**1. Introduction**

In recent years, twinned nanowires have attracted considerable attention for research due to their superior physical properties as compared to their perfect counterparts. Moreover, since twin boundaries (TBs) possess high symmetry and lowest interface energy, materials containing TBs exhibit better properties compared to those containing any other boundaries [1]. TBs can act both as barrier and carrier for dislocation motion, thus resulting in a simultaneous increase of strength and ductility [2]. TBs can also exhibit high thermal and mechanical stability [3], improve corrosion resistance, fracture toughness and strain rate sensitivity [4,5]. In view of this, the TBs are desirable planar defects in nanowires/nanocrystalline materials and many efforts have been made to synthesize and investigate the twinned nanowires and other nanocrystalline materials [6].

In order to understand the deformation behaviour of twinned FCC nanowires/nanopillars, many experimental and atomistic simulation studies have been carried out in the literature [2,6–9]. The results have shown that depending on the orientation of the TBs with respect to the loading direction, deformation mechanisms vary in nanowires and also many novel deformation mechanisms have been reported. For example, in nanopillars with TBs perpendicularly to the loading direction (transverse TBs), the dislocation–twin interactions like dislocation transmission, absorption, multiplication and stair-rod formations have been observed [2,6]. On the other hand, an extensive de-twinning is reported in nanopillars with slanted TBs [6]. The de-twinning is due to the easy glide of twinning partials along the TBs under finite shear stress. In addition to de-twinning, a novel pseudo-elasticity and shape memory effects have been discovered in nanowires with slanted TBs [10], which are distinct from those exhibited by the perfect nanowires [11]. The de-twinning mechanism

has also been observed in nanowires with TBs parallel to the loading axis [12]. Unlike the nanowires with slanted TBs, the de-twinning in nanopillar containing axial TBs is surprising due to zero resolved shear stress on the pre-existing twin boundary. However, using experiments and atomistic simulations, Cheng et al. [12] have reported a de-twinning mechanism in nanopillars with axial twin boundary placed closed to the surface. This de-twinning resulted in a complete annihilation of the existing twin boundary, giving rise to defect-free nanopillar. The observation of unique de-twinning mechanism has been attributed to the migration of a junction consisting of twin boundary and other high angle grain boundary [12]. Similar annihilation of twin boundary due to de-twinning mechanisms has been observed during the bending of Ni nanowires with twin thickness less than 1 nm [13]. These observations clearly indicate that the de-twinning is quite common in twinned FCC nanowires with the exception of traverse twin boundaries.

However, all the above studies were focused on twinned FCC nanowires/nanopillars and little attention has been paid towards twinned BCC nanowires. There are only a couple of investigations in the literature on BCC nanowires containing twin boundaries [14,15]. These studies on BCC nanowires with transverse TBs have shown a strong tension-compression asymmetry in deformation mechanisms. With respect to axial TBs, Li et al. [16] have investigated the role of six-fold twin on the torsional deformation of BCC Fe nanowires. Except this, no study exists on the role of axial TBs in BCC nanopillars. Further, it is interesting to examine the possibility of the occurrence of de-twinning mechanism in twinned BCC nanowires. In view of this, the present study aimed at understanding the deformation mechanisms in axially twinned BCC Fe nanowires using molecular dynamics (MD) simulations. The results show an intersecting de-twinning mechanism involving the migration of twin–twin junction, which results in a complete annihilation of pre-existing TBs and also leads to the reorientation of the nanowire.
