An Ab-initio Investigation of Environmental Impurities at a Crack Tip in Aluminum
Richard Zamora (Cornell University), Derek Warner (Cornell University)
From Atomistics to Reality: Spanning Scales in Simulations and Experiments Symposium B
Tue 2:40 - 4:00
CIT 227
The presence of environmental impurities is known to promote crack growth in a variety of technologically important materials. For example, when an aluminum alloy is subjected to cyclical loading in a controlled atmosphere, a rise in humidity will accelerate the crack growth process and lower the threshold load required for fatigue to occur. To date, empirical observations of environmental embrittlement have not clearly illuminated the fundamental underlying mechanisms due to length and time scale limitations of direct experimentation. Thanks to growing supercomputing resources and Density Functional Theory based multi-scale modeling techniques, it is now possible to explicitly simulate the chemo-mechanical processes governing crack growth in a realistic environment. Harnessing these advancements, this work investigates the effects of load and oxygen coverage on the energetics of hydrogen ingress at a crack tip in aluminum. Noting hydrogen’s likelihood of occupying surface sites in the absence of oxygen coverage, we use a concurrent multi-scale method to directly quantify the chemo-mechanical effects of environmental surface impurity atoms. For the crack-tip geometry simulated in this work, we have found that surface impurities capable of electro-negative behavior will promote brittle crack propagation by inhibiting dislocation nucleation and preventing crack blunting. This result illuminates the importance of considering realistic surface chemistry when simulating even simple fracture behavior in ductile metals.