Wasserstoffversprödung

Nanoindentation in solution

Because of the technological importance of hydrogen embrittlement, many people have explored the nature, causes and control of hydrogen-related degradation. We tried to answer the following questions using the in-situ nanoindentation technique:

i)          Which mechanism of hydrogen embrittlement is most likely to cause embrittlement for each material?

ii)         How do different mechanical properties like elastic modulus, hardness, energy needed for dislocation nucleation and etc. relate to the ductility of alloys?

iii)         How could we use our method for ranking different alloys based on their sensitivity to hydrogen embrittlement?

iv)        What is the contribution of various alloying element on the ductility of alloys?

Our results show that, in the case of iron aluminides especially, there are at least three main reasons for the high susceptibility of alloys to hydrogen embrittlement: (i) high reactivity of Al atoms with the moisture in air creates high fugacity of hydrogen atoms; (ii) existence of a high concentration of point defects and especially vacancies enhances the solubility of hydrogen in the intermetallics, and increases the influence of hydrogen on the mechanical properties; (iii) the iron aluminides intrinsically have a limited amount of ductility (and very high slip planarity) in comparison to bcc metals, and an additional reduction with hydrogen charging will decrease the ductility even more.

Atomic force microscopy in solution

Post-mortem SEM micrographs and in-situ AFM images show formation of slip traces in a super duplex stainless steel due to the hydrogen charging

Many materials exhibit a strong dependence of their surface physical and mechanical properties on the chemical nature of the test environment. Such effects are generically known as chemo-mechanical or Rehbinder effects or Westwood effect and may be particularly important in tribological situations where the nature of the environment may induce undesirable changes in the hardness, plasticity, surface tension and fracture behavior of a surface leading to a degradation in the performance of the component by enhancing wear. Rehbinder at 1928 related it to the influence of adsorbed atoms or molecules on weakening of the interatomic bonds at the surface. Afterward, much research into chemo-mechanical phenomena has been done on inorganic non-metallic solids such as MgO and the alkali halides , ceramics and glasses and it has been shown that the near-surface hardness or mobility of dislocations depends strongly on the test environment. One main important environmental item which origins the environment-dependence of the deformation and fracture of the near surface region of solids is hydrogen according to Cuthrell. In our department we can perform in-situ AFM and in-situ nanoindentation measurement in solutions and under different cathodic or anodic potentials. It gives us a powerful tool to study the environmental parameters like hydrogen on the deformation behavior and fracture mechanisms of various materials.