空位缺陷对单层MoS_2电子结构的影响

为了研究单层Mo S2中的空位缺陷形成及其对电子结构的影响,基于密度泛函理论框架下的第一性原理,采用平面波赝势方法分别计算了单层Mo S2中Mo空位和S空位的形成能、空位附近的晶格畸变、Mo S2层中的电子分布以及态密度(DOS)和能带结构。计算结果显示,2种空位缺陷都具有点缺陷特征,其附近的电子分布呈现出明显的局域化特点,且S空位比Mo空位更容易形成。通过与本征态Mo S2电子结构的对比分析,发现2种空位缺陷的存在对单层Mo S2的电子结构、尤其是对导带高能量区域的能态密度会产生十分明显的影响,这些影响可能与空位缺陷引入的缺陷能级有关。     

Creep-Rupture by Vacancy Condensation

The possibility that formation of voids under creep-rupture conditions may take place by the condensation of vacancies has been investigated theoretically. It has been concluded that nucleation of voids under creep-rupture conditions by vacancy condensation is highly improbable. However, growth of pre-existant voids by vacancy condensation is probable. A number of predictions made in this theory have been verified by the data. It has been predicted and checked that the product of rupture life and steady-state creep rate for preannealed metals and single phase alloys is an approximately invariant quantity, independent of stress, temperature, and atomic number for a given type structure. The direction of the effect of cold work on this product has been predicted and found in agreement with experiment. A number of experiments to evaluate the vacancy condensation mechanism further are described.     

Vacancy strengthening in Fe3Al iron aluminides

The room temperature strength of FeAl alloys can be increased significantly by freezing in the high thermal vacancy concentrations present at elevated temperatures. In contrast, because of their lower thermal vacancy concentrations, vacancy strengthening in quenched Fe₃Al alloys is believed to be much smaller and has not received much attention to date. In the present work, the influence of annealing time and quench temperature on the room temperature strength of extruded and recrystallized Fe₃Al alloys is evaluated. For aluminum concentrations between 28 and 32 at% and quench temperatures between 400 and 900 °C both the magnitude and the kinetics of strengthening are found to be consistent with reported values for the thermal vacancy concentrations and vacancy migration rates. To assess the potential contributions of other strengthening mechanisms, appropriate heat treatments will need to be designed in follow-on studies that alter microstructural features relevant to those mechanisms while maintaining a constant vacancy concentration.     

A positron investigation of electron-irradiation induced defects in γ-TiAl intermetallic compounds

The isochronal recovery of irradiation induced defects was investigated in γ-TiAl intermetallic compounds (with 50 and 54 at% Al) by positron lifetime measurements after 2.5 MeV electron irradiation at 21 K. In the as-irradiated condition, the analysis of the results and the comparison with published data led to a value τ_d=230±5 ps for the lifetime of vacancy-trapped positrons. The lifetime variations observed during isochronal anneals at increasing temperatures are consistent with vacancy migration around 450 K. The observation of a progressive decrease in the lifetime of trapped positron, during the migration and elimination of vacancies, suggests that they do not form unrelaxed three-dimensional clusters, and that another type of positron traps is simultaneously present.     

Variational calculations of positron characteristics in metals

Energies of the formation of monovacancies, as well as the binding energies and work functions of quasi-free and localized positrons and positroniums have been calculated in terms of the gradient version of the density-functional method using the stable-jellium model. The calculations show that for the metals possessing negative positron work function, the presence of vacancies in the subsurface region can lead to the change in the sign of the work function. The same effect of the change in the sign of the work function of a quasi-free positron can be caused by the presence of a dielectric coating on the metal surface. Comparison with the experiment is performed.     

TiAl中空位团的稳定构形及其对附近空位迁移行为的影响

用分子动力学方法,采用Finnis-Sinclair类型的多体势,对TiAl中小尺寸空位团（N0=2,3,4）的各种可能构形进行了模拟计算,计算了空位团的形成能、结合能,分析讨论了容位团最稳定的构形,在此基础上,研究了空位团对单空位迁移的影响。计算结果表明：在TiAl合金中,组成空位团的每个空位都尽可能地与其它空位保持最近邻关系,空位团附近有反位原子。当空位团较大时,空位团周围的原子会向空位中心埸陷,类似于空位团中包含有间隙原子。已有的空位团可作为空位的凝聚中心具有捕获或吸收附近空位的能力。     

Effects of Oxygen Vacancy on the Electronic Structure of Cubic LaAlO3

采用基于密度泛函理论和广义梯度近似的第一性原理研究了立方相LaAlO3中不同价态氧空位的电子性质。结果表明,氧空位的存在会在立方相LaAlO3的禁带中引入缺陷能级,在氧缺失的条件下,氧空位在立方相LaAlO3中是稳定存在的。随着费米能级位置的改变,具有0价和+2价的氧空位分别为最稳定的电荷状态。LaAlO3中的氧空位具有负U性质,当空位进入到氧化物中时,氧空位会俘获2个空位,达到稳定状态。因此,在LaAlO3高k栅介质中,氧空位为主要俘获电荷的陷阱。     

Vacancy mechanism of oxygen diffusivity in bcc Fe: A first-principles study

Diffusivity of interstitial oxygen (O) in bcc iron (Fe) with and without the effect of vacancy has been investigated in terms of first-principles calculations within the framework of transition state theory. Examination of migration pathway and phonon results indicates that O in octahedral interstice is always energetically favorable (minimum energy) with and without vacancy. It is found that vacancy possesses an extremely high affinity for O in bcc Fe, increasing dramatically the energy barrier (∼80%) for O migration, and in turn, making the predicted diffusion coefficient of O in bcc Fe in favorable accord with experiments.     

Role of vacancy–solute complex in the initial rapid age hardening in an Al–Cu–Mg alloy

We have investigated the origin of the initial rapid hardening of an Al–1.3 at.% Mg–1.7 at.% Cu alloy by coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopy. Quenched-in vacancies are bound to Mg atoms rather than Cu atoms initially and the vacancy–Mg complexes easily migrate to vacancy sinks at 150°C. Vacancy–Mg–Cu complexes form during the initial 1 min aging at vacancy sinks, meanwhile vacancy density decreases rapidly. These results support that the dislocation–solute interaction is the origin of the initial rapid hardening.     

Study on the recovery of quenched commercial pure Al and Al-1wt.% Mn alloys by positrons

Isochronal annealing of quenched commercial pure Al (99.5%) and Al-1wt.% Mn was performed between room temperature and 560 °C. The annealing of defects was studied using positron lifetime and Doppler broadening techniques. The behaviors can be divided into three stages: recovery, partial recrystallization, and complete recrystallization. The results suggest that a strong interaction between Mn atoms and vacancies in Al would retard the vacancy recovery from quenching. The Mn atoms enhance the nucleation of vacancy clustering in Al.     

镁偏析对铝合金晶界性能影响的计算机模拟

采用嵌入原子法对铝晶界上Mg的偏析性质及其对晶界性能的影响进行了计算机模拟。结果表明 ,Mg的晶界偏析具有自发性 ,并且降低基体的电荷密度和空位形成能 ,从而导致晶界脆化。     

Mg对三元Al-Cu-Mg合金位错分布组态的影响

对经均匀化退火，不同变形锻造，固溶处理的样品，借助TEM，研究了Mg对三元Al-Cu-Mg合金位错分布组态的影响。研究结果表明：Cu原子与Al原子在变形与时效过程中形成了Al2Cu沉淀相而降低了Cu元素在Al基体中的浓度，减少了Cu、Mg元素的交互作用。由于Mg元素有较大的原子半径，与基体产生了很大的晶格畸变能和内摩擦力，因此Mg元素在Al基体中对位错的分布式滑移（交滑移）起着明显的阻滞作用，限制了胞状组织的形成。与典型的高能层错金属与合金中位错呈胞状组织不同，Al-Cu-Mg合金中的位错呈准均匀的Taylor晶格分布，这种分布可以降低合金的自由能。     

First-principles study of vacancy formation and migration in clean and Re-doped γ′-Ni3Al

Using density functional theory calculations in conjunction with the climbing images nudged elastic band method, we studied the vacancy formation and migration in clean and Re-doped Ni₃Al. Both the chemical potential of the species and the magnetic effect are considered to determine the vacancy formation energy. We also simulated the vacancy migration in a complete set of migration paths. The evaluated vacancy formation energy and activation energy for the motion of vacancy compared well with the experimental results. Also, the obtained migration ways for the diffusion of Ni and Al atoms are consistent with previous theoretical predictions and experimental observations. Magnetism is found to influence both the vacancy formation and migration. Our results reveal that Re doping can inhibit the formation of Ni vacancies but facilitate the formation of Al vacancies, and can also inhibit the migration of neighboring vacancies. While the doped Re atom on the Al site is stable, the Re atom on the Ni site can diffuse within the Ni-sublattice mediated by Ni vacancies.     

Effect of alloying α-Fe with aluminum, silicon, cobalt, and germanium on the snoek relaxation parameters

Parameters of Snoek relaxation in binary alloys of iron with Co, Ge, Al, and Si have been studied. The concentration of Al was varied from 1.5 to 16 at. %, and that of Si, from 3 to 10 at. %, in binary (Fe-Al, Fe-Si) and ternary (Fe-Al-Si) alloys. It has been shown that Co and Ge added to α-Fe in amounts to ～3 at. % only wekaly affect the parameters of the Snoek peak, i.e., the height, width, and shape of the peak, as well as the activation energy for diffusion of carbon atoms. Aluminum and silicon lead to a qualitatively the same effect—a broadening of the Snoek peak on the high-temperature side, which indicates an increase in the activation energy for carbon diffusion and carbon redistribution in iron. The quantitative effects of Al and Si are different. To analyze these effects, a computer analysis of the curves of the temperature dependence of internal friction has been applied.     

Calculations of positron states in BEDT-TTF based organic superconductors

We have calculated positron distributions on a series of BEDT-TTF based organic conductors, whose anion species are I₃, Cu(NCS)₂, Cu(CN)[N(CN)₂], Cu[N(CN)₂]Br and KHg(SCN)₄. The results are discussed in connection with Fermiological studies by the positron annihilation method. Lifetimes and band masses of positrons have been calculated also.     

Ni3Al中Al含量对硼韧化作用的影响

本文利用“有序—无序”模型对Ni_3Al中的点阵空位与成分的关系进行了计算,结果证明,当Al含量超过化学计量值时,Ni_2Al中有大量的点阵空位存在,称之非化学计量空位。正电子湮没技术证实了这种空位的存在。由此解释了Al含量对硼韧化作用的影响。     

温度对六方系金属层错能影响的热力学模型

利用密排六方金属发生层错时在层错区域由密排六方(Hexagonal Close-packed,hcp)的AB排列转变为面心立方(Face-centered Cubic,fcc)的ABC排列的特点,通过计算hcp/fcc相变所需要的能量来计算hcp金属层错能.同时,在模型中引入了空位缺陷和温度等因素.利用此模型计算Mg,Zn,Ti等3种hcp纯金属的层错能,推导出层错能随温度变化的理论计算式,计算出变化值.计算结果表明:该热力学模型适用于hcp金属层错能的计算.Mg,Zn,Ti等hcp结构金属的层错能随着温度的升高逐渐降低,化学自由能变对层错能的影响占主导地位.空位有降低层错能的作用,随着温度的升高空位密度增大,层错能降低的幅度增大.     

The effect of platinum on Al diffusion kinetics in β-NiAl: Implications for thermal barrier coating lifetime

First-principles density functional theory calculations are used to study Al diffusion in β-NiAl. The activation energy and diffusion constant pre-exponential factors are calculated for five previously postulated Al diffusion mechanisms: next-nearest-neighbor Al jumps, the triple defect mechanism and three variants of the six-jump cycle mechanism beginning with an Al vacancy. We predict that the triple defect mechanism has the lowest activation energy and is the mechanism by which Al diffusion occurs in NiAl. In order to elucidate why Pt has a beneficial effect on thermal barrier coating lifetime, the effect of Pt on each of these mechanisms is also examined. In all cases, Pt decreases the diffusion activation energy, which should enhance Al diffusion in the coatings.     

Comparative behavior of vacancy and C,B, N,O atoms single defect on hardening the B2-FeAl structure: An atomistic study

Atomistic modeling based on the density functional theory (DFT) is used to study the behavior of single defects in B2-FeAl. A comparative study of vacancy, as well as boron, carbon, nitrogen, and oxygen substituting Fe or Al atoms is presented in order to give new insights in the understanding of hardening associated to the presence of small radius atoms. The effects of these defects on lattice parameters, bulk modulii, and density of states at the Fermi level are compared. Also, their potential for hardening is discussed in light of their site preference and chemical bindings with Fe and Al atoms.     

The effect of tin and nitrogen on liquid phase sintering of Al–Cu–Mg–Si alloys

Alloys of Al–3.8Cu–1Mg, Al–3.8Cu–1Mg–0.7Si, Al–3.8Cu–1Mg–0.1Sn and Al–3.8Cu–1Mg–0.7Si–0.1Sn were made using elemental powders and conventional press-and-sinter powder metallurgy techniques. The sintering of these alloys was studied at 590 °C under atmospheres of nitrogen or argon using a horizontally aligned, push-rod dilatometer. Sintered samples were characterized using scanning electron microscopy and positron annihilation lifetime spectroscopy. Under argon, shrinkage occurs in a single stage and the shrinkage rate decays monotonically over time. Sintering shrinkage under nitrogen occurs in three distinct stages and the rate increases over time. Tin decreases shrinkage under argon but activates shrinkage under nitrogen. Positron spectroscopy indicates that the presence of tin does not alter the vacancy concentration in the sintered alloys, suggesting that tin does not bind preferentially to vacancies in the aluminum matrix in these AlCuMgSi alloys. Rather, it appears that tin may limit the formation of aluminum nitride on specific surfaces.