Formation, structure and magnetism of the γ-(Fe,M)23C6 (M = Cr, Ni) phases: A first-principles study

The γ-(Fe,M)23C6 phases constitute an important class of iron carbides. They occur both as precipitates in steels and iron alloys, thereby increasing their strength, and as common minerals in meteorites and in iron-rich parts of the Earth's mantle. Here we investigate the composition-dependent relative stability of these phases and the role of magnetism therein. The γ-(Fe,M)23C6 phases have mineral names isovite (M = Cr) and haxonite (M = Ni), and have a complex crystal structure (116 atoms in the cubic unit cell) in which the metal atoms have a rich variety of atomic coordination numbers, ranging from 12 to 16. First-principles calculations show a narrow formation range for γ-(Fe1−xNix)23C6 (x = 0–0.043), while the formation range for γ-(Fe1−xCrx)23C6 is very broad (x = 0–0.85), in good agreement with available experimental data. The present study also shows the importance of magnetism on the formation and stability of these compounds. The conditions of formation and several factors enhancing or hampering the formation of γ-(Fe,M)23C6 in man-made steels and in meteorites are discussed.MvH acknowledges the Dutch Science Foundation NWO for a VIDI (Grant No. 723.012.006)

Interaction of Hydrogen with Graphitic Surfaces, Clean and Doped with Metal Clusters

Producción CientíficaHydrogen is viewed as a possible alternative to the fossil fuels in transportation. The technology of fuel-cell engines is fully developed, and the outstanding remaining problem is the storage of hydrogen in the vehicle. Porous materials, in which hydrogen is adsorbed on the pore walls, and in particular nanoporous carbons, have been investigated as potential onboard containers. Furthermore, metallic nanoparticles embedded in porous carbons catalyze the dissociation of hydrogen in the anode of the fuel cells. For these reasons the interaction of hydrogen with the surfaces of carbon materials is a topic of high technological interest. Computational modeling and the density functional formalism (DFT) are helping in the task of discovering the basic mechanisms of the interaction of hydrogen with clean and doped carbon surfaces. Planar and curved graphene provide good models for the walls of porous carbons. We first review work on the interaction of molecular and atomic hydrogen with graphene and graphene nanoribbons, and next we address the effects due to the presence of metal clusters on the surface because of the evidence of their role in enhancing hydrogen storage.Ministerio de Economía, Industria y Competitividad (Grant MAT2014-54378-R

The effectiveness of reference-free modified embedded atom method potentials demonstrated for NiTi and NbMoTaW

One of the effective potentials that has proven to be very versatile and useful for describing metals is the modified embedded atom method (MEAM) potential. The reference-free version of the MEAM (RF-MEAM) potential provides more flexibility for fitting than the 2NN-MEAM because it also describes the pair potential as an explicit function. In this work, we present a methodology to fit RF-MEAM potentials to DFT data. We then evaluate the performance of the fitted potential by comparing MD simulations with experimental and DFT data. As an example, the methodology is applied to a binary and a quaternary alloy, namely NiTi and NbMoTaW. In the case of the equi-atomic NiTi shape memory alloy, our attention focuses on designing a potential that properly captures its mechanical behavior, given that the existing potentials fail to predict elastic constants in agreement with experiments. To reach our aim, we included the stress tensors of different high temperature NiTi configurations in the fitting database. The obtained RF-MEAM potential outperforms existing EAM and MEAM potentials in predicting the lattice and elastic constants of austenitic and martensitic phases as well as the corresponding transformation temperatures. To demonstrate the suitability of this methodology also for more complex systems, a RF-MEAM potential is fitted to model the multi-component NbMoTaW high-entropy alloy. Validation is achieved through comparison between observables obtained through the MD output and ab initio data. The article also reports key improvements to the optimization code MEAMfit v2 and the freely-available LAMMPS implementation of the RF-MEAM formalism. Most notably, resorting to analytic derivatives of the objective function with respect to the potential parameters rather than derivatives through finite differences, the time necessary for fitting has decreased by an order of magnitude