The Alloying Elements Effects On The Formation Mechanism Of Hydrogenated Vacancy And Mechanical Properties Of γ-Ni/γ’-Ni₃Al Phase Interface:A First-principles Study

Ni-based superalloys contain a large amount ofγ-Ni/γ’-Ni₃Al phase interface and are widely used as the hot-end components of aviation,aerospace and marine equipment.In the service environment of Ni-based superalloys,fuel often generates free[H]and its compounds.Due to the smallest radius,light mass and high diffusion coefficient of hydrogen atom,it is easy to enter metal interior,which often causes the hydrogen embrittlement failure of metallic materials and affects the safe operation of equipment.In order to explain the failure mechanism of hydrogen embrittlement,the concept of hydrogenated vacancy（H-Vac）has been proposed as the smallest unit of hydrogen embrittlement.It is well known that alloying is an important strengthening strategy for current Ni-based superalloys.Although the formation mechanism of H-Vac in theγ-Ni/γ’-Ni₃Al phase interface and its influence on the interfacial properties have been studied by previous researchers,the presence state of alloying elements at theγ-Ni/γ’-Ni₃Al phase interface and their possible effects have not been considered.To further investigate the influence of alloying elements on the formation mechanism of H-Vac in theγ-Ni/γ’-Ni₃Al phase interface and the changes in interfacial properties under the combined action of alloying elements and H-Vac,the first-principles calculation method based on density functional theory was used in this thesis.In theγ-Ni/γ’-Ni₃Al interfaces containing alloying elements X（X=Cr,Co,Mo,Ru,W and Re）,the solution energy of hydrogen atom,the formation energy of H-Vac,interaction energy between H-Vac and alloying elements X as well as the Griffiths fracture work,theoretical tensile strength and crystal orbital Hamiltonian population（COHP）of phase interfaces in different states were systematically investigated,to elucidate the influence of alloying elements on the formation mechanism of H-Vac in the interface and the effect of the combined action of alloying elements and H-Vac on the mechanical behavior of the interface in depth.Our work is not only a useful supplement to the theoretical study of H-Vac,but also would provide a theoretical reference for the comprehensive understanding of H-Vac formation mechanism and its effects onγ-Ni/γ’-Ni₃Al phase interface.The main results are as follows:（1）In the Re-containingγ-Ni/γ’-Ni₃Al phase interface,the most favorable site for vacancy formation is the L3-S2 site,which is located in the Ni lattice site of the Re-containing layer and having a distance of 3.523（?）to Re atom.The presence of Ni vacancy increases the formation energy of the Re-containing phase interface and thus reduces the interfacial thermodynamic stability.In the first-nearest neighboring interstitials around the Ni vacancy,hydrogen atom has relatively lower solution energies.The presence of Re atom in the phase interface can not only promote the dissolution of hydrogen atom around the Ni vacancy and thus form stable H-Vac,but also reduce the formation energy of the phase interface,i.e.,the presence of Re atom can improve the stability of the H-Vac-containing phase interface.The calculation results of fracture work showed that the（001）γ’||（002）γfracture plane is the preferred fracture position in the Re-containing phase interface,and the presence of both Ni vacancy and H-Vac does not change the preferred fracture position of the interface.When the hydrogen atom enters the nearest neighboring octahedral interstitials around the Ni vacancy of the Re-containing phase interface and forms a stable H-Vac,the fracture work at the（002）γ’||（001）γfracture position decreases from 4.24 J/m² to 4.22J/m².The results of the electron localization function indicated that the dissolution of the hydrogen atom weakens the bond strength of the Re/Ni atoms to their surrounding atoms,resulting in the decrease of Griffith fracture work of（002）γ’||（001）γfracture plane.（2）For the γ-Ni/γ’-Ni₃Al phase interfaces containing different alloying elements X,the calculated results of H-Vac formation energy showed that the Co element can prevent the formation of H-Vac in the Co-containing phase interface,while the Cr element can promote the formation of H-Vac.With analyzing the interaction energy between Co/Cr atom and H-Vac in the phase interface,we found that Co/Cr atom and H-Vac always maintain the repulsive and attractive interactions,respectively.The effect of H-Vac on the interfacial stability is mainly embodied as follows:when the number of hydrogen atoms is less than three,the phase interface becomes more stable with the increase of hydrogen atoms;while when the number of hydrogen atoms is larger than three,the stability of the phase interface decreases with the increase of hydrogen atoms.The presence of alloying elements X slightly increases the fracture strength of the（001）γ’||（002）γfracture plane of the phase interface containing H-Vac,but significantly increases the fracture strength of（002）γ’||（001）γplane,thus changing the fracture site of the phase interface.