Project Summary Dynamic Fracture in Dealloying Induced Stress Corrosion Cracking Principle Investigator: Karl Sieradzki, Arizona State University This research program examines the role of unstable dynamic fracture processes in dealloying induced stress-corrosion cracking of face-centered cubic alloys. This form of stress corrosion cracking is particularly relevant in energy conversion systems (both nuclear and fossil fuel) as many failures in alloys such as austenitic stainless steel and nickel-based systems can result directly from dealloying. Corrosion of such alloys often results in the formation of a brittle nanoporous layer which we hypothesize serves to nucleate a crack that owing to dynamic effects penetrates into the un-dealloyed parent phase alloy. Thus, since there is essentially a purely mechanical component of cracking, stress corrosion crack propagation rates can be significantly larger than that predicted from electrochemical parameters. The main objective of this work is to examine and test this hypothesis under conditions relevant to stress corrosion cracking. Silver-gold alloys serve as a model system for this study since hydrogen effects can be neglected on a thermodynamic basis, which allows us to focus on a single cracking mechanism. Additionally, over the past ten years, there has been a considerable worldwide effort aimed at elucidating the mechanical properties of monolithic nanoporous gold structures which allows us to take advantage of this accumulated knowledge in designing our experimental protocols. In order to study various aspects of this problem we are characterizing the dynamic fracture properties of monolithic nanoporous gold as a function of the intrinsic length scale in the structure under electrochemical conditions relevant to stress corrosion cracking. We are also examining the detailed processes associated with the crack injection phenomenon by forming dealloyed nanoporous layers of prescribed properties on un-dealloyed parent phase structures and measuring crack injection velocities and crack penetration distances. Dynamic fracture in monolithic nanoporous gold and in crack injection experiments is examined using high-speed (106 frames s-1) digital photography. In the case of intergranular dealloying induced stress-corrosion cracking we are using orientationimaging microscopy to ascertain if there are any particular grain boundary types that are more susceptible than others to this form of cracking. The tunable set of experimental parameters includes the nanoporous gold length scale (5-40 nm), thickness of the dealloyed layer (10-3000 nm) and the electrochemical potential (0.1-1.5 V). The results of crack injection experiments are characterized using the dual-beam focused ion beam/scanning electron microscope and aberration corrected scanning transmission electron microscopy. Together these tools allow us to very accurately (sub nanometer spatial resolution) examine the detailed structure and composition of dealloyed grain boundaries and compare crack injection distances to the depth of dealloying. The results of this work should provide a basis for new mathematical modeling of dealloying induced stress corrosion cracking while providing a sound physical basis for the design of new alloys that may not be susceptible to this form of cracking. Additionally, our results should be of broad interest to researchers interested in the fracture properties of nano-structured materials. In this regard, we expect that our findings will open up new avenues of research apart from any implications our study may have for stress corrosion cracking.
|Effective start/end date||8/15/12 → 8/14/20|
- US Department of Energy (DOE): $1,217,163.00