return to ~70.5°, indicating a reversible fcc–bct phase transformation.

3.5. Direct Atomic-Scale Evidence of the Grain Boundary Formation through Lattice Bending

As the bending strain increases to a very high level (greater than ~18.5%), the formation of GBs was activated. Figure 6 shows three $\left[\overline{1}10\right]$ zone-axis HTEM images (captured at different deformation stages) detailing the GB formation mechanism. Figure 6a presents the HTEM image before the GB generation in the Ni NW under bending train. At this high-bending-strain stage, the lattices are highly curved, and the lattice strain is nonuniform in this local region. As shown in Figure 6b, with increasing bending strain, the original parallelogram-shaped lattice becomes highly distorted. With a further increase in the bending strain, the lattice in the local region becomes disordered, and a tilted GB with an angle of ~19.5° is induced, as shown in Figure 6c. This disordered lattice spreads in the entire GB region, and no disclination or well-arranged dislocations are observed. These results, in contrast to those of previous studies, indicate that GB generation can also occur through lattice collapse [41,42,43,44].

3.6. Effect of Strain Magnitude on the Deformation Mechanism of the Twin-Structural Ni NWs

More than 10 twin-structured Ni NWs were used to clarify the effect of bending strain on the plastic deformation mechanism. Our results revealed that a transition in the deformation modes occurred with increasing bending strain. Figure 7 shows the plastic deformation picture of the twin-structured Ni NWs at different strain stages. According to this picture, their plastic deformation can be fitted into three regions. The first plasticity region corresponds to the bending strain of ~2–10%. At this stage, the twin-structured NWs accommodate bending strain via the dislocation activities, consistent with most previous studies on twin-structured materials [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. The second region (~10–18.5%) is the plasticity governed by both phase transitions and dislocation activities. As the bending strain increases over ~18.5%, the deformation is in the third deformation region, in which GBs are observed. This effect of the magnitude of strain on the plastic deformation model of metallic NWs has rarely been studied in previous research.

4. Discussion

In the past few decades, there has been a dramatic increase in the number of studies on the nanomechanical testing of nanosized metals [2,8,10,20,21,22,35,44,45,46,47,48,49]. However, most of these studies were based on conventional tensile or compressive tests, which showed that the deformation of metals was governed by partial dislocation. In contrast, the present study demonstrated that the plastic deformation of the twin-structured Ni NWs under bending was governed by dislocation, reversible fcc–bct phase transition, and GB formation. The observed deformation mechanism depended on the deformation stage, which has rarely been reported [5,36,47,48,49,50]. The GB generation and phase transformation occurred at high strain and always after the dislocation, making them difficult to detect at not sufficiently large plastic strain [51,52,53,54].

5. Conclusions

The bending twin-structured Ni NWs were captured in situ at the atomic scale. The HTEM results revealed that the deformation model changed with increasing bending strain, which has rarely been previously reported. These findings provide valuable insights into plastic deformation mechanisms in metallic NWs and show a general picture of the deformation mechanism; some of the deformation modes are different from previous studies.