Hydrogen effects by in-situ nanoindentation
Mechanical degradation of materials due to hydrogen is a problem causing significant worldwide losses. The mechanisms leading to the material failure initiate at the atomic scale with hydrogen absorption and further interaction with trap binding sites or defects, ranging from vacancies or interstitial atoms to dislocations, grain/phase boundaries or precipitates. Nanoindentation is a valuable technique to study independently such mechanisms due to the small volume probed. Even more, in-situ testing while charging the sample with hydrogen prevents the formation of concentration gradients due to hydrogen desorption.
Two custom electrochemical cells were built to charge the sample with hydrogen while nanoindenting: “front-side” charging with the sample and the indenter tip immersed into the electrolyte, and “back-side” charging where the analyzed region is never in contact with the solution. Both approaches have advantages and disadvantages that we test during the study of the hydrogen effect on incipient plasticity and general mechanical behavior of iron alloys by nanoindention prior, during and after hydrogen charging. For instance, in bcc Fe-Cr alloys with a low dislocation density, a reduction in the pop-in load indicating the yield point with the increase of hydrogen content and formation of multiple pop-ins during nanoindentation provides evidence for the decrease in the resolved shear stress and enhanced dislocations nucleation.