Hydrogen at cracks

Motivation

If a metal is subjected to a hydrogen atmosphere, it can diffuse into it after dissociation at the surface. Materials typically possess many defects like voids, dislocations and cracks, and the hydrogen accumulates not only in the interstitial sites of the lattice but also near these defects. For a material under tension, the interstitial sites are widened, thus more space is available for the hydrogen, thus there is a higher tendency for hydrogen saturation. This effect is most pronounced in the tensile regions of dislocations and cracks, which can even be enhanced by an attractive H-H interaction, leading to local hydride formation. Since its cleavage behavior presumably differs from that of the matrix, one can expect serious influence on the fracture behavior.

Central questions are therefore:

• How do the elastic properties change by the presence of hydrogen?
• How fast can hydrogen diffuse towards a crack tip?
• Under which circumstances is the development of a hydride zone near a crack tip possible?
• How fast does this hydride zone grow and lead to embrittlement?

Fundamental principles

The understanding of hydrogen induced failure in metals and alloys is one of the central and unresolved problems in Materials Science. By hydrogen embrittlement one understands the change of ductility of metals by the incorporation of hydrogen in the lattice, which can cause crack formation. Hereby, atomic hydrogen (which has dissociated before via chemcial reactions at the surfaces) invades into the metal and can recombine to molecular hydrogen at defects. This can lead to a pressure increase and allows cracks to grow from inside the material. Many steels suffer from hydrogen embrittlement; austenitic steels are less sensitive, but in particular high strength steels are often strongly affected. This issue has gained additional attention by the technological advances in fuel cells for automotive applications.

To to its complexiity, hydrogen embrittlement cannot be attributed to just one mechanism. Instead, during the past 100 years of research on this issue, several phenomena have been identified as key mechanisms for hydrogen induced material failure. In brief, important concepts in the room temperature regime are

• HEDE hydrogen enhanced decohesion
• HELP hydrogen enhanced localized plasticity
• Hydride phase formation and brittle fracture

Recent achievements

We have developed a generic continuum model for the accumulation of hydrogen near a crack tip. It takes into account the long-range mechanical deformations, configurational entropy terms, and attractive H-H interaction, diffusion and lattice expansion through hydrogen. This complex coupled problem is then solved numerically using finite difference methods. As a result we have obtained predictions for the size of the hydride zone near a crack tip as function of the tensile load of a mode I crack, as well as the hydrogen partial pressure. The H-H interaction is a central ingredient for the hydride formation, as it allows to have phase coexistence between the hydrogen-poor matrix and the almost fully saturated H-rich region near the crack tip. The tensile strain effectively leads to an increase of the chemical potential of hydrogen, which favors the hydride formation. Analytical scaling laws have been derived for the size of the hydride cluster as function of the stress intensity factor and the hydrogen partial pressure.

This project is related to the DFG Collaborative Research Center 761 "Stahl - ab initio".