Stress Corrosion Cracking of a Transformation Induced Plasticity Fe39Mn20CO20Cr15Si5Al1 High Entropy Alloy

Thumbnail Image
Journal Title
Journal ISSN
Volume Title
University of Alabama Libraries

Stress corrosion cracking (SCC) is a materials degradation phenomenon in which a susceptible material (metal) fails below its yield strength in a brittle manner because of the concomitant effect of a tensile load and a corrosive environment. After achieving a critical crack length, the component fails catastrophically. As society continues to advance, the demand for advanced alloys is also increasing. High entropy alloys (HEAs) are recently developed alloys, with some of them exhibiting excellent mechanical properties. However, their SCC properties have yet to be explored. The present research focuses on understanding the SCC behavior of the Fe39Mn20Co20Cr15Si5Al1 (at. %) HEA in 3.5 wt.% NaCl solution at room temperatureThe Fe39Mn20Co20Cr15Si5Al1 (at. %) HEA exhibits transformation-induced plasticity (TRIP) behavior and shows the transformation from γ (face-centered cubic) to ɛ (hexagonal closed pack) phase. The TRIP Fe39Mn20Co20Cr15Si5Al1 (at. %) HEA consists of two phases: the ɛ – martensite phase (hexagonal close-packed structure) and the γ – austenite phase (face-centered cubic), with phase fractions of ~97% and ~3%, respectively. The alloy showed excellent resistance to uniform corrosion due to the formation of a stable and dense passivation film but exhibits susceptibility to pitting corrosion. The alloy shows high susceptibility to SCC at a relatively lower strain rate (10-6 s-1), resulting in more than 50% decrease in ductility. The susceptible sites for crack nucleation and propagation were found to be ɛ – γ interphase and ɛ – ɛ interlath boundaries. A Cr-rich passivation film was formed on the HEA surface during SCC investigated by energy dispersive spectroscopy (EDS) in high-resolution scanning transmission electron microscopy (STEM). Under a constant load experiment within the elastic regime (26% of yield strength), the passivation film formed exhibited differences in thickness and chemical composition. The passivation film formed under no-load and load conditions consisted of Cr hydroxide and Cr oxide rich, respectively. The research suggests that the passivation film behavior can be changed by changing the loading conditions, which influences the SCC behavior of the alloys. The cracks propagated through interphase and interlath boundaries, suggesting that reducing such boundaries may mitigate the alloy's SCC susceptibility.

Electronic Thesis or Dissertation
High Entropy Alloy, Mechanical Alloying, Slow Strain Rate Tensile Test, Stress Corrosion Cracking, Transition Metal Alloys and Compounds, Transmission Electron Microscopy