First-principles Study On The Occupation Behavior Of Hafnium In Ni-Ni₃Al System And Its Influence On The Occupation Behavior Of Oxygen

Hot corrosion of superalloy refers to the accelerated oxidation phenomenon caused by the deposition of sulfur sulfate on the device surface from the burning of fuel and saline environment, under which the superalloy parts experience more serious damage than under the pure high temperature oxidation condition, making the life of the superalloy reduced greatly.Recently it was reported that the adding of Hf could increase the creep life of Pt-doped nickel-based superalloy and the oxidation resistance of Ni-Ni3 Al system, making Hf one of important alloying elements for developing nickel base single crystal superalloy with high oxidation resistance performance.But the questions are: What are the mechanisms of the corrosion resistance and the oxidation resistance of Hf in superalloy? Does it come from the possibility that the strengthening effect of Hf on the mechanical properties and the stability of the superalloy can counteract the harmful effect caused by impurities, such as O, S and N? Or it originates from the prohibition effect of Hf on the entry of outside impurity elements(such as O, S and N) into superalloy, which can lower the concentration of the impurity elements and consequently maintain the stabilities of structure and mechanical properties of the superalloys?To answer above questions, in this paper, the first principle calculation method is used to study the site preferring and phase preferring of Hf and the influence of Hf on the occupancy behavior of O in Ni-Ni3 Al system with lattice misfit, hoping to know something about the influence of Hf on Ni-Ni3 Al system and the oxidation resistance mechanism of Hf. We get some meaningful results as following:(1)The addition of Hf can increase the stability of Ni-Ni3 Al alloy system, and Hf in Ni-Ni3 Al alloy system prefers to segregate to Ni phase region, and then the Ni/Ni3 Al interface region, the last Ni3 Al phase region; The occurrence of the lattice misfit will reduce the stability of Ni-Ni3 Al alloy system, but it has no influence on phase preferring of Hf in Ni-Ni3 Al alloy system.(2)Within most of the lattice misfit investigated, Hf in Ni3 Al phase would rather to occupy Al sublattice than Ni sublattice, but it will change from Al sublattice to Ni sublattce when the lattice misfit is larger than 5.1%.(3)Bond order analysis indicate that Hf may influences the mechanical properties of the Ni-Ni3 Al alloy system from two aspects: on one hand, the addition of Hf is beneficial to the ductility of Ni-Ni3 Al alloy system; on the other hand, the addition of Hf will change the response mode of ductility to lattice misfit in Ni3 Al phase region but not that in Ni phase region, which makes the abnormal strength-temperature phenomenon in Ni-Ni3 Al alloy system weaken or even disappear.(4)No matter whether there is Hf in Ni-Ni3 Al alloy system or not, O in Ni-Ni3 Al alloy system mostly would prefer to occupy the loose and Ni-rich interstitial site. Therefore, in the absence of lattice misfit or under small lattice misfit condition, O tends to segregate to Ni phase region, then the Ni/Ni3 Al interface region, the last the Ni3 Al phase region.(5)Although Hf will not influence the preferring of Hf to loose and Ni-rich interstitial site, but it will influence the formation energy of O, which affects the stability of O in the system. The degree of the influence of Hf on the formation energy of O ranks as follows: Hf1 > Hf2 > Hf0 > Hf3 > Hf4, indicating that Hf in both Ni phase region and Ni3 Al region phase will lower the stability of O in Ni phase region, which is manifested as oxidation resistance of Hf. The rank also shows that the degree of antioxidant effect of Hf in Ni- Ni3 Al alloy system is related to the occupy position of Hf in the system: the best when Hf occupy in the Ni phase region, the middle in the Ni/Ni3 Al interface region, the weakest in Ni3 Al phase region.