A multiscale study of the the H enhanced local plasticity (HELP) mechanism
J. von Pezold, L. Lymperakis and J. Neugebauer
Abstract
A multiscale approach towards an atomistic understanding of hydrogen-induced embrittlement of metals is introduced, combining ab-initio simulations, semiempirical atomistic potentials, as well as a simple lattice gas Hamiltonian. This hierarchical approach is used to investigate the hydrogen-enhanced local plasticity (HELP) mechanism that is based on the assertion that a local increase in plasticity induces macroscopic embrittlement.
Introduction
- Stainless Steel bar (a) before and (b) after exposure to hydrogen [2]
H-embrittlement can be loosely defined as a structural deterioration of materials under the influence of hydrogen. The effect has been observed in fcc, bcc and hcp materials and is particularly pronounced in high strength steels. The mechanisms underlying the observed embrittlement are still not fully understood, despite extensive research efforts in the field for over a century [1].
A range of proposed Mechanisms
A number of mechanisms have been suggested to account for the observed H-induced failure, including
- Stress induced hydride formation
- Hydrogen induced decohesion
- Hydrogen enhanced local plasticity.
The stress-induced hydride formation mechanism is relatively well-established for hydride forming metals and asserts that the formation of the hydride phase results in the embrittlement of the material. In particular, cracks tend to propagate through the generally brittle metal hydride phase or along the hydride-matrix interface.
The hydrogen induced decohesion theory is based on the proposition that solute H atoms reduce the cohesive energy of the metal matrix and hence lowers the energy to cleave the crystal along certain crystallographic planes, grain boundaries or phase boundaries.
The distinctly plastic nature of the fracture surface in a whole range of metals led to the development of the hydrogen enhanced local plasticity (HELP) mechanism. It is based on the assertion that the mobility and formation energy of dislocations is reduced in regions of high H content, such as notches or crack tips. The corresponding increase in plasticity is therefore localised to these regions, resulting in slip localisation, constrained plasticity and finally localised plastic fracture.
Stress shielding and modulus effect
On an atomistic level the HELP mechanism is based on the so-called stress shielding and modulus effects. The H induced stress shielding effect results in increased dislocation mobility by shielding the repulsive stress-mediated interaction between dislocations. The modulus effect, on the other hand, describes a reduction in the shear modulus, as well as in the stacking fault energy of metals in the presence of hydrogen, which results in a reduction in the Peierl’s and dislocation nucleation barrier.
The stress shielding effect is supported by experimental evidence based on in-situ TEM studies, which revealed increased dislocation velocities in the presence of hydrogen, as well as reduced dislocation-dislocation separations in dislocation pile-ups at dislocation barriers, such as grain boundaries. These experimental observations led to the development of a semi-quantitative model based on continuum mechanics simulations that predicted significantly reduced dislocation-dislocation interactions in the presence of hydrogen. The model treats the H-dislocation interaction using a mean field approach solely based on the elastic interaction between the dilatational stain field of interstitial H atoms and the compressive and tensile strain fields of the dislocation, while H-H interactions, which have been shown to significantly affect H distributions around dislocations [3], are disregarded.
- Hydrogen distribution in the strain field of an edge dislocation in Ni
Aims
The aim of the current study is to validate the stress shielding effect on an atomistic level, taking H-H interactions fully into account. NiH was used as a model system, as H-induced, locally plastic fracture has been observed in Ni [4] and well-established semi-empirical (EAM) interaction potentials [5,6] are available for this system.
A multiscale approach for a scale-transcending problem
The stress-shielding mechanisms is challenging to model on an atomistic level, as it involves interactions at various length scales, which necessitates a multi-scale approach. In particular, the atomic metal-hydrogen and hydrogen-hydrogen interactions require in principle an ab initio electronic structure calculation, while the long ranged strain fields of dislocations necessitate system sizes involving 1000s of atoms, which cannot be treated by standard ab initio approaches. Moreover, the distribution of H among several 1000 sites is demanding, even using the computationally efficient semi-empirical EAM potential.
In order to accommodate the scale-transcending nature of the physical phenomena underlying the stress shielding effect a hierarchical multiscale approach was adopted:
Density functional calculations were performed to validate H-H and M-H interaction parameters determined using the EAM potential, which in turn was used to parametrise a lattice gas Hamiltonian for the distribution of H within the metal matrix.
Using this setup the equilibrium hydrogen distribution around an edge dislocation in Ni and its effect on the dislocation stress field are determined.
References
[1] W. H. Johnson, Proc. R. Soc. 23, 168 (1875).
[2] L. E. Probert and J. J. Rollinson, Electroplating and Metal Finishing 14, 396 (1961).
[3] I. M. Maxelon, A. Pundt. W. Pyckhout-Hintzen, J. Barker and R. Kirchheim, Acta Mater. 49, 2625 (2001).
[4] I. M. Robertson and H. K. Birnbaum, Acta Metall. 34, 353 (1986).
[5] J. E. Angelo, N. R. Moody and M. I. Baskes, Model. Simul. Mater. Sci. Eng. 3, 289 (1995).
[6] M. I. Baskes, X. Sha, J. E. Angelo and N. R. Moody, Model. Simul. Mater. Sci. Eng. 5, 651 (1997).
