微观相场法研究高铝Ni75AlxV25-x中Ni3Al反位缺陷

采用微观相场法研究高Al浓度Ni75Akv25-x合金析出相Ni3Al的反位缺陷随Al浓度增高的变化规律.选取在1150 K温度下时效从8 at%Al至20 at%Al的共14个合金作为研究对象.研究结果显示:此类型合金主要反位缺陷类型是VAl、NiAl;随Al浓度增高,反位缺陷AlNi增高:而Ni Al、VAl、VNi 3种反位缺陷变化与Al浓度和Ni3V析出与否相关,Al浓度稍低时,有Ni3v相析出,Al浓度增高反位缺陷Ni Al降低,VHi升高,VAl没变化;Al浓度稍高时,无Ni3V相析出,Al浓度增高反位缺陷NiAl稍有降低,VNi、VAl明显降低.     

微观相场法计算Fe含量对NiAlFe三元合金中反位缺陷的影响

采用三元微观相场模型,对铝含量大于25%(原子分数,下同)与镍含量大于75%(原子分数,下同)的NiAlFe三元合金中反位缺陷NiAl、AlNi随Fe含量变化的规律进行模拟计算,其中NiAl(AlNi)表示Ni(Al)原子占据Al(Ni)格点产生的反位缺陷。结果表明:在一定温度范围内,随着Fe含量的增大,铝含量大于25%的NiAlFe合金中AlNi浓度明显上升,NiAl浓度略有上升,但小于AlNi浓度,相反在镍含量大于75%的NiAlFe合金中NiAl浓度明显上升且远大于AlNi浓度;同一温度下比较铝含量大于25%与镍含量大于75%的NiAlFe合金中反位缺陷受Fe含量影响的程度差异,发现前者的AlNi浓度比后者受Fe含量影响大,而后者的NiAl浓度比前者受Fe含量影响大。此外,反位缺陷NiAl和AlNi浓度随时间的演化规律均是逐渐由初始值降低至平衡值;温度升高促使反位缺陷演化变缓慢以及平衡时浓度增大。     

微弹性微观相场研究不同时效过程下L12和D022的原子占位

用包含弹性应变能的微观相场法研究L12-Ni3Al相和D022-Ni3V相在1000 K,1200 K单温度时效及1000 K和1200 K交变温度下时效的原子占位.研究表明:从1000 K单温度时效经由交变温度时效至1200 K单温度时效过程中,L12相中的正位原子NiNi和AlAl及D022相中的正位原子NiNi和VV的占位几率随时间延长而下降,两种结构中的反位缺陷NiAl,NiV,VNi和AlNi及替代缺陷AlNi,AlV和VNi的占位几率却随时间延长而上升.平衡时的L12相中的NiNi占位几率大于D022相中的NiNi,L12相中的AlAl占位几率小于D022相中的VV,反位缺陷和替代原子在L12相中的占位几率均大于D022相.在交变时效下,原子占位几率的时间演化曲线呈"长城"状,曲线顶点的占位几率值小于相应的单温度时效下的平衡值,但是曲线谷值大于相应的单温度时效下的平衡值.     

第一性原理研究点缺陷对B2结构NiAl合金性能的影响（英文）

采用基于密度泛函理论的第一性原理计算分析了含点缺陷结构NiAl晶胞的形成热、形成能、点缺陷平衡浓度、力学性能、电子结构等。将NiAl与其他B2结构的金属间化合物进行对比,发现NiAl拥有更好的塑性和成键强度。根据形成热、形成能和点缺陷平衡浓度的计算结果,发现Ni反位和空位缺陷是NiAl晶胞结构中主要的点缺陷。通过Pugh(G/B_0)和Cauchy(C_(12)–C_(44))准则预测出Ni空位和反位缺陷、Al反位缺陷能够提升NiAl合金的脆性,其中Ni空位缺陷作用最明显;而Al空位缺陷能够改善NiAl合金的塑性,但在NiAl合金中的浓度很低。态密度计算结果发现NiAl合金具有良好的导电性能,Ni空位缺陷、Al空位和反位缺陷能够提升NiAl合金的稳定性。     

V掺杂Ni3Al点缺陷结构及合金化效应的第一性原理研究

基于密度泛函理论的第一性原理平面波赝势方法研究了V掺杂Ni3Al合金的电子结构和点缺陷结构.通过计算与实验结果对比选择了适合Ni3Al合金计算的近似方法,计算了含有各个缺陷的晶胞的晶格常数,形成热和结合能,点缺陷的形成能和平衡浓度,态密度和电荷密度.计算结果表明:Ni3Al合金中反位缺陷较空位缺陷易形成,NiAl是Ni3Al合金中最主要的反位缺陷,Al位最易形成缺陷,在1400 K时,空位缺陷的浓度远远低于反位缺陷的浓度.V加入Ni3Al合金体系中能提高合金的稳定性.     

微量P对NiAl力学性能影响的理论研究

采用第一原理赝势平面波方法和基于虚拟晶体势函数近似(VCA),计算了P微合金化(浓度x<0.2at%)时完整与缺陷B2-NiAl晶体的弹性性质,并采用弹性常数C44、Cauchy压力参数(C12-C44)、杨氏模量E、剪切模量G及其与体模量B0的比值G/B0等,表征和评判了P微合金化浓度x对NiAl金属间化合物延性与硬度的影响。结果表明:无论是理想NiAl晶体,还是含Ni空位或Ni反位的NiAl缺陷晶体,P合金化不能有效的影响其硬度的变化趋势;随着P合金化浓度的稳步增加,理想NiAl晶体的Cauchy压力参数和G/B0比值的变化再现了0.07at%的P可以提高NiAl的室温压缩塑性的实验结果,而缺陷NiAl晶体的Cauchy压力参数和G/B0比值的变化比较好的解释了实验上出现0.03at%的P提高NiAl室温压缩塑性可能是因为Ni反位协同占据Al原子位的P相互作用的缘故。     

Fe对NiAl力学性能影响的第一原理计算

采用第一原理赝势平面波方法,基于虚拟晶体势函数近似(VCA),计算Fe合金化(浓度x<3.0%,原子分数,下同)时完整与缺陷B2-NiAl晶体的弹性性质,并采用弹性常数C44、Cauchy压力参数(C12-C44)、杨氏模量E、剪切模量G及其与体模量B0的比值G/B0等,表征和评判Fe合金化浓度x对NiAl金属间化合物延性与硬度的影响。结果表明:无论是无缺陷的理想NiAl晶体,还是含Ni空位或Ni反位的NiAl缺陷晶体,x<0.6%的Fe合金化均可使其硬度大幅提高。Fe合金化浓度低于0.5%时,虽然完整NiAl晶体的延性变差,但含Ni空位的缺陷NiAl晶体的延性却可明显改善,并以x=0.2%～0.4%时韧化效果最好。Ni空位或Ni反位降低B2-NiAl晶体的本征延性。实验中0.20%～0.25%的Fe合金化对NiAl晶体延性的改善很可能源于Fe原子与NiAl晶体中Ni空位间的关联与协同作用。     

Al3Li相反位缺陷演化的微观相场模拟

基于离散格点形式的微扩散方程(Langevin方程),模拟了Al3Li相反位缺陷随时间的演化特征及随组元浓度、温度的变化规律.结果表明,Al3Li相中主要以Al原子占据Li位形成的反位缺陷AlLi为主,同时存在少量的Li原子占据Al位形成的反位缺陷LiAl,2种反位缺陷浓度均随温度的升高而上升,且AlLi远大于LiAl上升速率;随Li浓度的增加,AlLi浓度缓慢降低,LiAl浓度略有上升,但仍远远小于AlLi浓度;浓度变化对反位缺陷的影响远不及温度对其影响大.     

Ni_(74.6)Al_xMo_(25.4-x)合金早期沉淀过程的微观相场模拟

采用三元微观相场动力学模型研究Ni74.6AlxMo25.4-x合金早期沉淀过程,对合金的微观组织演化图像、平均序参数和原子占位概率进行了分析。结果表明:合金首先析出L10和L12相,析出的L10相原位转变为L12相。随着Al浓度的增加,原子聚簇和有序化的进程加快。Ni原子倾向于占据αⅠ位和αⅡ位,Al原子和Mo原子倾向于占据β位。随着Al原子浓度的增加,Ni原子和Al原子在αⅠ和αⅡ位的占位概率增加,Mo原子在αⅠ和αⅡ位的占位概率降低,Al原子在β位占位概率增加,Ni原子和Mo原子在β位的占位概率降低。     

微观相场法研究应变能对Ni68.2Al22.7V9.1合金中两相沉淀及原子占位的影响

采用微观相场方法,以 Ni68.2Al22.7V9.1 合金为例研究了应变能对两相沉淀及原子占位的影响。研究结果显示:沉淀过程中应变能明显使 L12沉淀相增加,对DO22相的沉淀有一定的抑制作用;由于 DO22相减少,V原子在L12相中各个位置的占位几率增大,而且应变能越大占位几率越高;应变能对Al原子在L12相中的α1位的占位几率影响不大,只有小幅度的降低,但明显降低了Al原子在α2位和β位的占位几率;各种情况下V原子在 L12相β位的占位几率始终高于α位。     

First-principles investigation of migration barriers and point defect complexes in B2–NiAl

We perform a systematic first-principles investigation of atomic hop mechanisms in B2–NiAl and discover a low barrier collective hop that can mediate Al diffusion through the anti-structural bridge mechanism. We also find an alternative hop sequence for the migration of a triple defect than that proposed previously. To shed light on the dominant hop mechanisms that mediate diffusion in B2–NiAl, we study point defects and defect clusters in B2–NiAl at high temperature by combining a cluster expansion with Monte Carlo simulations. Going beyond the mean field approximation, we find that the inclusion of interactions among the various point defects is crucial to predict the concentration of defect complexes, such as the triple defects of B2–NiAl. Interactions among point defects also introduce an important degree of short-range order between Al antisite defects and Ni antisite defects. We find an increasing probability between pairs of Al antisite atoms and Ni vacancies as the alloy concentration of B2–NiAl becomes both Al rich and Ni rich, suggesting that the anti-structural bridge mechanism should play an important role in facilitating Al transport.     

The effect of platinum on defect formation energies in β-NiAl

First-principles density functional theory is used to examine the effect of Pt on point defects and defect clusters in NiAl. It is found that Pt promotes the formation of Ni and Al vacancies and Ni and Al antisite atoms. Defect clusters that are minima in postulated Ni diffusion mechanisms in NiAl are also found to be stabilized by the presence of Pt. By decreasing defect formation energies, Pt may decrease the overall activation barrier to the diffusion of Ni and Al in NiAl. The results provide clues as to how Pt enhances thermal barrier coating lifetime.     

Possibilities of random vacancy distribution and antisite atom recovering in the point defect mechanism in B2-type intermetallics

Point defect behavior in B2-type intermetallic compounds is investigated from thermodynamic point of view based on the Bragg–Williams method. The model is developed by taking new point defect formation mechanism, random vacancy distribution (RVD) and antisite atom recovering (ASAR) processes, into consideration, which was proposed based on the current findings in X-ray and in situ neutron diffraction studies for B2 FeAl. Free energy expressions for pure states of the antisite defect (ASD), RVD, triple defect (TRD) and ASAR and also those for hybrid state between the RVD, TRD and ASAR are obtained. From these expressions, the condition for appearance of the RVD and ASAR behavior is considered. Numerical results are given for three cases which show the different point defect behaviors due to composition and temperature. The first case indicates a creation of a substantial high concentration of the A-vacancy only in the B-rich composition region, as observed in B2 NiAl. This situation, however, is interpreted by the ASAR process by B antisite atom, not by the TRD process. In the second case, RVD like behavior appears in the A-rich region. This resembles the observation in B2 FeAl, but it is not by an appearance of the pure RVD state. ©     

Point Defect Concentrations and Interactions in D0(19)-Ti3Al from First- Principles CalculationsEI北大核心CSCD

The intermetallics D0(19)-Ti3Al has low specific density and high thermal resistance for both bulk and coating applications in engineering area. The point defects such as thermal vacancy, compostion vacancy and antisite defect have great influence on the properties of D0(19)-Ti3Al, but are usally neglected. According to available research data from both theory and experiment, it is commonly considered that the thermal vacancies in D0(19)-Ti3Al provide paths for atomic migration and diffusion, the antisite defects play an important role in the disordering of D0(19)-Ti3Al, and the interaction between composition vacancy and antisite defect may have important influence on atomic diffusion and dislocation movement. So it is necessary to explore the mechanism of interaction between composition vacancy and antisite defect for more accurate understanding of the atomic diffusion, dislocation movement and plastic deform in D0(19)-Ti3Al. In this work, the formation enthalpy, equilibrium concentration, and binding energy of composition vacancy and antisite defect in D0(19)-Ti3Al intermetallics were calculated by using both the Wagner-Schottky model of point defect thermodynamics and the plane wave pseudopotential method in first-principles. Results suggest that, in the whole composition range of interest, the point defect concentrations increase with the increase of temperature. In particular, the concentrations of antisite defects are higher than those of vacancies, and the vacancy concentration of Ti is higher than that of Al. At the stoichiometric composition, the concentrations of antisite defects of Ti and Al are very close. At the Ti-rich side of component, the antisite defect of Ti dominates in concentration, while at the Al-rich side, that of Al dominates in concentration. For the calculated results of 3 types of point defect pairs, AlTi-TiAl, TiAl-TiAl and VAl-AlTi, they may have the strong trend to aggregate, while others may show the tend to diffuse into the matrix.     

ATOMISTIC SIMULATION OF POINT DEFECTS IN NiAl ALLOY

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Many-body central force potentials and properties of grain boundaries in NiAl

Empirical many-body central force potentials of the Finnis-Sinclair type have been constructed for B2 NiAl by fitting a number of equilibrium properties of this alloy and reproducing the asymmetric behaviour of constitutional point defects in off-stoichiometric NiAl. At the same time these potentials ensure the structural and mechanical stability of the B2 lattice and reproduce quite adequately the equilibrium properties of Ni₃Al. Using these potentials, grain boundaries in NiAl have been studied by computer simulation. It was found that in stoichiometric NiAl alloy boundaries with a surplus of aluminium have appreciably lower cohesive strength than the stoichiometric boundaries or boundaries with a surplus of nickel. From the structural point of view, boundaries with a surplus of aluminium possess the largest expansions and large ‘holes’ usually occur in the boundary regions. On the other hand, boundaries with the stoichiometric configuration or with a surplus of nickel have more compact structures. The interaction of the antisite defects and vacancies with grain boundaries was also studied and segregation of nickel and aluminium in off-stoichiometric alloys discussed with the help of these results.     

Site Occupation of Nb in γ-TiAl: Beyond the Point Defect Gas Approximation

Microalloying is an effective approach to improve the mechanical properties of γ-TiAl intermetallic compound. Knowledge about the site occupancy of the ternary alloying element in the crystal lattice of γ-TiAl is highly demanded in order to understand the physics underlying the alloying effect. Previous first-principle methods-based thermodynamic models for the determination of the site occupancy were based on the point defect gas approximation with the interaction between the point defects neglected. In the present work, we include the point defect interaction energy in the thermodynamic model, which allows us to predict the site occupancy of the ternary alloying element in γ-TiAl beyond the point defect gas approximation. The model is applied to the γ-TiAl-Nb alloy. We show that, at low temperature, the site occupancy of Nb atoms depends on the composition of the alloy: Nb atoms occupy the Al sublattice for the Ti-rich alloy but occupy Ti sublattice for the Al-rich alloy. The fraction of Nb atoms occupying Al sublattice in the Ti-rich alloy decreases drastically, whereas the fraction of Nb atoms on the Ti sublattice in the Al-rich alloy decreases slightly with increasing temperature. At high temperature, Nb atoms occupy dominantly the Ti sublattice for both the Ti-rich and Al-rich alloys. The interaction between the point defects makes the Ti sublattice more favorable for the Nb atoms to occupy.