arXiv subjects

Ivaylo H. Katzarov

Publications and source records attributed to Ivaylo H. Katzarov.

4 recordsLinked to original sources

arXiv2019

The influence of hydrogen on plasticity in pure iron-theory and experiment

Tensile stress relaxation is combined with transmission electron microscopy to reveal dramatic changes in dislocation structure and sub structure in pure alpha iron as a result of the effects of dissolved hydrogen. We find that hydrogen charged specimens after plastic deformation display a very characteristic pattern of trailing dipoles and prismatic loops which are absent in uncharged pure metal. We explain these observations by use of a new self consistent kinetic Monte Carlo model, which in fact was initially used to predict the now observed microstructure. The results of this combined theory and experimental study is to shed light on the fundamental mechanism of hydrogen enhanced localised plasticity.

cond-mat.mtrl-sci↗

arXiv2019

The influence of hydrogen core force shielding on dislocation junctions in iron

The influence of hydrogen on dislocation junctions was analysed by incorporating a hydrogen dependent core force into nodal based discrete dislocation dynamics. Hydrogen reduces the core energy of dislocations, which reduces the magnitude of the dislocation core force. We refer to this as hydrogen core force shielding, as it is analogous to hydrogen elastic shielding but occurs at much lower hydrogen concentrations. The dislocation core energy change due to hydrogen was calibrated at the atomic scale accounting for the nonlinear inter-atomic interactions at the dislocation core, giving the model a sound physical basis. Hydrogen was found to strengthen binary junctions and promote the nucleation of dislocations from triple junctions. Simulations of microcantilever bend tests with hydrogen core force shielding showed an increase in the junction density and subsequent hardening. These simulations were performed at a small hydrogen concentration realistic for bcc iron.

cond-mat.mtrl-sci↗

arXiv2014

Quantum rate theory of the trapping of hydrogen and deuterium by a vacancy in iron

We apply quantum rate theory to calculate the transition rates as hydrogen or deuterium atoms escape from a vacancy trap in iron into a neighbouring metastable site. We determine transition rates and corresponding activation energies over a wide range of temperatures covering both the quantum and classically dominated regimes. We find that quantum effects lead to an increase of the transition rate activation energy and to very significant recrossing of the transition state dividing surface. As a result of recrossing quantum transition state theory overestimates the rate of proton transfer by more than an order of magnitude and the rate of deuteron transfer by a factor of two.

cond-mat.mtrl-sci↗

arXiv2010

Is the pinning of ordinary dislocations in gamma-TiAl intrinsic or extrinsic in nature? A combined atomistic and kinetic Monte Carlo approach

We address the question of the observed pinning of 1/2 <110] ordinary screw dislocations in gamma-TiAl which leads to the characteristic trailing of dipoles in the microstructure. While it has been proposed that these may be variously intrinsic or extrinsic in nature, we are able to rule out the former mechanism. We do this by means of very large scale, three dimensional atomistic simulations using the quantum mechanical bond order potential. We find that the kink-pair formation energy is large: 6eV, while the single kink migration energy is conversely very small: 0.13eV. Using these, and other atomistically derived data, we make kinetic Monte Carlo simulations at realistic time and length scales to simulate dislocation mobility as a function of stress and temperature. In the temperature range of the stress anomaly in gamma-TiAl, we determine whether one or several of the pinning and unzipping processes associated with generation of jogs are observed during our simulations. We conclude that the pinning of ordinary dislocations and anomalous mechanical behaviour in gamma-TiAl must be attributed to a combination of extrinsic obstacles and extensive cross-slip in a crystal containing impurities.

cond-mat.mtrl-sci↗