Mechanics of hydrogen-dislocation-impurity interactions: part 1- increasing shear modulus

Abstract

The effect of hydrogen on dislocation-dislocation and dislocation-impurity atom interactions is studied under conditions where hydrogen is in equilibrium with local stresses and in systems where hydrogen increases the shear modulus. In the case of two edge dislocations (plain strain) the effect of hydrogen is modeled through a continous distribution of dilatation lines whose strength depends on the local hydrogen concentration. The hydrogen distribution in the atmospheres is adjusted to minimize the energy of the system as the dislocations approach each other. The iterative finite element analysis used to calculate the hydrogen distribution accounts for the stress relaxation associated with the hydrogen induced volume and the elastic moduli changes due to hydrogen. The interactions between the dislocations are calculated accounting for all the stress fields due to dislocations and hydrogen atmospheres. An analytical formula is suggested for the hydrogen induced reduction in the magnitude of the shear stress exerted between the dislocations along the slip system. Modeling of the hydrogen effect on the edge dislocation-interstitial solute atom interaction is discussed using a finite element analysis and a formula is developed for the calculation of the dislocation-solute atom interaction energy in the presence of hydrogen. In this paper the numerical results are presented for the case where hydrogen increases the shear modulus of the metallic system. A significant decrease of the edge dislocation-intersititial solid atom interaction energy was observed when the dislocation-solute distance is approximately less than 2 Burgers vectors. This can be attributed almost entirely to the modulus change due to hydrogen. The effect of hydrogen on the screw dislocation-intersitial solute interaction was investigated. Numerical results indicate that, depeneding on the orientation of the tetragonal axis of the carbon distortion field, hydrogen may strengthen or weaken the interaction. The present model provides strong support for the hydrogen shielding mechanism wherby hydrogen diminishes the local stress fields from dislocation and solutes which act as barriers to the dislocation motion