Hydrogen-Induced Transformations in Metastable High Entropy Alloys

Abstract

Hydrogen embrittlement (HE) presents a critical challenge to application of structural alloys in hydrogen (H) environments. Recently, development of high-entropy alloys (HEAs) has opened a new avenue for alloy design against HE: not only do some HEAs indicate resistance to HE, but the immense composition spaces associated with these alloys provide endless prospects for tuning composition and corresponding mechanical behavior. 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 under the right conditions. One alloy system, FeMnCoCr, has been previously shown to include metastable alloys which are of special interest due to the high tunability of deformation mechanisms with respect to composition. For example, tuning just the Mn content enables switching between dislocation slip, twinning, and martensite transformation mechanisms. Thus, in this work, we further explore alloys in the FeMnCoCr system to discover H effects and their interactions with metastability.

In the first part of this work, we explore 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. Through these analyses, we find that the hydrogen induces ε-martensite that preferentially forms in <101> and <111> oriented grains and along Σ3 coincident site lattice boundaries. Further addition of hydrogen induces extension twinning within the martensite. We examine the microstructural factors influencing these transformations to better understand the hydrogen-microstructure interactions.

In the second part of this work, we address the compositional complexity of the FeMnCoCr-H system by developing a method to efficiently screen this composition space for interactions between H and metastability. We apply this method to Fe₈₈₋ₓ₋ᵧMn₁₂CoₓCrᵧ alloys, with a focus on microstructure and H effects. To this end, we first select three alloys using predictions from Thermo-Calc, then produce these alloys by suction casting and apply three thermo-mechanical treatment routes to further vary microstructure. Indentation and scanning electron microscopy are employed to screen for deformation mechanisms and cracking. We identify two particular samples which exhibit extreme cases of indentation response and can provide a starting point for future iterations of this investigation.