A Study Of Deleterious Influences Of Trace Dopants In Ni-based Single Crystal Superalloys On The Basis Of ECO-degsin Idea

Ni-based single crystal （NSC） superalloy is a key material of hot end componentsin advanced aeroengines. In order to cotrol costs of the superalloy preparation, reducethe usage of the rare and noble metals, enhance recovery rate of superalloys and carryout the ecological design of the NSC superalloy, the doping effects and deleteriousinfluences of trace dopants doped during the preparation and molding production ofthe superalloy must be examined. Thus, based on the principle of ecological design,some typical dopants （N, S, P） and their deleterious influence as well as mechanism isstudied by using the first principle calculation in this paper.As well known, the high-temperature mechanical properties of NSC superalloyslargely derive from the interactions of γ′precipitates with the γ-matrix, and morespecifically, from the γ/γ′interface. The structure and characteristics of the γ/γ′interface control the performance of the alloys to a large extent. Cambridge serial totalenergy package （CASTEP）, a first principle plane-wave pseudopotential method basedon the density functional theory （DFT）, is employed to investigate the site preferenceand deleterious effect of the representative non-metallic impurity elements i.e., S, Pand N on the rupture strength, toughness of the γ-Ni/γ′-Ni₃Al interface. The resultssuggest that, in energetics, S-and P-doping are found to be permissive either atsub-lattice sites or at octahedral interstitial centers, gaseous N2is difficult to be joinedinto the interfacial region, but solid N impurity can be easily doped into the Ni/Ni₃Alinterface. Comparing with the octahedral interstitial centers, S and P prefer tosubstitute for the host atoms, firstly. Whereas, N-doping has greater trend to occupythe octahedral interstitial sites. In comparison with the clean γ-Ni/γ′-Ni₃Al system, theinterface with S-doping is not stable, P-doping at the sub-lattice sites is stable but thatat the octahedral interstitial sites is not as stable as the clean interface, and theN-doped interface system can stably exist either for substitution for host atoms or foroccupation at octahedral interstices. P and S have some similarities in their sitepreference. N-, S-and P-doping at the γ/γ′interface lead to a change of host atoms’location and lattice distortion. Weakening of interlayer electronic interactioncombined with increases of elastic strain energy induced by the local lattice distortionshould be responsible for the harmful effect of the impurities. The N-dopingsubstituted for Ni and Al host atoms dose not change the geometric structure of the interface, obviously, though, it can not stay at the sub-lattice sites, stably, and wouldmove to the interstitial site in the nearby γ or γ′block, leaving a Frenkel point defectat the original sub-lattice site. The depletion of the valence charge at the Frenkelvacancy weaken the bonding strength at the Ni/Ni₃Al interface. The calculation ofGriffith rupture work deduces that, N-, S-and P-doping have deleterious effect on thefracture property of the γ/γ′interface and may vary the fracture site. The interface withsubstitution of impurities for host atoms has relatively better fracture strengthcomparing with the system with impurity atoms at the octahedral interstitial sites. N, Sand P generally prefer to occupy the sub-lattice site and octahedral interstitial site withobviously deleterious effect on the interface and are thought to be typical deleteriousimpurities in the NSC superalloy. In the doped γ/γ′system, S and P mainly releaseelectrons to form ionic bonding, while N only obtain electrons from other host atomsin the γ/γ′interfacial region. The strong bonding effect between impurity atoms andhost Ni or Al atoms makes partial valence charges transfer to the low energy levelregion, which result in the impairment of the metallic and covalent bonding in theinterfacial region, and the depletion of the electrons in the interlayer region. It finallylead to the reduce of the bonding strength in the γ/γ′interface. The Frenkel vacancy inthe N-doped γ/γ′interface, which results in the depletion of bonding electrons in theinterfacial region, is another reason of the N-induced deleterious effect on theinterfacial fracture performance. To sum up, the embrittlement induced by theimpurity elements can be attributed to the variation of electronic structure in theinterfacial region combined with the change of local elastic strain energy induced by alarge lattice distortion.The influence of impurity elements on the Re-alloyed γ-Ni/γ′-Ni₃Al interface isfurther examined in this paper. The typical impurity P is doped at the sub-lattice siteby substituting for Ni or Al host atom or the octahedral interstitial centers in theRe-alloyed γ-Ni/γ′-Ni₃Al interface. The synergetic effect of Re and P on Griffithrupture works of the γ-Ni/γ′-Ni₃Al is investigated. In the duplex doping system, Reand P can coexist in the γ/γ′interfacial region, and the site preference of P at the γ/γ′interface is almost not changed by Re-addition. The synergetic effect of P and Re onthe rupture strength of the γ/γ′interface has been found. As P being close to Re, therupture strength of the doped interface is lower than that in the case of P apart from Re.In some cases, the synergetic effect Re and P on the interface strengthening is evenbetter than that achieved by the individual Re-addition. The calculation for thecorrelative energy between P-Re shows a strong correlation and repulsive interaction between P-doping and Re-addition within the range ofd Re P≤2.75. In this region,the P-Re correlation increases rapidly as P and Re approach. The rupture strength ofthe γ/γ′interface with duplex doping of Re and P ascends sharply at first, and thenfalls down rapidly with increasing P-Re correlation energy, viz a suitable strength ofP-Re correlation is favorable to the strengthening of the γ/γ′interface.Lastly, the correlation between S and P and their synergetic effect on the fractureproperty of the γ/γ′interface are examined. The multiple S-and P-doping at theinterface weakens the fracture character, obviously, the fracture strength of theinterface with both S-and P-doping is weaker comparing with the the interface withindividual S-or P-doping. It is harmful to the strengthening of the Ni/Ni₃Al interface.The correlation between S and P is relate to the S-P atomic separation, there is arepulsive interaction between S and P, when they are close to each other, and aattractive interaction when S and P are widely separated. The strength of thecorrelation reduces with decrease of the S-P atomic separation. The replulsiveinteraction is harmful to the interface, while the attractive interaction can relieve theembrittlement induced by non-metallic impurities. The change of the fracture site andstrength stems from the varieties of the local electronic structure and geometricstructure in the γ/γ′interface induced by multiple S-and P-doping.Consequently, the individual doping of the non-metallic impurity is detrimentalto the γ-Ni/γ′-Ni₃Al interfacial properties involving rupture strength and toughness.The doping effect on the interface depends on the site occupancy of the impurityatoms in the γ/γ′interfacial region. The doping of the non-metallic impurity undercertain conditions has a little influence on the interface, and it can even improve somecharacters of the γ/γ′interface. In addition, the doping effect of the non-metallicimpurity on the strengthening alloy element in the alloy is not purely “deleteriouseffect”. The synergetic effect of the non-metallic impurity element and strengtheningalloy element can be advantageous for the γ-Ni/γ′-Ni₃Al interface and even better thanthe strengthening effect induced by an individual alloy element. Undoubtedly, theconclusions above is significant and scientific interest to the reasonable control of thetrace dopants based on the ecological desighn and cost accounting of NSCsuperalloys.