Hydrogen segregation by local chemical ordering structure in CrCoNi medium-entropy alloys: A first principle study

The solution energy (E_S) of the hydrogen (H) atom in the octahedral interstitials (OIs) of the CrCoNi medium entropy alloys (MEAs) is investigated based on the random solid solution (RSS) and local chemical ordering (LCO) models. Results show that the distribution of E_S can be well represented by a Gaussian distribution with an average energy of 0.033 eV in the RSS model with a maximum value of E_S = 0.14 eV. In comparison, the distribution of E_S is separated into three parts in the LCO model. The first part is the value of E_S higher than 0.5 eV, where Co–Cr ordering around these OIs. The second part is the lower part in which the E_S value is lower than 0.17 eV. The atomic environment around these OIs are occupied by H atoms exhibit local Ni bias. The third part is E_S between 0.17 eV and 0.5 eV, which hardly exists and accounts for only 4%. The separation of the E_S comes from the effect of the LCO structure, in which two heterogeneous chemical environments appear (localized Ni bias and Co–Cr ordering). When H atoms lie in the OIs with E_S in the first part of the LCO model, the diffusion energy barrier disappears and the H atoms spontaneously move to lower energy sites, leading to the segregation of H atoms in the LCO model. However, when H atoms lie in the OIs of the lower second part, the migration barriers of the H atom between energetically stable OIs in the LCO model (0.3 eV to 1.38 eV) are higher than those in the RSS model (0.4 eV to 0.6 eV). Our study discloses the atomic-scale mechanisms of good resistance to hydrogen embrittlement (HE) of CrCoNi MEAs to some extent.