DMSE Master's Thesis Presentation - Maria Ronchi
Friday, April 30, 2021 at 4:30pm to 5:30pmVirtual Event
DMSE Master’s Thesis Presentation
Hydrogen-Induced Transformations in Metastable High Entropy Alloys
Friday, April 30, 2021
4:30 – 5:30 PM EST
Contact email@example.com for Zoom link.
Hydrogen embrittlement (HE) presents a critical challenge to application of structural alloys in hydrogen (H) environments. Recently, development of high-entropy alloys (HEAs) has presented a new avenue for alloy design against HE: not only do the immense composition spaces associated with these alloys provide endless prospects for tuning composition and corresponding mechanical behavior, but some HEAs also indicate resistance to HE. In particular, metastable alloys—those that exhibit a mechanically-induced austenite-to-martensite phase transformation—pose an interesting opportunity for HE resistance, where the toughening mechanisms associated with this transformation could counter HE effects. Specifically, while α’ martensite transformations are known to worsen HE, ε martensite could instead provide the desired toughening effects. One alloy system, FeMnCoCr, is of special interest in this respect: the system has been previously shown to include metastable alloys whose deformation mechanisms are highly tunable with composition, and these exhibit an ε martensite transformation. Thus, in this work, we explore alloys in the FeMnCoCr system to discover H effects and their interactions with metastability. First, we approach the complexity of these interactions by investigating H-induced transformations in one metastable alloy, Fe45Mn35Co10Cr10. To this end, we electrochemically introduce H to the samples, quantify the hydrogen evolution by thermal desorption spectroscopy, and observe microstructural transformations by scanning electron microscopy techniques. We examine the microstructural factors influencing these transformations, which include ε-martensite and twinning within the martensite, to better understand the role of H. Second, we approach the compositional complexity of the FeMnCoCr-H system by developing a method to efficiently screen this composition space. We apply this method to Fe88-x-yMn12CoxCry alloys, with a focus on microstructure and metastability. To this end, we first select and produce three alloys using predictions from Thermo-Calc, then apply thermo-mechanical treatments to further vary microstructure. Indentation and scanning electron microscopy are employed to analyze metastability and H effects. We find that these alloys vary widely in microstructure and properties, and exhibit metastable transformations that are influenced by H.
C. Cem Tasan, Thomas B. King Associate Professor, Materials Science and Engineering, Massachusetts Institute of Technology