【作者】 彭黎；

【导师】 刘云国； 彭平；

【作者基本信息】 湖南大学 ， 环境科学与工程， 2012， 博士

【摘要】 镍基单晶高温合金是先进航空发动机热端部件的关键材料，为了控制合金制备成本、减少稀贵金属的用量、提升合金回收利用率、实现产品的生态设计，必须对合金制备与成型过程中引入的迹量杂质元素的作用效果与危害行为进行分析，为此，本文采用第一原理计算方法，针对镍基单晶高温合金中的几种常见掺杂（N、S、P），基于生态设计的原则，对其危害作用及其作用机理进行了研究。鉴于镍基单晶高温合金的高温蠕变性能很大程度上取决于γ相与γ′相之间的相互作用，特别是γ-Ni/γ′-Ni₃Al相界的结构性质，本文首先采用赝势平面波方法与CASTEP总能计算程序，分析了非金属迹量杂质元素S、P与N在γ-Ni/γ′-Ni₃Al相界中的存在许可及其择优占位趋势，进而考察了其对γ-Ni/γ′-Ni₃Al相界断裂强度与韧性的有害作用。结果表明：从能量角度上看，S、P无论是置换γ-Ni/γ′-Ni₃Al相界中的某个基体Ni或Al原子还是占据其中的八面体间隙位都是许可的；以气态形式存在的N不易掺杂到γ-Ni/γ′-Ni₃Al相界区域中，而以固态形式存在的N则较易掺杂至相界中。与占据γ/γ′相界中的某个八面体间隙位相比，S、P均具有较大的趋势优先置换相界中的基体原子，而N则将优先偏聚到γ/γ′相界中的八面体间隙位。与掺杂前清洁界面相比，S掺杂使得相界的稳定性下降；P的置换掺杂能在相界中稳定存在，在相界区域八面体间隙中心的掺杂则并不稳定，而N在相界中的置换型掺杂与间隙位掺杂则都能稳定存在。可见，S与P在相界中的占位具有一定的相似性。S、P、N对γ-Ni/γ′-Ni₃Al相界的掺杂，使得相界区域原子位置发生了一定的变化，引起晶格畸变，使相界区域厚度增加，增加了相界区域原子层间距，从而削弱了层间电子作用强度，同时也导致了相界区域局部弹性应变能的增加。置换相界区域中基体Ni或Al原子的杂质N，虽然没有引起明显的晶格畸变，但其不能在晶胞格点处稳定存在，会向邻近的γ-Ni相或γ′-Ni₃Al内部的八面体间隙发生非常明显的迁移，从而在初始格点处形成Frenkel点缺陷，造成大量成键电子的缺失，削弱相界层间结合强度。N、S、P对γ-Ni/γ′-Ni₃Al相界的掺杂在不同程度上削弱了相界的断裂强度，并有可能改变相界的断裂位置。其中，置换型掺杂γ-Ni/γ′-Ni₃Al相界的断裂强度相对于间隙位掺杂时较好。掺杂原子对γ-Ni/γ′-Ni₃Al相界中具有显著不利影响的晶格点阵位置与八面体间隙位置具有较强的占位趋势与结构稳定性，从而表现出显著的有害作用。本文进一步计算、分析了掺杂γ-Ni/γ′-Ni₃Al相界的电子能态结构，以考察非金属杂质元素在相界的行为作用机理，掺杂在相界中的S、P以失去电子为主，N得到电子的趋势非常强。杂质原子与其周围近邻原子形成了较强的电子相互作用，使得部分价电子由高能级向低能级附近聚集，在一定程度上削弱了相界区域中的金属键与共价键作用，层间成键电子减少，导致了相界结合强度的降低。置换相界区域中基体原子的杂质N，会向γ相或γ′相内部间隙扩散，在初始位置处形成Frenkel空位缺陷，导致相界价电子的缺失，降低结合强度。可见，N、S、P等杂质元素掺杂引起相界发生晶格畸变以及相界区域局部电子作用发生改变，这最终导致了其对相界的脆化作用效果。进一步本文考察了非金属杂质元素P对Re合金化γ-Ni/γ′-Ni₃Al相界的影响效果。在Re合金化相界超胞模型的基础上，将P掺杂到相界中的晶格点阵位置或八面体间隙位，从而建立了一系列P、Re复合掺杂合金化的γ-Ni/γ′-Ni₃Al相界超胞模型，考察了P、Re交互作用对γ-Ni/γ′-Ni₃Al相界断裂性质的影响。P在相界区域中的占位趋势几乎不受到Re合金化的影响，并能够与Re在相界区域共存，因此P与Re间的交互作用及其对相界性质的作用效果不可忽视。P-Re交互作用对相界断裂强度产生了显著影响，当P与Re相隔较近时，相界的断裂强度相对较差，而当P与Re之间具有一定间隔时，相界的断裂强度相对较好，甚至比Re单独合金化时更好。关联能计算结果表明P-Re之间普遍存在着排斥作用，当P-Re原子间距小于2.75时，排斥作用明显，且随着原子间距的减小而迅速攀升。随着P-Re相互关联作用强度的增加，γ/γ′相界的断裂功先增大后迅速减小，即适当的P-Re交互作用强度对于相界的强化是有利的。最后，本文考察了共存于γ-Ni/γ′-Ni₃Al相界中的两种非金属杂质元素S与P的复合掺杂对相界断裂性能的有害作用。S与P的复合掺杂在很大程度上削弱了γ/γ′相界的断裂强度，S与P复合掺杂的Ni/Ni₃Al相界的断裂强度比S或P单独掺杂时的断裂强度更低，可见，相界中共存的多种杂质元素对相界断裂强度的改善是非常不利的。S与P之间的关联作用效果与S、P杂质原子的原子间距有关：当S与P相隔较近时，S与P之间主要是排斥作用，而当S与P相隔较远时，其间存在相互吸引作用，相互作用强度随着S、P原子间距的增大而减小。S与P之间的排斥作用会加剧相界的脆化，而S与P之间的相互吸引作用则能在一定程度上缓解其对相界强度的有害影响。复合掺杂导致相界的几何结构发生改变，相间原子层间距增大，层间电子相互作用减弱，局域弹性应变能增加，这都将导致相界断裂强度的削弱，引起相界的脆化。由此可见：非金属元素的单独掺杂，在很大程度上不利于相界断裂强度与区域韧性的提高。非金属杂质的危害作用与其在相界区域的占位有关，某些占位下，其对相界的危害较小，对相界强化元素的影响也不是单纯的妨害与削弱作用。迹量掺杂元素与合金化元素间的交互作用在某些特殊的占位与分布条件下，甚至比强化元素单独合金化时的强韧化效果还好。无疑地，该结果对于基于生态设计与成本核算的镍基单晶高温合金中迹量掺杂元素的优化与控制具有一定的科学价值与现实意义。更多还原

【Abstract】 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.更多还原