CAREER: Understanding the Role of Cu-Containing Secondary Phase Particles in Enhancing the Resistance to the Environmental Acceleration to Fatigue in Age-Hardenable Al Alloys

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Non-Technical Summary: In order to address the existential crisis of global climate change and our aging infrastructure, dramatically improving the sustainable use of metals utilized in our infrastructure (examples include aluminum used for aircraft, automobiles, and bridges and steels used for ships, automobiles, bridges, and nuclear waste storage) is required. A major problem when considering the sustainable use of metals is environmental degradation through corrosion processes and the need for engineers educated in both environmental degradation (specifically corrosion and related cracking) and materials science. In everyday life, corrosion is most easily seen as rust; but in aging infrastructure it can go unseen and cause catastrophic failures that severely limit sustainable long-term use of engineered metal structures. This research pushes to understand why some aluminum based metals have inherently better resistance to the environmental acceleration of cracking and associated failures than others. Knowledge that could be utilized to improve the performance and the long term sustainable use of metals that have lower environmental resistance to cracking. Through these research endeavors, a graduate student and 5 years of REU (Research Experience of Undergraduates) and RET (Research Experience for Teachers) participants are being introduced to and trained in corrosion and materials science. Additionally, a wide range of students and future engineers are being exposed to materials science and corrosion as the research team develops corrosion related demos and social media videos. Finally, this research is establishing bonds between the PI’s research lab at The Ohio State University, a local Columbus K-12 school, and an education focused university in Ohio, Wittenberg University, which will go far beyond the short-term goals. Technical Summary: Corrosion and environment assisted cracking (EAC) play critical roles in sustainability. Research and education in corrosion and EAC is critical to ensuring our society addresses the sustainable use of metals to save energy and reduce consumption. Research has established that some age-hardenable aluminum alloys have better resistance to EAC than others, but an underlying mechanism for this has yet to be confirmed. Specifically, Al-Cu based Al alloys are inherently more resistant to the environmental acceleration of fatigue crack growth than Al-Zn based Al alloys. This research tests the hypothesis that age-hardenable Al alloys with metallurgical secondary phases that promote Cu re-depostion during corrosion (Al-Cu based Al alloys) have an intrinsic resistance to the environmental acceleration to fatigue crack growth as a result of crack wake surface Cu enrichment catalyzing cathodic reaction(s) within the crack environment. This self-mitigates the adverse acidic environment established to drive EAC in 7xxx age-hardenable Al alloys, which are of higher susceptibility. The less acidic crack solution pH reduces environmental sensitivity and corrosion fatigue susceptibility by reducing crack tip H uptake through the stabilization of a crack tip passive film. The following approaches are being used to test this hypothesis. First, fracture mechanics approaches are being utilized to probe corrosion fatigue sensitivity of Al alloys with specific Zn, Mg, and Cu concentrations as a function of fatigue loading frequency and load waveform. In addition, near crack tip pH is being directly probed utilizing mini-flexible pH electrodes inserted into the fracture mechanics samples. Finally, crack tip pH is being altered to either promote or mitigate corrosion fatigue as a final confirmation. This research is being performed by a team that includes the principal investigator, a sponsored graduate student, and RET and REU participants with the goal of training and engaging a diverse team throughout the research program. Together this research team is also collaboratively developing corrosion related materials science demos that will be deployed at the REU’s and RET’s home institutions and in the Introduction to Materials Science course at The Ohio State University. Social media videos are also being created to showcase the developed demos and the diverse research team with the mission of educating on the science of corrosion and how it impacts sustainability.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.