CAREER: Microstructural Influences on the Diffusion Paths Controlling Nucleation and Growth of the Grain Boundary Precipitates Responsible for Sensitization in Al-Mg Alloys

About This Grant

NON-TECHNICAL SUMMARY Aluminum-Magnesium (Al-Mg) alloys are commonly used as a lightweight alternative to steel in transportation applications and exposed infrastructure due to their excellent strength-to-weight ratio, weldability, and some of the highest resistances to corrosion damage amongst aluminum alloys. Prolonged exposure to elevated temperatures, however, leads these alloys to develop a serious problem known as sensitization that can eventually result in catastrophic failures. This project investigates the fundamental mechanisms controlling the sensitization process in Al-Mg, which are governed by complex atomic interactions with defects in the material’s internal structure created during manufacturing. These findings will ultimately guide the development of new material processing strategies that mitigate the sensitization process and extend the lifetime of these materials by several times their current limit. As Al-Mg alloys are already used extensively in critical transportation and energy infrastructure, this research presents immediate potential for large economic and sustainability improvements. The education and outreach efforts included in this project introduce materials science education in venues and contexts classically associated with the humanities. In an ongoing collaboration with the Cincinnati Art Museum, educational Science of Art programming is being introduced via QR-code links located alongside select objects within the museum, and a new interdisciplinary undergraduate course is being offered that uses the unique and publicly intriguing lens of sword-making to introduce fundamental concepts in physical metallurgy and materials science to undergraduates outside science and engineering majors. TECHNICAL SUMMARY This project tests three major hypotheses regarding the fundamental mechanisms by which microstructural features control the nucleation and growth of grain boundary precipitates in 5xxx series Al-Mg alloys, a defect-controlled precipitation process governed by complex diffusion pathways. Precipitation of the anodic Mg-rich beta-phase along grain boundaries in these super-saturated Al-Mg alloys leads to increased susceptibility to intergranular corrosion in a process known as sensitization, which can compound over time at environmental temperatures and result in catastrophic failures. Carefully designed experiments are employed that take advantage of novel characterization techniques to make statistically relevant direct observations of the nanoscale precipitates along the grain boundaries, addressing uncertainties regarding the underlying mechanisms and diffusion modes dictating the nucleation and growth, along with the precipitate distribution’s consequent impact on the sensitization response. Establishing the contributions that specific microstructural features play in the progression of this type of precipitation, notably as a function of temperature and configuration, enables the development of new processing strategies that lower long-term sensitization response in these alloys and increase service life. This research answers pertinent questions inherent to many systems possessing detrimental nanoscale precipitates that nucleate preferentially upon defect structures, and provides critical new experimental studies exploring the interplay between microstructural elements and multi-stage diffusion pathways. 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.