BCC Fe Nanowires Research Articles

Atomistic simulation of structural transition and grain refinement in Fe nanowires driven by high strain rate compression

The compression-induced structural transition (ST) and grain refinement of BCC Fe nanowires have been investigated based on atomistic simulations. It is found that high strain rate compression can cause nanowires to reach higher pressures and meet ST conditions, and the threshold stress of ST is found to be much lower than that of bulk materials. Thus, the compression process of nanowires at high strain rates includes elastic deformation, ST and its reverse process, grain refinement, and buckling instability. The occurrence of grain refinement is due to the fact that after undergoing ST and its reverse process, the (001) cross section of the nanowire can be transformed into different (111) planes. Furthermore, the dependence of stress threshold and nucleation structure on strain rate is revealed. When the strain rate increases to a certain threshold, HCP nucleation is found to occur on the side surface of the nanowire, but its reverse process will quickly occur due to the pressure release on the side. With the strain rate increase, the stress threshold of ST no longer satisfies a constant power-law change, and the power-law index will increase. When the strain rate exceeds 5 × 1010 s−1, the elastic deformation prior to ST also exhibits strong nonequilibrium characteristics, causing a sharp increase in the number of HCP nuclei. Especially, the nanowires will ultimately be in a disordered state, rather than a nanocrystalline structure. Also, the cylindrical and prismatic nanowires are both considered to understand the influence of boundary morphology, and the differences in nucleation and the similarity in deformation are explained. Note that, as the strain rate increases, the cylindrical nanowires undergo HCP nucleation directly, while the prismatic nanowires undergo significant twinning deformation first.

Competition between full slip and twinning in BCC-Fe: Effect of preloaded stress and temperature

Slip or twinning is one of the fundamental questions in the deformation studies of metals and alloys. Internal parameters such as generalized stacking fault energy and size and external parameters such as pressure, strain rate, and temperature influence the competition between the full slip and twinning, thus dictating the predominance of one mechanism over the other. In the present investigation, we studied the influence of preloaded stress and temperature on the deformation behavior of BCC-Fe nanowires using molecular dynamics simulations and theoretical analysis. Based on detailed investigations into the energetics associated with slip and twinning, we observed that twinning is the preferred deformation mechanism in BCC-Fe. However, this has been modified by preloaded stresses applied in normal, transverse, and both directions on the nanowire. We observed a slip on {110}, on {112}, and even on {123} planes. The temperature did not alter the inherent twinning nature but linearly decreased the various fault energies.

Plasticity through De-Twinning in Twinned BCC Nanowires

The deformation behaviour of twinned FCC nanowires has been extensively investigated in recent years. However, the same is not true for their BCC counterparts. Very few studies exist concerning the deformation behaviour of twinned BCC nanowires. In view of this, molecular dynamics (MD) simulations have been performed to understand the deformation mechanisms in twinned BCC Fe nanowires. The twin boundaries (TBs) were oriented parallel to the loading direction [110] and the number of TBs is varied from one to three. MD simulation results indicate that deformation under the compressive loading of twinned BCC Fe nanowires is dominated by a unique de-twinning mechanism involving the migration of a special twin–twin junction. This de-twinning mechanism results in the complete annihilation of pre-existing TBs along with reorientation of the nanowire. Further, it has been observed that the annihilation of pre-existing TBs has occurred through two different mechanisms, one without any resolved shear stress and other with finite and small resolved shear stress. The present study enhances our understanding of de-twinning in BCC nanowires.

Twinning to slip transition in ultrathin BCC Fe nanowires

We report twinning to slip transition with decreasing size and increasing temperature in ultrathin <100> BCC Fe nanowires. Molecular dynamics simulations have been performed on different nanowire size in the range 0.404–3.634 nm at temperatures ranging from 10 to 900 K. The results indicate that slip mode dominates at low sizes and high temperatures, while deformation twinning is promoted at high sizes and low temperatures. The temperature, at which the nanowires show twinning to slip transition, increases with increasing size. The different modes of deformation are also reflected appropriately in the respective stress–strain behaviour of the nanowires.

Atomistic simulations on ductile-brittle transition in ⟨111⟩ BCC Fe nanowires

Molecular dynamics simulations have been performed to understand the influence of temperature on the tensile deformation and fracture behavior of ⟨111⟩ BCC Fe nanowires. The simulations have been carried out at different temperatures in the range 10–1000 K employing a constant strain rate of 1 × 108 s−1. The results indicate that at low temperatures (10–375 K), the nanowires yield through the nucleation of a sharp crack and fails in brittle manner. On the other hand, nucleation of multiple 1/2⟨111⟩ dislocations at yielding followed by significant plastic deformation leading to ductile failure has been observed at high temperatures in the range 450–1000 K. At 400 K, the nanowire yields through nucleation of crack associated with many mobile 1/2⟨111⟩ and immobile ⟨100⟩ dislocations at the crack tip and fails in ductile manner. The ductile-brittle transition observed in ⟨111⟩ BCC Fe nanowires is appropriately reflected in the stress-strain behavior and plastic strain at failure. The ductile-brittle transition increases with increasing nanowire size. The change in fracture behavior has been discussed in terms of the relative variations in yield and fracture stresses and change in slip behavior with respect to temperature. Further, the dislocation multiplication mechanism assisted by the kink nucleation from the nanowire surface observed at high temperatures has been presented.

Formation of pentagonal atomic chains in BCC Fe nanowires

For the first time, we report the formation of pentagonal atomic chains during tensile deformation of ultra thin BCC Fe nanowires. Extensive molecular dynamics simulations have been performed on 〈100〉/{110} BCC Fe nanowires with different cross section width varying from 0.404 to 3.634 nm at temperatures ranging from 10 to 900 K. The results indicate that above certain temperature, long and stable pentagonal atomic chains form in BCC Fe nanowires with cross section width less than 2.83 nm. The temperature, above which the pentagonal chains form, increases with increase in nanowire size. The pentagonal chains have been observed to be highly stable over large plastic strains and contribute to high ductility in Fe nanowires.

Influence of twist boundary on deformation behaviour of 〈1 0 0〉 BCC Fe nanowires

Molecular dynamics simulations revealed significant difference in deformation behaviour of 〈1 0 0〉 BCC Fe nanowires with and without twist boundary. The plastic deformation in perfect 〈1 0 0〉 BCC Fe nanowire was dominated by twinning and reorientation to 〈1 1 0〉 followed by further deformation by slip mode. On the contrary, 〈1 0 0〉 BCC Fe nanowire with a twist boundary deformed by slip at low plastic strains followed by twinning at high strains and absence of full reorientation. The results suggest that the deformation in 〈1 0 0〉 BCC Fe nanowire by dislocation slip is preferred over twinning in the presence of initial dislocations or dislocation networks. The results also explain the absence of extensive twinning in bulk materials, which inherently contains large number of dislocations.

Finite size effects on the magnetocrystalline anisotropy energy in Fe magnetic nanowires from first principles

The geometric and the electronic structures, the magnetic moments, and the magnetocrystalline anisotropy energy of bcc-Fe nanowires with z-axis along the (110) direction are calculated in the framework of ab initio theories. In particular, we report a systematic study of free standing nanowires with geometries and sizes ranging from diatomic to 1 nm wide with 31 atoms per unit cell. We found that for nanowires with less than 14 atoms per unit cell, the ground-state structure is body-centered tetragonal. We also calculated the contributions of the dipolar magnetic energy to the magnetic anisotropy energy and found that in some cases, this contribution overcomes the magnetocrystalline part, determining thereby the easy axis direction. These results emphasize the importance and competition between both contributions in low dimensional systems.

A new look at the atomic level virial stress: on continuum-molecular system equivalence

The virial stress is the most commonly used definition of stress in discrete particle systems. This quantity includes two parts. The first part depends on the mass and velocity (or, in some versions, the fluctuation part of the velocity) of atomic particles, reflecting an assertion that mass transfer causes mechanical stress to be applied on stationary spatial surfaces external to an atomic‐particle system. The second part depends on interatomic forces and atomic positions, providing a continuum measure for the internal mechanical interactions between particles. Historic derivations of the virial stress include generalization from the virial theorem of Clausius (1870) for gas pressure and solution of the spatial equation of balance of momentum. The virial stress is stress‐like a measure for momentum change in space. This paper shows that, contrary to the generally accepted view, the virial stress is not a measure for mechanical force between material points and cannot be regarded as a measure for mechanical stress in any sense. The lack of physical significance is both at the individual atom level in a time‐resolved sense and at the system level in a statistical sense. It is demonstrated that the interatomic force term alone is a valid stress measure and can be identified with the Cauchy stress. The proof in this paper consists of two parts. First, for the simple conditions of rigid translation, uniform tension and tension with thermal oscillations, the virial stress yields clearly erroneous interpretations of stress. Second, the conceptual flaw in the generalization from the virial theorem for gas pressure to stress and the confusion over spatial and material equations of balance of momentum in theoretical derivations of the virial stress that led to its erroneous acceptance as the Cauchy stress are pointed out. Interpretation of the virial stress as a measure for mechanical force violates balance of momentum and is inconsistent with the basic definition of stress. The versions of the virial‐stress formula that involve total particle velocity and the thermal fluctuation part of the velocity are demonstrated to be measures of spatial momentum flow relative to, respectively, a fixed reference frame and a moving frame with a velocity equal to the part of particle velocity not included in the virial formula. To further illustrate the irrelevance of mass transfer to the evaluation of stress, an equivalent continuum (EC) for dynamically deforming atomistic particle systems is defined. The equivalence of the continuum to discrete atomic systems includes (i) preservation of linear and angular momenta, (ii) conservation of internal, external and inertial work rates, and (iii) conservation of mass. This equivalence allows fields of work‐ and momentum‐preserving Cauchy stress, surface traction, body force and deformation to be determined. The resulting stress field depends only on interatomic forces, providing an independent proof that as a measure for internal material interaction stress is independent of kinetic energy or mass transfer.