**3. Results And Discussion**

Figure 2a–h shows a typical deformation behaviour in axially twinned [110] BCC Fe nanowires containing two pre-existing twin boundaries. Similar behaviour has been observed in nanowires containing one and three TBs and therefore not shown here. It can be seen that the plastic deformation initiates by the nucleation of a twin from the nanowire corners (Figure 2a). With increasing deformation, the leading front of the twin approaches the existing twin boundary and penetrates into neighbouring grain (Figure 2b). Here, it is interesting to see that the twin penetration happens in two different ways; (1) it can penetrate directly to the next grain without any deviation in twinning plane and (2) it can pass onto a plane symmetrical to the original twinning plane (Figure 2b). Following the penetration, the leading twin front again passes through the second pre-existing twin boundary and results in the formation of twinned rhombohedron, which is enclosed by twin boundaries (Figure 2c). Also, the initially nucleated twin (Figure 2a) has grown significantly cutting across the pre-existing TBs (Figure 2c). It can be seen that the twin penetration and its growth has led to the annihilation and also the migration of pre-existing TBs (Figure 2c,d). With increasing strain, the twin growth dominates the deformation leading to the complete annihilation of pre-existing TBs (Figure 2e,f). Following the annihilation, the growing twin completely sweeps the nanowire length and leads to the reorientation of the nanowire from initial <110>/{111}{112} orientation to <001>/{010}{310} orientation (Figure 2g,h). Thus, in axially twinned BCC Fe nanowires there is an annihilation of pre-existing TBs. In other words, the twinned nanowires become twin free single crystalline nanowires with different orientation.

The deformation by twinning and reorientation in [110] twinned BCC Fe nanowires is similar to that observed in perfect [110] nanowires [19]. However, the annihilation of pre-existing TBs and the observation of reorientation in twinned nanowires is interesting. Similar to the present study, the annihilation of twin boundary due to de-twinning has also been observed recently in FCC nanowires [12]. However, this annihilation in FCC system has been observed only when the nanowire contains a single axial twin boundary located close to surface [12]. In the present study, the annihilation of TBs has been observed in nanowires containing one, two and three TBs irrespective of their location with respect to the surface. Further, there is an experimental evidence for the annihilation of twin boundaries due to de-twinning in FCC nanowires [12]. However, due to limited experimental studies on BCC nanowires compared to FCC, the annihilation of twin boundaries as a result of de-twinning has not been reported experimentally in BCC nanowires. Interestingly, a reversible twinning, which is also called de-twinning has been reported experimentally in BCC W nanowires [21]. Using in situ high-resolution transmission electron microscopy, it has been demonstrated that during loading, the W nanowire undergo deformation by twinning leading to twin growth. However, upon unloading, deformation proceeds by de-twinning, which is same as twinning, but in the reverse direction. As a result, the twin thickness is gradually reduced [21].

**Figure 2.** The atomic snapshots displaying the typical deformation behaviour in twinned BCC Fe nanowires at different strains; (**a**) twin nucleation, (**b**,**c**) twin penetration through existing TBs, (**c**,**d**) twin boundary migration and annihilation, (**d**–**g**) twin growth, and (**h**) reoriented nanowire. The atoms are coloured according to their centro-symmetry parameter (CSP). The insets in (**a**,**h**) shows the top view of the nanowire.

The annihilation of pre-existing TBs in Figure 2a–h has occurred through two different mechanisms, one without any resolved shear stress (Figure 3) and other with finite and small resolved shear stress (Figure 4). Figure 3 shows a mechanism where a pre-existing twin boundary gets annihilated step by step due to the glide of 1/6<111> partial dislocations on newly formed TBs. The 1/6<111> partial dislocations nucleate from a twin–twin junction (Figure 3b,f,j) and glide on TBs (Figure 3c–d,g–h,k–l) whose migration results in twin growth. As a result of twin boundary migration and twin growth, the twin–twin junction moves step by step and leads to the annihilation of pre-existing twin boundary (Figure 3a,e,f). During this process, there is no apparent resolved shear stress on the pre-existing twin boundary as it is completely parallel to the loading axis (Figure 3a,e). It remains parallel till the partial dislocation activity is equal on both left and right TBs (Figure 3a–h). However, with increasing strain, due to the presence of some barriers, the partial dislocation activity on one twin boundary (left in this case) dominates over the other (Figure 3k,l). As a result, the pre-existing twin boundary slightly bends with respect to the loading axis (Figure 3i) which introduces a small resolved shear stress. Due to this shear stress, the partial dislocation activity also commences on the pre-existing twin boundary (Figure 4a,e). This partial dislocation activity further bends the twin boundary towards the other pre-existing twin boundary (Figure 4b,c,f,g). As a result, they meet each other and annihilate over time (Figure 4d,h). This annihilation occurs under small resolved shear stress. Thus, two different mechanisms combinedly contribute to de-twinning in twinned BCC Fe nanowires (Figure 2). In contrast, the de-twinning in FCC nanowires occurs through a single mechanism of twin embryo nucleation followed by its expansion due to the migration of a special junction consisting of two TBs and one high angle grain boundary [12]. During this complete process, the existing twin

boundary is always parallel to the loading axis, thereby getting annihilated under zero resolved shear stress [12].

**Figure 3.** The annihilation of pre-existing twin boundary due to the step-by-step migration of the twin–twin junction (**a**,**e**,**i**). The movement of twin-twin junction is aided by the continuous nucleation (from the twin–twin junction) (**b**,**f**,**j**) and glide of 1/6<111> partial dislocations on newly formed twin boundaries(**c**,**d**,**g**–**h**,**k**,**l**). The atoms are coloured according to their centro-symmetry parameter (CSP). For clarity, the atoms in perfect BCC structure were removed in (**b**–**d**), (**f**–**h**) and (**j**–**l**).

**Figure 4.** The annihilation of pre-existing twin boundary due to the bending and subsequent migration of pre-existing twin boundaries (**a**–**d**). This process is aided the by the continuous glide of 1/6<111> partial dislocations on the existing TBs. The atoms are coloured according to their centro-symmetry parameter (CSP). For clarity, the atoms in perfect BCC structure were removed in (**e**–**h**).

An investigation on the deformation behaviour of twinned nanowires also offers valuable insights into twin–twin interactions. Figure 5 shows two different types of twin–twin junctions observed in the present study. In one junction, three TBs meet at an angle of 120◦ with respect each other as shown in Figure 5a (Y-junction), while in other junction, they meet at an angle of 60 and 240◦ forming an arrow (↓) like junction (Figure 5b). Further, the annihilation of twin boundary with zero resolved shear stress has occurred mainly near arrow like twin–twin junctions (Figure 3), which participates more actively in de-twinning mechanisms. In the past, a junction containing six TBs (six-fold twin) has been inserted and studied for its evolution under torsion in *α*-Fe [16]. However, no Y-junction or arrow like junctions have been observed. It is well known that in BCC systems, the TBs lie on {112} planes and also three of the {112} planes have the same <111> zone axis as depicted in Figure 5c. As a result of three {112} planes having the same zone axis, the twin–twin junctions such as those shown in Figure 5a,b and also the six fold twins in Ref. [16] were feasible in BCC systems. In contrast, the six-fold

twins were not compatible in FCC systems, where only up to five-fold twins were observed [24]. The arrangement of three {112} planes as shown in Figure 5c is also responsible for the direct as well as symmetrical transmission of twin across the existing twin boundary as seen in Figure 2b. In a previous study [14], it has been shown that, like twins, dislocations can also either directly transmit through the twin boundary without any deviation in slip plane or they can transmit to symmetrical plane.

**Figure 5.** Two different twin–twin junctions observed in the present study; (**a**) Y-junction, where three TBs meet at an angle of 120◦ with respect each other, (**b**) arrow (↓) like junction, where three TBs meet at an angle of 60 and 240◦, (**c**) the arrangement of {112} slip planes, when viewed along <111> direction. The atoms are coloured according to their common neighbour analysis (CNA).
