The single-phase, FCC high-entropy alloy FeNiCoCrMn, comprising five elements in equi-molar amounts, has recently been shown to exhibit desirable mechanical properties, including excellent fracture toughness, strength, and ductility, particularly at cryogenic temperatures. FeNiCoCrMn has also been shown to be resistant to environmental degradation such as irradiation and was recently reported to be resistant to hydrogen embrittlement, which would place it in a unique class of metals.
In the present experiment, FeNiCoCrMn subjected to gaseous hydrogen charging at 120 MPa at 200°C was found to be susceptible to hydrogen embrittlement under tensile loading at slow strain rates (1.7 × 10−6 s−1). The presence of hydrogen caused a reduction in strain to failure and a transition in failure mode from ductile microvoid coalescence to intergranular fracture. Deformation microstructures in uncharged and hydrogen-charged specimens were examined across length scales in the absence and presence of hydrogen to investigate the mechanism for intergranular failure. Analysis included SEM fractography; EBSD orientation image mapping of the free surfaces, before and after deformation; and STEM diffraction contrast imaging and orientation image mapping of subsurface microstructures. At the microscale, the presence of hydrogen led to an advanced dislocation structure accompanied by deformation twins indicative of significant plasticity. At larger length scales, orientation analysis indicated that the presence of hydrogen reduced the communication of strain between grains and reduced the elongation of grains along the loading direction, suggesting that the presence of hydrogen modified how compatibility constraints are accommodated between grains. Secondary microcracks formed along grain boundaries on the hydrogen-induced fracture surface and on the free surface.
The effects of hydrogen on the microstructure at different length scales and their relationship to the failure mechanism are interpreted through the hydrogen-enhanced localized plasticity mechanism. It was found that the effects of hydrogen on plasticity are a necessary component in the hydrogen-induced transition in failure mode, along with hydrogen-enhanced decohesion, and that the microstructural development must be considered to understand hydrogen embrittlement.