Nanomechanical probes as new approaches to hydrogen/deformation interaction studies

Continuous developments into the understanding of hydrogen/deformation interactions have been facilitated by experimental advances and improved computational capabilities. Interactive fracture problems have enjoyed the assistance of fracture mechanics theory and methodology. Regarding hydrogen embrittlement (HE) the current study leans toward a generic hydrogen enhanced decohesion model (HEDE), allowing quantitative engagement with experiments. These include degradation in terms of fracture criteria, subcritical slow crack growth, alternating ductile/brittle transition, threshold values and fine scale fracture surface assessments. In this context the HEDE model remains illuminating, physically based, and experimentally consistent. However, one of the main points of contention, if not confusion, has been the use of thermodynamically based vs. kinetically based arguments to define the threshold and crack growth regimes. We give our view of the general process which we believe to have greater applicability to time-dependent interactive processes. In this context, the description applies to some aspects of stress corrosion cracking and liquid metal embrittlement as well. Following a brief overview regarding HE in the bulk, the paper centers on new avenues of exploring small volume problems. As such this investigation was assisted by nanomechanical testing and high resolution observations. Here, mainly two examples are described in polycrystalline FCC systems. For example, it is shown that hydrogen increases the load onset for dislocation nucleation in a metastable austenitic stainless steer by about a factor of two or more whereupon the yield point recovers to a value near 200 muN after hydrogen outgasses. In a different set of indentation experiments, it is shown that hydrogen decreases both the practical and true works of adhesion at Cu/Ti/SiO2 interfaces by about 50%. Besides evaluating interfacial strengths, the nanomechanical approach should allow additional critical experiments of hydrogen/deformation interactive effects. (C) 2001 Elsevier Science Ltd. Al rights reserved.