Hydrogen is the most abundant element, both in the universe and on earth. As an element with the simplest atomic structure and smallest volume, hydrogen can easily dissolve into many solid materials, changing their properties. In many industrially important metals during processing or service, hydrogen often has deleterious consequence on mechanical properties that is commonly referred to as hydrogen embrittlement (HE), thus has been a wide concern in industry and academia for over a century.

Despite numerous efforts over the past century, the exact mechanism of hydrogen effects on the ability of the material to plastically deform remains controversial, thus knowing how hydrogen interacts with dislocation – the primary carriers of plasticity is essential. Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys.

But our latest experimental discovery subverts the established cognition in the past decades.

 

The effect of vacuum aging on dislocation movements in hydrogen-free sample
Effect of hydrogenation on dislocation movements.
 By quantitative mechanical tests in an environmental transmission electron microscope, here Dr. Degang Xie, a young faculty in CAMP-Nano, demonstrates that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ~103 s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking and strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. Vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment.
 
  Side view of dislocation core decorated by hydrogen and hydrogen-vacancy complex, respectively. Atoms with golden and black colours refer to aluminum and hydrogen, respectively
Atomistic simulation of the pinning effect of hydrogen-vacancy

The project is supervised by Prof. Zhiwei Shan and Prof. Ju Li. Besides vice Prof. Zhangjie Wang and Dr. Meng Li from our faculty and post Dr. Suzhi Li, Prof. Peter Gumbsch, Prof. Jun Sun, Prof. Evan Ma also made a significant contribution to this work.

This work has been published on the top research journal, Nature Communications.

The article can be accessed at http://www.nature.com/articles/ncomms13341