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.     

Influence of chemical composition and alloying elements on microdefects and electron density in Ni—Al alloys

1　ＩＮＴＲＯＤＵＣＴＩＯＮＬｌ2 Ｎｉ3ＡｌａｎｄＢ2 ＮｉＡｌａｒｅｉｍｐｏｒｔａｎｔｉｎｔｅｒｍｅｔａｌ ｌｉｃｃｏｍｐｏｕｎｄｓｉｎＮｉＡｌａｌｌｏｙｓ .ＡＮｉ3Ａｌａｌｌｏｙｈａｓｒｅｌａ ｔｉｖｅｌｙｈｉｇｈｓｔｒｅｎｇｔｈａｎｄｐｒｅｓｅｎｔｓｒｅ     

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.     

Molecular dynamics simulation of diffusion in a (110) B2-NiAl film

We report on the first direct molecular dynamics study of diffusion in B2-NiAl using one of the most reliable embedded-atom method potentials for this phase. The simulation is performed for near the stoichiometric composition at a temperature just below the melting temperature of the model. In the molecular dynamics simulation, an equilibrium point-defect concentration is generated and maintained by using a film sample with periodic boundary conditions only in two directions and free surfaces in the third direction. Two types of point defects – Ni vacancies and Ni antisites – are found in the bulk of the model. It is demonstrated that isolated Ni vacancies strongly dominate in concentration over all of their bound complexes with Ni antisites. Although we predict that some attractive interactions should occur between point defects to form bound Ni vacancy-Ni antisite pairs and bound Ni antisite-Ni vacancy-Ni antisite complexes, only about 2% of Ni vacancies and 1% of Ni antisites statistically randomly associate to form bound Ni vacancy-Ni antisite-Ni vacancy complexes (the so-called bound triple-defect complex). As a result, it is deduced that the triple-defect diffusion mechanism is not likely to be the dominant diffusion mechanism in the bulk of the model because this diffusion mechanism effectively requires that the Ni vacancies and Ni antisites must form bound triple-defect complexes. Furthermore, it is found that Ni atoms diffuse in the bulk of the model on average about 2.5 times faster than Al atoms. Therefore, we suggest that isolated Ni vacancies are likely to play a key role in atomic diffusion of both Ni and Al near the stoichiometric composition of B2-NiAl and, consequently, the most plausible and widely accepted candidate for dominant diffusion mechanism in B2-NiAl can be considered to be six-jump cycles of a Ni vacancy. Furthermore, we can suppose that additional next-nearest-neighbor jumps of a Ni vacancy may cause that Ni atoms diffuse faster than Al atoms.     

Effects of Ni vacancy,Ni antisite,Cr and Pt on the third-order elastic constants and mechanical properties of NiAl

Effects of Ni vacancy, Ni antisite in Al sublattice, Cr in Al sublattice, Pt in Ni sublattice on the second-order elastic constants (SOECs) and third-order elastic constants (TOECs) of the B2 NiAl have been investigated using the first-principles methods. Lattice constant and the SOECs of NiAl are in good agreement with the previous results. The brittle/ductile transition map based on Pugh ratio G/B and Cauchy pressure P_c shows that Ni antisite, Cr, Pt and pressure can improve the ductility of NiAl, respectively. Ni vacancy and lower pressure can enhance the Vickers hardness H_v of NiAl. The density of states (DOS) and the charge density difference are also used to analysis the effects of vacancy, Ni antisite, Cr and Pt on the mechanical properties of NiAl, and the results are in consistent with the transition map.     

基于马儿可夫链B2-NiAl反位缺陷的模型推导与计算

基于马儿可夫链理论和Bragg-Williams型方程,建立了描述反位缺陷占位几率的基本方程和转移几率;推导出了平衡时空位与反位缺陷浓度的表达式。利用所建模型结合第一性原理平面波赝势法系统研究了NiAl中各种点缺陷,从定量计算和电子结构角度论证了平衡状态下,在Ni:Al=1附近,Ni和Al原子的占位服从Fermi-Dirac统计,并且当温度从800增加到1300K时,NiAl几率比AlNi几率大106～109倍,反位缺陷以NiAl为主。VNi浓度比AlNi浓度大105～107倍、而NiAl浓度比VAl浓度大106～1010倍。对于同一原子而言,NiAl比VNi稳定,AlNi比VAl稳定。     

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.     

NiAl热诱发马氏体相变的分子动力学模拟

运用ＮｉＡｌ合金的镶嵌原子势，进行了Ｂ２结构ＮｉＡｌ中热诱发马氏体相变的分子动力学模拟，从而研究马氏体形核和长大的微观机理．首先进行了０Ｋ等体积条件下ＮｉＡｌ的相稳定性分析，发现０Ｋ时点阵参数ｃ与ａ之比约为１．３１的ｂｃｔ结构是稳定结构，Ｂ２结构只是一种亚稳结构．在热诱发马氏体相变的模拟过程中，由于ＮｉＡｌ中Ｎｉ原子和Ａｌ原子的振动性质存在着差异，造成了马氏体的不均匀形核．对另两种不同初始构型的系统进行的模拟进一步证实了不均匀形核的存在，并通过计算Ｂ２结构ＮｉＡｌ中Ｎｉ原子和Ａｌ原子的热运动均方位移解释了其原因．模拟中，Ｂ２结构奥氏体经热诱发马氏体相变转变为ｆｃｔＬ１_０结构的马氏体．马氏体在长大过程中内部形成了若干转变孪晶．     

Bonding characteristics of micro-alloyed B2 NiAl in relation to site occupancies and phase stability

A recently developed mean-field model has been combined with first principles calculations of binding energy to investigate the site occupancies of micro-alloying elements and vacancies in NiAl as well as the stability of the micro-alloyed B2 phase with respect to disordering and second-phase formation. The theoretical results suggest that the transition metal elements in the same row of the periodic table increasingly tend to the Ni sublattice with increasing atomic number. Micro-alloying addition tends to decrease the vacancy concentration of NiAl alloys. Alloying with X that substitutes for Ni is predicted to have the sides of its solubility lobe parallel to the Ni–X side of the isotherm, but parallel to the Al–X side if X substitutes for Al. Micro-alloying was shown to raise the ordering temperature of the B2 phase over the corresponding binary alloy, in contrast with the effect of vacancies. Alloying effects on ordering temperature and the formation of point defects appear independent of the site substitution behaviour, and are less significant for 3d than for 4d and 5d transition metal elements.     

B2-RuAl点缺陷结构的第一原理计算

采用第一原理赝势平面波方法,计算了B2-RuAl金属间化合物的基本物性及其点缺陷结构的几何、能态与电子结构,通过对不同点缺陷结构形成热与形成能的计算与比较,分析和预测了RuAl金属间化合物中点缺陷结构的种类与存在形式.结果表明：RuAl金属间化合物的点缺陷主要是Ru空位和Al反位,在富Ru合金中主要为Ru反位,在富Al合金中则主要是Al反位.这些点缺陷主要以Ru-Ru双空位和Al-Al双反位的组态结构形式出现,并且双空位以Ru-Ru为第一近邻时其点缺陷结构最稳定,而双反位则是以Al-Al为第三近邻时稳定性最高.进一步通过对NiAl和RuAl不同点缺陷结构Cauchy压力的比较,发现点缺陷对RuAl塑性的降低程度比NiAl低,因而含有点缺陷的实际合金的室温塑性RuAl比NiAl好.     

Modelling of defects and amorphisation by ball milling of γ-TiAl

The formation and site preference energies and volume changes of single and pair of defects of ternary alloying elements in γ-TiAl intermetallic compound were studied by the density functional theory. Slight tendency to clusterization of antisite defects has been found. This may lead to disorder in the system. The V and Cr atoms prefer to reside in the Ti sublattice. The formation energy for Cr–Cr, Cr–V and V–V nearest neighbour pairs are in the 1.3–2 eV range. The Al antisite in Ti sublattice requires much less energy than the Ti antisite in Al sublattice. The amorphisation process of TiAl alloy was studied by means of high energy ball milling of Ti and Al elemental powders, which produces amorphous structure after 40 h. The amorphous states were studied by the DFT calculations of many random atomic configurations and the results were compared with the NiAl compound. Possible explanation for the amorphisation of the TiAl compound is presented.     

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.     

Effective formation energies of atomic defects in D03Fe3Al: an ab-initio study

The effective formation energies of atomic defects in D0₃---Fe₃Al are determined within the framework of a grand-canonical statistical method. The grand-canonical defect formation energies entering this method are calculated by the ab-initio mixed-basis pseudopotential method. Thermal vacancies appear on the sublattice of the Fe atoms with an effective formation energy of 1.25 eV and on the Al sublattice (about 1.4 eV). The structural defects are Al antisite atoms on the γ sublattice of the Fe atoms or Fe antisite atoms on the Al sublattice.     

Mean field theory of point defects in β-NiAl

We have used a mean field theory to study the equilibrium concentrations of vacancies and antisite defects in each sublattice of β-NiAl in the composition range between 45 and 55 at% Al and in the temperature range 800–2000 K. The interatomic potentials, which are inputs to the theory, are extracted from recently reported (continuous) pair potentials of Finnis–Sinclair type. The predominant defect in the stoichiometric alloy is a combination of an Ni-antisite defect and two vacancies on the Ni sublattice. The Al-rich alloys of composition (50+Δ) at% contain 2Δ% vacancies; since the alloys are almost perfectly ordered, these vacancies predominantly occupy the Ni sublattice. Similarly, the Ni-rich alloys of composition (50−Δ) at% contain Δ% Ni antisites. Both the vacancies on the Ni sublattice (in Al-rich alloys) and Ni-antisites (in Ni-rich alloys) show negligible temperature dependence, and hence owe their origin to the off-stoichiometry. In all the alloys, the Al-antisites have the lowest concentration (of the order 10⁻⁶ even at 2000 K). We have critically compared our theoretical results with available experimental ones.     

Existence patterns of Dy in β-NiAl from first-principles calculations

Intermetallic compound β-NiAl is a promising material in high temperature applications due to its high melting temperature,high strength,low density,and good oxidation resistance.However,its application remains limited because of its relatively poor cyclic oxidation resistance.Addition of reactive element(RE)Dy can improve the cyclic oxidation of NiAl alloys significantly.However,the mechanism of Dy addition is not clear.Even the existence pattern of Dy in NiAl is unspecified.Therefore,in the present study,the impurity formation energies of Dy in stoichiometric NiAl,Ni-rich,and Al-rich NiAl for the substitution cases were studied by first-principles density functional theory.The results show that Dy could hardly substitute for either Ni or Al atoms in NiAl.However,calculations for dissolution energies show that Dy could be easily dissolved in Al vacancies in all three types of NiAl,which provides a new existence pattern of Dy in NiAl beyond experimental detection.     

Microscopic phase-field modeling of atomic anti-site behaviors in precipitation progress of Ni3(AlFe)

The effects of temperature on atomic anti-site behaviors in L12-Ni3(AlFe) phases were studied using microscopic phase-field dynamic model in precipitation progress of Ni75Al20Fe5 alloy.The results show that with the increase of temperature,the formation of NiAl and AlNi anti-sites is much easier in Ni3(AlFe),and Ni and Al anti-site atoms show clearly stronger temperature-dependent than Fe anti-site atoms.The evolution progress of anti-site atoms is completed at the initial growth stage of L12-Ni3(AlFe) phases.The site occupation probabilities of Ni atoms on the sublattice A(NiNi,face centers sites of FCC),and Al and Fe atoms on the sublattice B(AlAl and FeAl,corners sites of FCC) all present the degressive tendency with the temperature increasing.Fe atoms mainly prefer to occupy the Al sublattice at the whole temperature range.     

Formation of NiAl Intermetallic Compound by Cold Spraying of Ball-Milled Ni/Al Alloy Powder Through Postannealing Treatment

Ni/Al alloy powders were synthesized by ball milling of nickel-aluminum powder mixture with a Ni/Al atomic ratio of 1:1. Ni/Al alloy coating was deposited by cold spraying using N₂ as accelerating gas. NiAl intermetallic compound was evolved in situ through postspray annealing treatment of cold-sprayed Ni/Al alloy coating. The effect of annealing temperature on the phase transformation behavior from Ni/Al mechanical alloy to intermetallics was investigated. The microstructure of the mechanically alloying Ni/Al powder and NiAl coatings was characterized by scanning electron microscopy and x-ray diffraction analysis. The results show that a dense Ni/Al alloy coating can be successfully deposited by cold spraying using the mechanically alloyed powder as feedstocks. The as-sprayed alloy coating exhibited a laminated microstructure retained from the mechanically alloying powder. The annealing of the subsequent Ni/Al alloy coating at a temperature higher than 850 °C leads to complete transformation from Ni/Al alloy to NiAl intermetallic compound.     

Manufacture of Raney Ni catalyst with metastable Ni2Al3 by high-energy milling

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