Hardening effect of diffusible hydrogen on BCC Fe-based model alloys by in situ backside hydrogen charging

Hydrogen embrittlement is common in metallic materials and a critical issue in industries involving hydrogen-related processes. Here we investigate the mechanical response upon hydrogen loading of ferritic Fe-16Cr, Fe-21Cr and Fe-4Al alloys. We use a novel in situ setup for electrochemical backside hydrogen charging during nanoindentation. Single-phase ferritic Fe-Cr binary alloys with high hydrogen diffusivity and low solubility, are ideal for in situ studies during hydrogen charging, particularly the effect of diffusible and lightly trapped hydrogen is targeted. The hardness increases linearly with increasing hydrogen content until a quasi-equilibrium state between hydrogen absorption and desorption is reached while Young's modulus remains unaffected. Above this transient region, the slope of the absolute hardness experiences a drastic decrease. The hardness variation in Fe-21Cr is anisotropic as determined for (100), (110) and (111) oriented grains. Increasing the Cr content enhances the hardening effect in (100) orientation: a 16.7 % hardness increase is observed in Fe-21Cr, while Fe-16Cr, shows an increment of 10.8 %. A Fe-4Al alloy increases slightly in hardness by only 4.3 % at the applied current density of 3 mA/cm2. The hardening effect is caused by enhancing dislocation density, as revealed by studying the cross-section underneath the nanoindentation imprints.