**2. MD Simulation Details**

The deformation behaviour of [110] BCC Fe nanowires with twin boundaries oriented parallel to the loading direction were considered in this study (Figure 1). The nanopillar is enclosed by [1-11] and [1-1-2] type side surfaces. The number of TBs is varied from one to three (Figure 1a–c). All the nanowires had a square cross section width (*d*) = 8.5 nm and length (*l*) was twice the cross section width, i.e., *l* = 17 nm. In order to mimic an infinitely long nanowire, periodic boundary conditions were chosen only along the wire axis, while the other directions were kept free. On these twinned nanowires, compression loading has been simulated using molecular dynamics (MD) simulations. MD simulations have been carried out in LAMMPS package [17] employing an embedded atom method (EAM) potential for BCC Fe given by Mendelev et al. [18]. The Mendelev EAM potential has been chosen mainly because several predictions obtained using this potential are in good agreement with experimental observations. For example, this potential correctly predicted deformation by twinning and dislocation slip based on nanopillar orientation in BCC Fe [19,20], which is quite close to the experimental findings in ultra-thin BCC W nanopillars [21]. Before applying the compressive load, energy minimization has been performed by conjugate gradient method followed by an equilibration to a required temperature of 10 K in NVT ensemble with a Nose-Hoover thermostat. Velocity verlet algorithm has been used to integrate the equations of motion with a time step of 2 femto seconds. The compressive deformation was carried out at a constant strain rate of <sup>1</sup> × <sup>10</sup><sup>8</sup> <sup>s</sup>−<sup>1</sup> with respect to initial box length. The visualization of TBs and dislocations is accomplished in AtomEye [22] and OVITO [23] packages using centro-symmetry parameter (CSP) and common neighbour analysis (CNA).

**Figure 1.** The initial configuration of BCC Fe nanowires containing (**a**) one, (**b**) two, and (**c**) three twin boundaries. The corresponding twin boundary spacings is of 4.25, 2.83 and 2.12 nm, respectively. The nanowire orientation and cross-section width (*d*) are indicated in (**a**). The atoms are coloured according to their centro-symmetry parameter (CSP).
