Abstract The coupled diffusion and deformation characterizing diffusive processes on the nanoscale provide significant challenges to many simulation methods due to the combination of fast atomic vibrations and slow diffusion. Diffusive molecular dynamics (DMD) models such processes by a scale separation modeling the thermal vibrations as a spatial position probability distribution and diffusion as atom sites with variable concentrations, which evolve based on kinetic equations. This study introduces a novel type of fully anisotropic spatial probability distribution to improve the approximations for atom sites with anisotropic potential energy wells, e.g. close to surfaces or lattice defects and atoms in lattices with a relatively high degree of intrinsic anisotropy. A comparative study between the classic isotropic and newly introduced anisotropic probability distributions is conducted for the case of magnesium hydride with molecular dynamics simulations using the same interatomic potential serving as reference solution. Tests range from lattice and elastic constants of perfect crystals to more complex cases like the bending of nanobeams and reveal previously unreported shortcomings of isotropic DMD, leading to a loss of lattice symmetry and nonphysical behavior. The results obtained using anisotropic shape functions avoid this behavior and follow the reference solution significantly better while maintaining comparable computational effort. This confirms the requirement for DMD to take anisotropic vibrations into account when simulating magnesium hydride and other systems with anisotropic atom environments.
Abstract Elucidating the mechanism of electroplasticity is essential for the widespread application of electrically assisted forming technology to hard-to-deform metallic components. In this study, a two-dimensional discrete dislocation dynamics (DDD) model incorporating the electroplastic effect is developed by introducing non-uniform temperature fields induced by Joule heating. The effects of Joule heating and the electron wind on dislocation nucleation, glide, and thermal activation escape are systematically quantified from both thermal and energetic perspectives. Dislocation evolution in titanium alloy single crystals subjected to electric current during uniaxial tensile deformation is investigated across both high and low strain rate regimes using this electroplastic-DDD framework. The results show that the non-uniform temperature fields modify the dislocation source strength and drag coefficient, thereby influencing dislocation nucleation and glide. Furthermore, the additional Joule heating energy E Joule and electron wind energy E wind supplied to dislocations reduce the energy barrier for thermal activation escape, promoting dislocation depinning. It is demonstrated that the contribution of E wind is approximately seven orders of magnitude smaller than that of E Joule , with Joule heating identified as the dominant mechanism governing the electroplastic response in titanium alloys.
Abstract Generally, models of spin-wave transport through magnetic junctions only consider the exchange between nearest neighbors. However, real thin films also exhibit coupling between second-order neighbors ( NNN ), ferromagnetic (FM) in ordinary films and antiferromagnetic (AFM) in frustrated ones, the influence of which at the device level has been observed. In this work, we examine how the NNN -contribution reshapes the spin wave response of a four-terminal asymmetric cross junction, with slabs of four, three, three and two atomic layers wide (the 4–3–3–2 system). The device is treated within the Heisenberg ferromagnet and solved by two methods: the analytic phase field matching theory (PFMT) and the tight-binding scattering solver Kwant. At a realistic strength, diagonal bonds leave the transport band gap-free and strictly reciprocal, while renormalizing it in a sign-dependent manner. FM NNNs raise the top of the conduction band, increasing the cutoff frequency of the wider slab from 7.41 to 7.57 J , and slow down the antisymmetric channel of the narrow slab by destructive interference of its two longitudinal hopping paths; AFM NNNs lower the band top and accelerate this channel. The frequency at which each slab closes, and thus ceases to route a signal, shifts accordingly. Whereas, if the coupling is stronger, the same diagonal links open a width-selective stopping band that transforms the cross into a frequency-selective reflector, a qualitatively new transport function. The sign of the second neighbor exchange thus shapes the spectral landscape of the junction, quantitatively at the realistic strength and qualitatively when driven harder, and is not a negligible correction.
Abstract The shear slip deformation mechanism of 4H‐SiC was investigated at various shear velocities and temperatures. A novel fine structure was observed following the dissociation of perfect dislocations, involving displacement and reconstruction on the high‐atomic‐density {0‐110} planes, with a graphene‐like transient intermediate configuration serving as a bridging intermediate. Elevated temperatures promote an increase in number of dislocations; however, lattice distortion and rotation are more pronounced under high‐temperature and high‐strain‐rate conditions than at room temperature. The dominant behaviour of basal dislocations remains glide of perfect dislocations at 300 K. In contrast, high temperatures facilitate dislocation dissociation and stacking fault formation. Thermal softening can effectively reduce the shear slip resistance of 4H‐SiC, but the extent of reduction is closely related to the shear velocity, and the relationship between the two is not monotonic.
Abstract Molecular dynamics is used to establish how the sign of the Burgers vector controls the interaction of a Frank dislocation loop with a supersonic solitary wave train in L1 2 Pt 3 Al. Interstitial loops ( b = + a /3〈111〉, radius R = 8, 12 and 16 Å) are exposed to the same [001] wave source, in the same 1.15-million-atom cell, as the vacancy loops of our previous study, so the two signs are compared one to one. Neither sign scatters the wave appreciably (normalised transmission 0.98–1.02 over A = 0.18–0.50 Å). The energy balance behaves differently. Vacancy loops absorb a little energy, +0.6 to +4.5 eV, growing with both R and A . The R = 16 Å interstitial loop does the opposite on another scale entirely: arrival of the wave front at t ≈ 11.5 ps triggers an avalanche-like relaxation that releases 50–58 eV (0.41–0.48 eV per inserted atom, about 1.1 J m −2 ) almost independently of amplitude, while the faulted ring is rebuilt 32–39 Å away within its 〈110〉 glide prism by a conservative mechanism. At R = 8 Å the strongest wave partially unfaults the loop (Frank → perfect), and R = 12 Å loops remain stable weak absorbers. Three further radii place the change of role between 14 and 16 Å. Detectors 60–300 Å away record the released energy leaving at 4.6 km s −1 , and the 20 Å analysis region captures only 69% of the potential-energy drop, so the quoted release is a lower bound. At 300 K piston heating masks the calorimetric signature, but the enhanced downstream displacement survives. The Burgers-vector sign thus switches a dislocation loop between two roles: an accumulator of wave energy and a wave-triggered emitter of stored energy. The asymmetry is geometric, whereas the absolute energies and the threshold radius are specific to the potential employed.
