Hydrogen transport in hydride and non-hydride forming metals and the mechanistic implications for fracture behavior

Abstract

Despite intense research, a complete mechanistic understanding of the hydrogen embrittlement phenomenon has yet to be achieved. Regardless of the specific mechanism of hydrogen embrittlement, a better understanding of hydrogen transport, hydride formation and their mechanistic effects on stress and strain is needed to elucidate the role of hydrogen in the mechanics of fracture.

The thesis consists of five distinct projects, thereby allowing for the analysis of stress induced hydrogen transport in different systems under varying degrees of model complexity. The metal-hydrogen systems are modeled using a continuum mechanics approach and the solutions are obtained via iterative numerical methods. The first project examines the interaction of solute hydrogen atoms with the stress field of a sharp crack in an elastic material under equilibrium conditions. The effect of hydrogen induced volume dilatation and modulus softening on the crack tip stress fields is examined. In the next project, transient hydrogen transport and elastically accommodated hydride formation near a stationary sharp crack in an elastic material are analyzed.

The third project considers the competition between hydrostatic stress and plastic strain in determining the interstitial and trapped hydrogen concentrations near a stress concentration in an elastoplastic material. Recent experimental data are considered in the fourth project, in which parameter studies are performed to gain insight into the factors responsible for the development of enhanced hydrogen concentrations in the nickel superalloy, X-750. In the final project, transient hydrogen diffusion and hydride formation near a stationary crack tip in an elastoplastic material are examined. A formulation for determining the effect of an external stress on the terminal solid solubility of hydrogen in an elastoplastic material is presented. A criterion for fracture via hydride formation and subsequent brittle cleavage is proposed and the fracture toughness of a cracked specimen in the presence of hydrogen is shown to be dependent on the tensile loading rate.