Toughness Scale from First Principles

We correlate the experimentally measured fracture toughness of 24 metals and ceramics to their quantum mechanically calculated brittleness parameter. The brittleness parameter is defined as the ratio of the elastic energy density needed to spontaneously break bonds in shear versus in tension, and is a primitive-cell property. Under 300 GPa hydrostatic pressure, the model predicts that diamond has smaller brittleness than molybdenum at zero pressure, and thus should deform plastically without cracking at room temperature

Hydrostatic compression and high-pressure elastic constants of coesite silica

Using density-functional theory, we computed all the independent elastic constants of coesite, a high-pressure polymorph of silica, as functions of pressure up to 15 GPa. The results are in good agreement with experimental measurements under ambient conditions. Also, the predicted pressure-dependent elastic properties are consistent with x-ray data in the literature concerning lattice strains at high pressures. We find that coesite, like quartz, exhibits a gradual softening of a shear modulus B44 with increasing pressure, in contrast to the rising bulk modulus

Shuffling-controlled versus strain-controlled deformation twinning: The case for HCP Mg twin nucleation

The atomistic pathways of deformation twinning can be computed ab initio, and quantified by two variables: strain which describes shape change of a periodic supercell, and shuffling which describes non-affine displacements of the internal degrees of freedom. The minimum energy path involves juxta-position of both. But if one can obtain the same saddle point by continuously increasing the strain and relaxing the internal degrees of freedom by steepest descent, we call the path strain-controlled, and vice versa. Surprisingly, we find the {101¯2}〈101¯1¯〉 twinning of Mg is shuffling-controlled at the smallest lengthscale of the irreducible lattice correspondence pattern, that is, the reaction coordinate at the level of 4 atoms is dominated by non-affine displacements, instead of strain. Shuffling-controlled deformation twinning is expected to have different temperature and strain-rate sensitivities from strain-controlled deformation twinning due to relatively weaker strength of long-range elastic interactions, in particular at the twin nucleation stage. As the twin grows large enough, however, elastic interactions and displacive character of the transformation should always turn dominant

“Conjugate Channeling” Effect in Dislocation Core Diffusion: Carbon Transport in Dislocated BCC Iron

Dislocation pipe diffusion seems to be a well-established phenomenon. Here we demonstrate an unexpected effect, that the migration of interstitials such as carbon in iron may be accelerated not in the dislocation line direction , but in a conjugate diffusion direction. This accelerated random walk arises from a simple crystallographic channeling effect. is a function of the Burgers vector b, but not , thus a dislocation loop possesses the same everywhere. Using molecular dynamics and accelerated dynamics simulations, we further show that such dislocation-core-coupled carbon diffusion in iron has temperature-dependent activation enthalpy like a fragile glass. The 71° mixed dislocation is the only case in which we see straightforward pipe diffusion that does not depend on dislocation mobility.Ishii A, Li J, Ogata S (2013) “Conjugate Channeling” Effect in Dislocation Core Diffusion: Carbon Transport in Dislocated BCC Iron. PLOS ONE 8(4): e60586. https://doi.org/10.1371/journal.pone.006058

Highly efficient and transferable interatomic potentials for {\alpha}-iron and {\alpha}-iron/hydrogen binary systems using deep neural networks

Artificial neural network potentials (NNPs) have emerged as effective tools for understanding atomic interactions at the atomic scale in various phenomena. Recently, we developed highly transferable NNPs for {\alpha}-iron and {\alpha}-iron/hydrogen binary systems (Physical Review Materials 5 (11), 113606, 2021). These potentials allowed us to investigate deformation and fracture in {\alpha}-iron under the influence of hydrogen. However, the computational cost of the NNP remains relatively high compared to empirical potentials, limiting their applicability in addressing practical issues related to hydrogen embrittlement. In this work, building upon our prior research on iron-hydrogen NNP, we developed a new NNP that not only maintains the excellent transferability but also significantly improves computational efficiency (more than 40 times faster). We applied this new NNP to study the impact of hydrogen on the cracking of iron and the deformation of polycrystalline iron. We employed large-scale through-thickness {110} crack models and large-scale polycrystalline {\alpha}-iron models. The results clearly show that hydrogen atoms segregated at crack tips promote brittle-cleavage failure followed by crack growth. Additionally, hydrogen atoms at grain boundaries facilitate the nucleation of intergranular nanovoids and subsequent intergranular fracture. We anticipate that this high-efficiency NNP will serve as a valuable tool for gaining atomic-scale insights into hydrogen embrittlement

Adaptive-boost molecular dynamics simulation of carbon diffusion in iron

We have developed an accelerated molecular dynamics (MD) method to model atomic-scale rare events. In this method, a smooth histogram of collective variables is first estimated by canonical ensemble molecular dynamics calculations, and then a temperature-dependent boost potential is iteratively constructed to accelerate the MD simulation. This method not only allows us to observe the rare events but also to evaluate the profile of free energy and trial frequency along the reaction coordinate. We employed this method to study carbon diffusion in bcc iron and evaluated carbon's temperature-dependent diffusivity. The obtained diffusivities agree well with the experimental measurements. Even at low temperature for which, to the best of our knowledge, no experimental data are available, the diffusivity can be evaluated accurately. Additionally, we study carbon diffusion inside the edge dislocation core in bcc iron, and demonstrate the applicability of the method to rare events on a rugged free-energy surface.Ishii A., Ogata S., Kimizuka H., Li J. Adaptive-boost molecular dynamics simulation of carbon diffusion in iron (2012) Physical Review B - Condensed Matter and Materials Physics, 85 (6), 064303, DOI: 10.1103/PhysRevB.85.064303