First-principles Study Of Solid Solution And Migration Of Hydrogen In The CoCrNi Medium Entropy Alloy

High(or medium)entropy alloys are single-phase solid solutions composed of multiple equimolar or nearly equimolar alloying elements.Their excellent mechanical properties,such as good strength-conductivity combination and high fracture toughness,have attracted a lot of attention.Recent studies have shown that CrCoNi medium entropy alloys have good resistance to hydrogen embrittlement,but the physical mechanism of hydrogen embrittlement is still unclear,and it is clear that the migration behavior of hydrogen atoms plays an important role in hydrogen embrittlement.Meanwhile,the local chemistry ordering structure is an intrinsic feature of the atomic structure in the high-entropy alloy,which has an important influence on the hydrogen embrittlement properties.The local chemistry ordering structure is difficult to be determined experimentally because of the complex composition of high entropy alloys,so the degree of local chemistry ordering of the structure can be well predicted by means of simulation.In addition,twinning is important for plastic deformation of materials in practical engineering applications,and experiments have shown that hydrogen atoms accumulate at the twins,thus weakening the material cohesion and affecting the stability and longevity of the material,but the physical mechanism of the interaction between twins and hydrogen atoms is still unclear.Therefore,in this paper,the effects of local chemistry ordering structure and twinning structure on the migration and solid solution behavior of hydrogen atoms in CrCoNi medium entropy alloys are investigated at the atomic scale by combining Monte Carlo methods and density function theory calculations,and the main findings of the paper are as follows:(1)The effect of local chemistry ordering structure on the solid solution behavior of hydrogen atoms in CrCoNi medium entropy alloy was systematically investigated by constructing a random solid solution model and a local chemistry ordering structure model of CrCoNi.The results show that the distribution of the solution energy of hydrogen atoms in the random solid solution model is in accordance with the Gaussian distribution,and the average energy of the solution energy of hydrogen atoms is 0.033 eV with a maximum value of 0.14 eV.In contrast,the distribution of the solution energy of hydrogen atoms in the local chemistry ordering model is divided into three parts.The first part is the solution energy of hydrogen atoms greater than 0.5 eV,where Co-Cr ordering occurs around these octahedral interstitial positions;the second part is the lower part of hydrogen atoms with solution energy values below 0.17 eV,and the atomic environment around these octahedral interstitial positions occupied by hydrogen atoms exhibits local Ni segregation;the third part is the hydrogen atoms between 0.17 eV～0.5 eV solution energy,which is almost non-existent and accounts for only 4%.The separation of the hydrogen atom solution energy comes from the local chemistry ordering structure with two non-homogeneous chemical environments(localized Ni bias and Co-Cr ordering).(2)The diffusion energy barriers of hydrogen atoms between the octahedral interstitial positions of the face-centered cubic structure are systematically investigated.The diffusion energy barrier disappears when the hydrogen atoms are located at the octahedral interstitial positions in the first part of the local chemistry ordering structure model,when the hydrogen atoms spontaneously move to the lower energy positions,leading to hydrogen atom bias in the local chemistry ordering structure model.However,when the hydrogen atoms are located at the octahedral interstitial positions in the lower half,the migration energy barriers of hydrogen atoms between the energy-stabilized octahedral interstitial positions in the local chemistry ordering structure model(0.3 eV～1.38 eV)are higher than those in the random solid-solution model model(0.4 eV～0.6 eV).(3)By constructing a twinning structure model of random solid solution and a local chemistry ordering twinning structure model,the joint mechanism of the twinning and local chemistry ordering structures on the solution and migration behavior of hydrogen atoms was systematically investigated.It is found that the dissolution energy of hydrogen atoms at the octahedral interstices of the twin structure is smaller overall,and the minimum value of dissolution energy of hydrogen atoms in the twin structure of random solid solution is-1.38 eV;correspondingly,the average value of solution energy in the local chemistry ordering twinning structure is-0.59 eV due to the higher degree of segregation of Ni and Co atoms and Co-Cr ordering in the local chemistry ordering twinning structure.The migration energy barriers of hydrogen atoms in the octahedral interstices at the twin boundaries of both models are relatively higher,and two scenarios are formed simultaneously.In the first case,the migration energy barrier is distributed around 5 eV(5.02 eV～6.53 eV in random solid solution twinning structure)similar to that in local chemistry ordering twinning structure(4.51 eV～6.11 eV),which is due to the introduction of the twinning structure resulting in a small overall solution energy of hydrogen atoms.In the second case the average value of the migration energy barrier is 2-8 times higher than in the previous case(17.58 eV～21.02 eV for the twin structure with random solid solution and 11.04 eV～40.54 eV for the twin structure with local chemistry ordering).The results indicate that the twin structure has a decisive effect on the migration energy barrier,while the local chemistry ordering one has a smaller effect.