Abdelali Oudriss

Contribution of the entropy on the thermodynamic equilibrium of vacancies in nickel.

The equilibrium vacancy concentration in nickel was determined from ab initio calculations performed with both generalized gradient approximation and local density approximation up to the melting point. We focus the study on the vacancy formation entropy expressed as a sum of a vibration and an electronic contribution, which were determined from the vibration modes and the electronic densities of states. Applying a method based on the quasi-harmonic approximation, the temperature dependence of the defect formation energy and entropy were calculated. We show that the vibrations of the first shell of atoms around the defect are predominant to the vibration formation entropy. On the other hand, the electronic formation entropy is very sensitive to the exchange-correlation potential used for the calculations. Finally, the vacancy concentration is computed at finite temperature with the calculated values for the defect formation energy and entropy. In order to reconcile point-defects concentration obtained with our calculations and experimental data, we conducted complementary calorimetric measurements of the vacancy concentration in the 1073-1273 K temperature range. Close agreement between theory and experiments at high temperature is achieved if the calculations are performed with the generalized gradient approximation and both vibration and electronic contributions to the formation entropy are taken into account.

Displacement field induced by a vacancy in nickel and some implications for the solubility of hydrogen

The displacement field induced by a vacancy in nickel and its possible implications for the hydrogen solubility in the interstitial sites is investigated up to 1100 K. The displacement field is extracted from our previous atomistic-scale density functional theory calculations and is compared with continuous isotropic models with and without image forces due to the distribution of the defects. This procedure put in forward the contribution of the elastic anisotropy and the vacancy concentration on the elastic displacement field. Then, we calculate the dilatation of the interstitial sites due to the presence of the defect and the local hydrostatic stress acting on them. This local hydrostatic stress modifies the hydrogen solubility in the interstitial sites. Elastic distortions contribute to reduce the local solubility inside the vacancy core and the opposite out of it. Finally, we extract the elastic contribution of the trapping energy of hydrogen in the displacement field of the vacancy. We find that the elastic energy is significant only for the closest interstitial sites of the defect core and limits the attractive electronic contribution to the trapping energy. In addition, we show that the elastic anisotropy and the vacancy concentration have moderate effects on the local solubility in the displacement field close to the defect core.

Segregation energy of the hydrogen at Ni Σ3 grain boundaries: some implications of the atomic volume and the interstitial self-stress

Atomistic simulations performed for the hydrogen segregation in different Ni

The diffusion and trapping of hydrogen along the grain boundaries in polycrystalline nickel

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Hydrogen solubility and vacancy concentration in nickel single crystals at thermal equilibrium: New insights from statistical mechanics and ab initio calculations

Σ3 〈110〉 tilt grain boundaries (GBs), including symmetrical and asymmetrical configurations, highlight a decreasing functional relationship between the segregation energy and the atomic volume of the GB interstitial site. The analysis of the local deformations around the hydrogen atoms showed that this relationship is not depending on the geometric form of the GB interstitial sites. For small contents of the volume change, a linear correlation was found between the segregation energy of the hydrogen solutes and the volume occupancy of the GB trapping sites. However, when interstitial volumes reach a certain critical value this proportionality presents some limits, especially for GB trapping sites that undergo considerable volume distortions. A general thermo-mechanical framework based on the atomic self-stress calculations is stated to understand locally the connection between the volume occupancy and the elastic relaxed energy from the insertion of hydrogen interstitials in

The influence of hydrostatic stress states on the hydrogen solubility in martensitic steels

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