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。     

定量计算Ni_3(Al_(1-x)Fe_x)沉淀中原子占位的微观相场研究（英文）

采用微观相场方法,模拟研究了化学计量比为Ni_(75)Al_(25-x)Fe_x(x=0,5~10)系列合金在时效温度为1273 K时的原子占位情况。通过该方法,定量计算了L1_2-Ni_3(Al_(1-x)Fe_x)沉淀相中各原子的占位几率(SOP),并获得了其随Fe含量变化的动态响应规律。研究结果表明:随着Fe浓度的增加,Fe原子优先占据B格点位置(FCC结构的角位),且其原子占位几率数值逐渐增高;Al原子在B格点位置的占位几率则明显降低。同时,在L1_2相沉淀过程中出现了Al_Ni和Fe_Ni反位现象;且随着Fe含量的升高,Al_Ni和Fe_Ni原子反位的形成将会变得更加容易。原子占位几率的瞬时动态演化早在L1_2相长大的初期阶段就已完成。     

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.     

D0_(19)-Ti_3Al中点缺陷浓度与相互作用的第一性原理研究

采用Wagner-Schottky点缺陷热力学模型和第一性原理平面波赝势方法,计算研究了D019-Ti3Al金属间化合物中空位和反位2种类型点缺陷的形成焓、平衡浓度及相互作用。结果表明,这些缺陷的平衡浓度均随温度升高而增大,反位缺陷浓度均高于空位缺陷,Ti原子空位的浓度高于Al原子空位。在理想化学计量比成分下,Ti原子反位与Al原子反位缺陷浓度基本相当;在略偏离计量比的富Ti成分端,Ti原子的反位缺陷浓度高于Al原子;在富Al成分端则情形相反。计算结果表明,3种点缺陷对(Al_(Ti)-Ti_(Al)、Ti_(Al)-Ti_(Al)、V_(Al)-Al_(Ti))在基体中具有较强的聚集趋势,而其它类型的点缺陷对则有向基体扩散的趋势。     

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。     

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.     

低碳钢表面抛丸加速制备纳米铝化物涂层的研究

将粉末包埋渗铝与抛丸过程相结合,在440-600℃相对较低的温度范围内,在低碳钢表面制备纳米结构的铝化物涂层.涂层为单层结构,均匀致密,呈现纳米结构特征,晶粒尺寸20nm左右.涂层由富铝相Fe-Al化合物组成,主要有η-Fe2Al5相和少量的θ-FeAl3相和β-FeAl相.合金球的冲击作用导致表面纳米化,加速表面原子扩散过程,使铝化物涂层可以在相对较低的温度下形成.     

Ce与Cr协同作用B2-NiAl合金力学性能的理论研究

本文采用第一性原理赝势平面波方法,基于虚拟晶体势函数近似(VCA),研究了不同浓度x (x=0~2.2%,原子百分比)的Ce和Cr 单独及协同合金化对B2-NiAl晶体弹性模量E、剪切模量G、 Cauchy压力参数及G/B0比值的影响。结果表明：Cr占据Ni原子位能显著提高B2-NiAl合金的剪切模量G和弹性模量E, 而Ce占据Al原子位则能降低其合金的G和E；随着合金化浓度提高到2.1%,Cr占据Al原子位能改善B2-NiAl合金的延性,而当Ce取代Al原子位协同Cr取代Al原子位时,对其延性的改善效果比各自单独合金化的效果更显著,且以合金化浓度为2.0%时为最佳效果。这一理论计算很好的重现了实验中Ce与Cr协同合金化明显提升B2-NiAl合金室温延性的现象。电子态密度分析显示,Cr原子或者Cr协同Ce原子能够削弱B2-NiAl晶体当中的Ni(d)-Ni(d)主要成键峰的杂化效应,降低主要成键峰的方向性。     

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。     

Microstructure of aluminized stainless steel 310 after annealing

The aluminized coating on type 310 stainless steel prepared by high-activity Al pack cementation method has been annealed at 900 °C for 12 h to transform the brittle δ-Fe₂Al₅ phase into the more ductile β-FeAl phase. The microstructure is studied in detail with transmission electron microscopy. The thick outer layer has β-(Fe, Ni)Al as matrix with cube-like Cr₂Al precipitates. The interfacial layer has a thin layer of metastable FCC phase (layer I) and then mixed β-(Fe, Ni)Al grains and α-(Fe, Cr) grains (layers II and III). The Cr₂Al precipitates are present in the β-(Fe, Ni)Al grains in layer II but not in those in layer III, while β-FeAl precipitates are present in the α-(Fe, Cr) grains in both layers. The orientation relationships between various phases, the formation of the layers, and the precipitation of Cr₂Al in β-(Fe, Ni)Al are discussed.     

A positron study of the defect structures in the D03 and B2 phases in the Fe–Al system

The positron annihilation technique was used to identify the nature of the vacancy-type defects in the D0₃ and B2 phases of the Fe–Al system. Seven alloys with Al concentrations in the range 22.7–48 at.% Al and with different thermal treatments were examined. Positron lifetime calculations for the expected defects in the two phases were also performed in order to facilitate the defect identification. In the B2 phase, two types of defects were identified: a thermal complex formed by a Fe-divacancy and an Al antisite, and a Fe-vacancy. No constitutional vacancies were found in the D0₃ phase.     

Fe-Al系金属间化合物中的微观缺陷和电子密度

对二元Fe-Al合金，含Cr和Si的Fe3Al合金的正电子寿命谱测量表明：随着二元Fe-Al合金中Al含量的增加，空位浓度增加，微孔洞的开空间增大。在Al含量高于40％原子分数）的B2－FeAl合金中存在着较高的空位浓度和开空间相当于Fe中的10－15个空位聚集体的微空洞。在B2－FeAl和D03－Fe3Al合金中，晶格中最邻近的Fe-Al原子对之间发生Fe-d-Alp杂化使用。Al的3p电子与Fe的3d电子被局域化并形成共价键。导致合金中的自由电子密度降低。二元Fe-Al合金中的平均电子密度随着Al含量的增加而下降。用Cr元素对Fe3Al进行合金化。合金基体和晶界处的自由电子密度均增加；而加入Si元素，合金基体和晶界处的自由电子密度均减小。讨论了Fe-Al合金的微结构对其力学性能的影响。     

Site occupancies,point defect concentrations,and solid solution hardening in B2 (Ni,Fe)Al

A novel approach was used to characterize the site occupancies and point defect concentrations in the B2-ordered (Ni,Fe)Al single phase field over a wide range of composition and temperature. This approach combined atom location by channeling enhanced microanalysis (ALCHEMI) determinations of atomic site occupancies and vacancy concentration measurements. The triple defect structure was observed across the entire phase field. Qualitative thermodynamic predictions such as that Fe anti-site are more stable than Ni anti-sites were confirmed. However, in some portions of the phase field it was found that Ni anti-site concentrations were higher than expected from simplistic thermodynamic predictions of site preference. Such predictions are clearly inadequate if a quantitative determination of site occupancies is desired. The importance of this type of characterization in interpreting solid solution hardening (SSH) in ordered compounds was illustrated by considering three portions of the (Ni,Fe)Al phase field. In all three cases the solute atoms did not directly harden, but affected the hardness by altering the concentrations of other point defects.     

Microstructures and morphologies of B2-Ordered NiAl(Co) and FeAl(Co)

Fine dispersion of disordered phases is obtained in a Ni-Al-Co and Fe-Al-Co ternary system. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Fe in B2-ordered FeAl(Co) with different stoichiometries. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and B2-FeAl(Co) increased appreciably by the fine precipitation of (Ni,Co)₂Al, α-Fe, and overage softening occurred after prolonged aging. In case of B2-ordered (Ni,Co)Al, the (Ni,Co)₂Al phase had a hexagonal structure and took a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appeared in the hexagonal (Ni,Co)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix was (0001)_p//(111)_B2 and $[\bar 12\bar 10]_p //[\bar 110]_{B2}$ , where the suffix p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardened appreciably by the fine precipitation of the (Ni,Co)₂Al phase. On the other hand, in case of B2-FeAl(Co), the disordered α-Fe phase was present as a precipitate in a B2-FeAl(Co) matrix and had a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix was typically hardened by the precipitation of α-Fe.     

Effect of Mn on Fe containing phase formation in high purity aluminium

In this study, the effect of varying Mn additions on Fe phase formation in high purity Al and its corresponding effect on the resulting mechanical properties have been investigated. Thermodynamic simulations have shown that in the Al–Fe–Mn ternary system two intermetallic phases (namely Al₆(Fe,Mn) and Al₁₃Fe₄) form. Findings indicated that a relatively high amount (>1 wt-%) of Mn was required to achieve Al₆(FeMn) phase formation which was congruent with experimental results. Both the Al–Fe and Al–Fe–Mn phases observed displayed fibre/platelet type morphologies and were found to exist at α-Al grain/cell boundaries. Results indicate that the Fe phases coarsen with increasing Mn content. In the Al–Fe system the Mn addition improves yield strength (YS) and ultimate tensile strength (UTS) but reduces elongation beyond the reduction resulting from the Fe addition. The further decrease in elongation with Mn was attributed to the increase in volume fraction of the intermetallic phases.     

Fabrication of dense Al2O3-FeAl composites by using solid-state sintering

Highly densified alumina-iron aluminide (Al₂O₃-FeAl) composites consisting of ubiquitous elements were fabricated by using pulse current sintering technique under a certain uni-axial pressure. The solid-state sintering without melting FeAl was the highlight in this study. The mechanical properties of the Al₂O₃-FeAl composites were much greater than previously reported ones fabricated by reaction sintering technique. The poor wettability of FeAl against Al₂O₃ strongly influenced the mechanical properties and made it difficult to be highly densified Al₂O₃-FeAl composites by liquid phase sintering especially when volume fraction of FeAl to Al₂O₃-FeAl was high (>30.5 vol%). However, highly densified Al₂O₃-FeAl composites were obtained by solid-state sintering with control of Al₂O₃ grain size and sintering temperatures. It was concluded that highly controlled powder metallurgy made it possible to fabricate dense ceramic-metal (intermetallic) composites from the combination of materials having poor wettability.     

Thermodynamic properties of the Al–Fe–Ni system acquired via a hybrid approach combining calorimetry,first-principles and CALPHAD

A reaction calorimeter coupled with first-principles calculations was employed to obtain enthalpies of formation for τ₁ (Al₉FeNi) and τ₂ (Al₁₀Fe₃Ni) compounds. The previous thermodynamic model for describing the disorder/order transition (fcc_A1/L1₂) in the Al–Fe–Ni system was modified to extrapolate this model to quaternary and higher-order systems. The first-principles energy calculations for the end-members of sub-lattice models in ternary compounds and L1₂ phase were performed to facilitate subsequent modeling. The existence of the experimentally observed miscibility gap for ternary B2-ordered phase is detected by the present calculation. Such a feature cannot be identified with available thermodynamic software due to the tiny difference between the Gibbs energies associated with different phase assemblages. A set of thermodynamic parameters for the Al–Fe–Ni system was obtained via thermodynamic modeling. Numerous experimental data including phase diagram, thermodynamic properties and site occupation of Fe in B2 phase are well accounted for by the present modeling.     

Phase formation in mechanically alloyed AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys

Alloying behavior and phase transformations in Al_xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) multi-component high entropy alloys that are synthesized by mechanical alloying were studied. Two FCC phases along with a BCC phase were formed in Al_0.45CoCrCuFeNi and AlCoCrCuFeNi, while a single B2 phase was observed in higher Al containing alloys Al_2.5CoCrCuFeNi and Al₅CoCrCuFeNi. DSC analysis indicates that BCC phase present in the alloys could be Fe–Cr type solid solution. A detailed analysis suggests that two melting peaks observed during DSC in lower Al containing alloys can be attributed to that of Cu–Ni and Fe–Ni FCC solid solutions. The BCC phase disappears in Al_0.45CoCrCuFeNi and AlCoCrCuFeNi at high temperatures during DSC. However, Al₅CoCrCuFeNi retains its B2 structure despite of heating in DSC. Further, phases present in these alloys retain nanocrystallinity even after exposure to high temperatures. A critical analysis is presented to illustrate that solid solution formation criteria proposed for high entropy alloys in the literature are unable to explain the phase formation in the present study of alloys. Besides, these criteria seem to be applicable to high entropy alloys only under very specific conditions.