Molecular Dynamics Simulation of the Effect of Twin Thickness on the Mechanical Behavior of Nanotwinned BCC Fe

Nanotwinned BCC Fe exhibits a high density of grain boundaries, which act as efficient sinks that promote the recombination of irradiation-induced defects; thus, it has the potential to be applied in irradiation environments. Given that the twin thickness of BCC Fe governs the density of grain boundaries, it represents a critical parameter for practical applications. In this study, molecular dynamics simulations were used to investigate the mechanical properties of nanotwinned BCC Fe and to explore the effects of twin thickness on microstructural evolution at the atomic scale under tensile deformation. The simulations were performed at room temperature (300 K) using the parallel MD package LAMMPS. The results indicated that the peak stress (σpeak) increased with increasing twin thickness, gradually approaching saturation for twin thicknesses ranging from 20 to 44.8 nm. During tension, the atomic fraction with an FCC structure within twin boundary (TB) regions gradually increased with increasing strain before the peak strain (εpeak) was reached. Once the strain exceeded εpeak, the atomic fraction with an FCC structure increased sharply to a value approaching its maximum value, accompanied by dislocation nucleation from the TBs, and subsequently decreased sharply to its minimum value, accompanied by an increase in dislocation emission and the formation of a new BCC phase within the twin interiors. The atomic fraction with a BCC structure was calculated in the models before loading, and the atomic fraction with an FCC structure was calculated as the strain approached its maximum value. The analysis indicated that the variations of the BCC and FCC fractions with twin thickness paralleled those of σpeak and εpeak with twin thickness, respectively.