Prof. Stan Lynch (Monash University, Australia)

Atomistics of Hydrogen Embrittlement: HEDE versus HELP versus AIDE
When Oct 12, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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There are long-standing disagreements regarding the mechanisms of hydrogen embrittlement of metals such as iron and nickel (where hydrides do not form), and the pros and cons of proposed mechanisms will be discussed, taking all the evidence into account and trying to avoid ‘confirmation bias’. Metallographic and fractographic observations are described in particular since they enable the processes occurring during crack growth to be ‘reconstructed’ with some confidence, especially when single crystals are used. Studying the effects of variables such as temperature and strain rate, and comparisons of hydrogen embrittlement (in gaseous hydrogen and in hydrogen-charged material) with adsorption-induced liquid-metal embrittlement, also provide valuable insights into the mechanisms involved. Atomistic modelling is not yet sufficiently advanced to cope with all the complexities of hydrogen assisted cracking, but does provide additional insights.

The evidence suggests that the Adsorption-Induced Dislocation-Emission (AIDE)/void-coalescence model, based on adsorbed hydrogen weakening interatomic bonds at crack tips, predominates for cleavage-like and nano-dimpled intergranular fractures, while the Hydrogen-Enhanced DE cohesion (HEDE) model, also based on weakening of bonds by hydrogen (and impurity segregation), can account for atomically brittle intergranular fractures [1]. The Hydrogen-Enhanced Localised Plasticity (HELP) hypothesis, based on solute hydrogen facilitating dislocation activity in the plastic zone ahead of cracks, could promote slip-band fractures but, contrary to widespread opinion in the literature [2] appears to play little role for other fracture modes.

[1] S.P. Lynch, ‘Mechanisms and Kinetics of Environmentally Assisted Cracking: Outstanding Issues and Suggestions for Future Work’, Metall. Mater. Trans. A, 44A, 1209, 2013.

[2] I.M. Robertson, P. Sofronis et al., ‘Hydrogen Embrittlement Understood’, Metall. Mater. Trans A, 46A, 2323, 2015.