杂质对镁合金耐蚀性影响的电子理论研究

利用大角重位点阵模型建立了Mg合金［0001］对称倾斜晶界模型，应用实空间的连分数方法计算了杂质在晶界的偏聚能，杂质原子间相互作用能和不同体系的费米能级，讨论了杂质在晶界的偏聚行为，杂质间的相互作用与有序化的关系及杂质对镁合金腐蚀性能影响的物理本质. 计算结果表明，杂质原子偏聚于晶界，且主要偏聚于晶界的压缩区；杂质原子间相互排斥，因此在晶界区形成有序相；费米能级与材料的腐蚀电位存在这样的关系：材料的费米能级越高，其腐蚀电位就越低，容易被腐蚀，相反费米能级低，其腐蚀电位就高，不容易腐蚀. 体系中成分不同区域的费米能级差导致电子从费米能级高的区域流向费米能级低的区域，正是费米能级差构成了镁合金电化学腐蚀的电动势. 关键词： 电子理论 晶界偏聚 镁合金 腐蚀机理     

La、Al在Fe晶界上协同作用的第一性原理研究

采用基于密度泛函理论的第一性原理,研究了稀土La和Al在bcc-Fe中Fe晶界处的协同作用影响.计算了La和Al在晶界、表面、晶内的形成能,建立La、Al共存于晶界模型,通过电荷密度、布居分布手段分析了La、Al原子对晶界的影响.结果表明:La、Al原子易偏聚在晶界处,且La-Al的原子间距与体系结合能成正比关系. La的掺杂改善了晶界处的电荷分布情况,促进了Al原子与周围Fe原子间的相互作用,态密度曲线的计算结果显示,La原子的加入能够使La、Fe及Al原子间的键的结合力更强,从而提高了界面结合强度.     

难熔元素钨在NiAl位错体系中的占位及对键合性质的影响

用第一性原理离散变分方法研究了难熔元素钨(W)在金 属间化合物NiAl<100>(010)刃型位错体系中的占位以及对键合性质的影响, 计算了纯位错体系和掺杂体系的能量参数(结合能、 杂质偏聚能及原子间相互作用能)、 态密度和电荷密度分布. 体系结合能和杂质偏聚能的计算结果表明: 难熔元素W优先占据Al格位. 此外,由于难熔元素W的4d轨道与近邻基体原子Ni的3d轨道和Al的3p轨道的杂化, 使得掺杂体系中难熔元素W与近邻基体原子间的相互作用能加强; 同时难熔元素W与位错芯区近邻基体原子间有较多的电荷聚集, 这表明W与近邻基体原子间形成了较强的化学键. 难熔元素W对NiAl化合物的能量及电子结构有较大的影响, 从而影响位错的运动及NiAl金属间化合物的性能. 关键词： 电子结构 位错 金属间化合物 杂质     

掺氮碳化硅纳米管电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算,对本征碳化硅纳米管和掺氮碳化硅纳米管的电子结构进行了计算.计算表明本征（8,0）碳化硅为直接带隙半导体,能带间隙为0.94 eV;掺氮浓度为1.56%和3.12％的碳化硅纳米管的能带间隙减小为0.83 eV和0.74 eV.从差分电荷密度可以看出,能带间隙的减小是氮硅键与碳硅键相比共价成键能力降低的结果. 关键词： 碳化硅纳米管 掺氮 第一性原理 电子结构     

Mg,O共掺杂实现p型AlN的第一性原理研究

采用密度泛函理论下的第一性原理平面波超软赝势方法研究了纤锌矿本征AlN,Mg单掺杂AlN和Mg,O共掺杂AlN体系的晶格参数、能带结构、电子态密度、差分电荷密度及电子布居数.计算结果显示:在Mg,O共掺杂AlN体系中,激活施主O原子的引入能使受主能级降低,形成浅受主掺杂.同时,体系的非局域化特征显著,受主能带变宽.因而提高了Mg原子的受主掺杂浓度和系统的稳定性.Mg,O共掺杂更有利于制备p型AlN.     

Cr含量对Ti-Nb-Cr合金抗腐蚀性影响的电子结构计算

许多实验报道中表明合金化元素Cr能够提高Ti基合金的抗腐蚀性.为了解Cr元素含量对Ti-Cr-Nb合金的影响,本文计算了不同Cr含量的Ti-Cr-Nb合金的内聚能、形成能、费米能级和态密度等参数.分析了Cr含量对合金的电子结构稳定性以及腐蚀性能的影响.结果表明:随着Cr含量的增加,体系内聚能升高,形成能增加,体系稳定性略有下降,且材料形成条件变得苛刻;费米能级明显降低,体系不易失去电子,抗腐蚀性能增强;体系金属键增强,失电子能力降低,抗腐蚀性能提高;态密度与差分电荷密度研究表明, Cr含量的增加使得体系金属键增强,表明体系抗腐蚀性的提高.从费米能级和态密度图中发现,当Cr含量约为18.75 at.%时,合金的耐腐蚀性最优.     

Al晶界的第一性原理拉伸试验

基于密度泛函理论和局域密度近似的第一性原理方法，进行了Al晶界的第一性原理拉伸试验.得到Al晶界的理论拉伸强度为9.5 GPa，对应的应变为16%.根据价电荷密度、键长和原子构型随应变的变化，我们证实断裂发生在晶界面，其特征是所有界面键的断裂.同时还发现在周围原子键的数目减少的情况下，界面重构的Al-Al原子键具有共价键的性质.因此Al晶界依然保持着较高的界面强度. 关键词： Al晶界 第一性原理拉伸试验 理论拉伸强度     

稀土La在α-Fe中占位倾向及对晶界影响的第一性原理研究

本文采用重合位置点阵理论构建了α-Fe的Σ3[110](112)对称倾转晶界模型,通过基于密度泛函理论的平面波超软赝势方法研究了稀土La元素在α-Fe中的占位倾向.结果表明,La在α-Fe晶界的杂质形成能最低,因而La原子倾向于占据晶界区;掺杂La前后的α-Fe晶界电子结构计算结果显示,La占位于α-Fe晶界会使体系中的电荷发生重新分配,将提供更多电子用于晶界区成键,使得Fe原子得到更多的电子,这将导致掺杂区原子间结合有离子化趋势,从而使La与晶界区相邻Fe原子之间的相互作用加强,也使晶界原子与晶界两侧Fe原子的键合加强,从能量角度解释了材料宏观力学性能变化的原因;计算同时发现,La加入后,也使晶界上的原子成键区态密度左移,降低了体系的总能量,使晶界结构更为稳定.     

含扭转晶界位错Al金属拉伸强度第一性原理预测

本文基于密度泛函理论第一原理方法,从影响力学性能本质的电子结构计算上,对含Σ 5{001}扭转晶界位错Al金属拉伸强度进行了预测,发现其理论拉伸强度达到8.73 GPa,临界应变为 24%.拉伸强度低于文献报道(Phys. Rev. B 75, 174101 (2007))的倾斜晶界位错Al金属的理论拉伸强度9.5 GPa,但其临界应变却远大于倾斜晶界的16%.本研究结果表明,通过工艺参数控制,改变缺陷形态,可极大地改变其力学性能.进一步地,从电子结构层次上, 分析了含晶界位错Al金属拉伸断裂行为的实质,通过分析电荷密度分布、键长变化等,发现其断裂处发生在晶界处;理论计算结果将对Al金属结构设计及力学性能改善具有重要的指导作用.     

ZA27合金晶界处铁、稀土元素的有序化与交互作用

为从理论上揭示铁、稀土元素在锌铝合金晶界处的行为本质，建立了ZA27合金中α相大角度重位点阵晶界模型，利用递归法(Recursion)计算了晶界的电子结构(状态密度、费米能级、结构能).用晶界结构能定义合金的团簇能(有序能)，并计算了偏聚铁及稀土晶界的团簇能.计算结果表明：铁、稀土元素在锌铝合金晶界处团簇能为正值，不能形成团簇，具有有序化倾向，趋于形成稳定的金属间化合物.铁与稀土元素在晶界形成负电中心，降低晶界的费米能级. 关键词： 稀土 晶界 电子结构 有序化     

Solute segregation to grain boundaries and free surfaces in an Fe---Si multicomponent alloy

Solute segregation was measured at both the {310} symmetrical tilt grain boundary and the (310) free surface of a sample of an Fe-6at%Si alloy containing traces of P, S, N and C at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation in spite of a different bulk concentration of nitrogen. The surface segregation in nitrogen-containing samples is controlled by strong cosegregation of Si and N, resulting in the formation of a stable Si_xN_y 2D surface compound, whereas pronounced surface segregation of sulphur dominates in denitridized samples. The differences of grain boundary and surface segregation are discussed as a kind of “anisotropy of interfacial segregation” on the basis of Guttmann's theory with different values of free energies of segregation to grain boundary and free surface. They also suggest that the measurements of surface segregation cannot be unambiguously used for predicting the grain boundary segregation. In some non-brittle multicomponent systems, a better way of predicting segregation behavior at grain boundaries would be the measurement of grain boundary segregation in a related system with solute concentrations that cause embrittlement. The findings can then be applied to the required alloy composition on the basis of Guttmann's theory.     

Computer simulation of solute segregation to a Σ = 5 tilt boundary in Au,Cu and Ni

A Lattice Energy Function that combines a Mie type interatomic potential and a free electron gas volume dependence has been applied to the study of grain boundary energy and structure of a Σ = 5 tilt boundary in Au, Cu and Ni and of solute segregation to the same. Interatomic potentials and volume dependencies of the solvent and solute were adjusted to fit the relative partial molar enthalpy and volume at infinite dilution order to construct a AB type potential and volume dependence. This AB interaction is then applied to calculate the binding energies of solute to various grain boundary sites and the resulting change in grain boundary energy. A relationship between the binding energy and change in grain boundary is derived. The relative values of the grain boundary energy are in agreement with experimental values of the average grain boundary energies. The relative binding energies of the tested solvent-solute systems are in agreemnet with expectations that certain systems should have larger binding energies than others. The behavior of solute binding energies and local relaxations are in agreement with other studies of grain boundary segregation which use different Lattice Energy Functions and relaxation algorithms. The change in grain boundary energy is shown to be directly proportional to the binding energy.     

Diffusion, Solute Segregations and Interfacial Energies in Some Material: An Overview

We have shown a connection among the three important properties of interfaces, namely, the free energy, diffusion and solute segregation through the conjecture that the interface free energy is the difference between those responsible for diffusion in the lattice and the interface itself. The interface energy is known to decrease upon solute additions. We discuss the methodology and the thermodynamical analysis of the diffusion parameters which enable extraction of the interfacial energies and illustrate them by results obtained in a wide variety of materials. Investigations carried out in pure polycrystalline metals have yielded grain boundary energies comparable to those directly measured. Furthermore, we discuss the role of solute segregation at grain boundaries in alloys in altering diffusion. From the perturbations caused, the solute segregation parameters—the enthalpy and the entropy of binding—have been extracted and levels of solute concentrations estimated. It is shown that similar analyses when applied to complex materials, e.g. the Pb–Sn eutectic alloy, several intermetallic compounds, and oxide systems, also result in acceptable values of interface energies and segregation factors. Finally, some ad-hoc guidelines are provided to alter diffusion in interfaces through solute additions in order to achieve some end use engineering objectives.     

Theoretical aspects of solute segregation to high-angle grain boundaries

A high-angle grain boundary is modeled as a planar defect characterized by its thickness and atomic density. We successively examine the elastic and electronic contributions to the solute/grain boundary binding energy. We deduce the effect of the grain boundary physical parameters on its propensity for segregation. The thickness of high-angle grain boundaries is not a fundamental parameter for segregation. The atomic density in the grain boundary controls the electronic binding energy. The rate of change of elastic constants with the density is the important factor in the elastic contribution to segregation. We conclude that segregation to boundaries with small excess volumes is not precluded.     

Proton-induced grain boundary segregation in stainless steel

Abstract

A technique is developed which addresses the problem of irradiation assisted stress corrosion cracking of stainless steels in light water reactors using high energy protons to induce grain boundary segregation. These results represent the first grain boundary segregation measurements in bulk produced by proton irradiation of stainless steel. The technique allows the study of grain boundary composition with negligible sample activation, short irradiation time, rapid sample turnaround and at minimal cost. Scanning Auger electron microscopy is used to obtain grain boundary composition measurements of irradiated and unirradiated samples of ultra high purity (UHP) type 304L stainless steel and UHP type 304L steels with the additions of phosphorus (UHP + P) and sulphur (UHP + S). Results show that irradiation of all three alloys causes significant Ni segregation to the grain boundary and Cr and Fe away from it. Irradiation of the UHP + P alloy also results in segregation of P at the grain boundary from 5.3 to 8.7 at %, over 80 times the bulk value. No radiation-induced grain boundary segregation of S was measured in the UHP + S alloy. Results also indicate that the presence of P or S may enhance radiation-induced segregation of major alloying elements at the boundary. Comparison of irradiated and unirradiated regions of the UHP + P alloy indicate that while a prior thermal treatment segregates P to the grain boundary to 5.3 at %, the major element concentrations at the grain boundary are completely different from those under irradiation.     

SmCo7纳米晶合金晶粒组织热稳定性的热力学分析与计算机模拟

推导出了单相纳米晶合金的晶界过剩体积与晶粒尺寸之间的定量关系, 建立了纳米晶合金的晶界热力学性质随温度和晶粒尺寸发生变化的确定性函数. 针对SmCo₇纳米晶合金, 通过纳米晶界热力学函数计算和分析, 研究了单相纳米晶合金的晶粒组织热稳定性. 研究表明, 当纳米晶合金的晶粒尺寸小于对应于体系中晶界自由能最大值的临界晶粒尺寸时, 纳米晶组织处于相对稳定的热力学状态; 当纳米晶粒尺寸达到和超过临界尺寸时, 纳米晶组织将发生热力学失稳, 导致不连续的快速晶粒长大. 利用纳米晶合金热力学理论与元胞自动机算法相耦合的模型对SmCo₇纳米晶合金在升温过程中的晶粒长大行为进行了计算机模拟, 模拟结果与纳米晶合金热力学模型的计算预测结果一致, 由此证实了关于纳米晶合金晶粒组织热稳定性的研究结论. 关键词： 纳米晶合金热力学 7纳米晶合金')" href="#">SmCo₇纳米晶合金 热稳定性 计算机模拟     

Grain boundary ridge formation during initial high temperature oxidation of Mn/Al TRIP steel

Confocal scanning laser microscopy (CSLM) was used in real-time observation of alloy element oxidation of a Mn/Al TRIP steel in an Ar–O₂ atmosphere. CSLM images reveal a marked role of grain boundaries in the overall initial oxidation kinetics of the alloy, and consequently in the morphology of the initial surface oxide. The oxidation on the alloy surface is dominated by the formation of Mn-rich oxide ridges along grain boundary traces on the surface. Oxide ridge formation kinetics was quantified by measurements on images extracted from real-time recordings of surface oxide evolution. Oxide ridge growth was found to take place at a constant rate. Scanning electron microscopy (SEM) images of the oxidized surfaces showed homogenous oxide ridges along straight grain boundary traces and heterogeneous oxide ridges along non-straight grain boundary traces. A transport mechanism of Mn to the surface is proposed, which relies on grain boundary segregation of Mn and on a relationship between grain boundary diffusivity and grain boundary character. It is suggested that when regarding alloys with significant grain boundary segregation of a solute, separate Wagner balances for internal vs. external oxidation is required for the grain lattices and the grain boundaries, respectively.     

Solute segregation at grain boundaries

This feature article summarizes the present art and science of grain boundary segregation from the viewpoint of the authors activities in this field. In the part on equilibrium segregation, fundamental effects on grain boundary segregation are discussed such as the nature of the solute/matrix binary system, presence of additional elements, temperature, grain boundary orientation and type of interface. In addition, the predictive capabilities of grain boundary segregation diagrams are outlined. The present models of segregation kinetics are reviewed and discussed in connection with recent experiments. The last part of the paper is focussed on the most important consequences of grain boundary segregation, i.e., grain boundary cohesion and fracture.     

Thermodynamic stability of binary nanocrystalline alloys: analysis of solute and excess vacancy

Regarding that the excess volume in grain boundaries (GBs) is released as the vacancies which are accommodated by the crystal bulk during grain growth, a free-energy function for binary nanocrystalline solid solution is proposed, based on the pairwise nearest-neighbor interactions. The model, for the given composition and temperature, predicts an equilibrium grain size, subjected to a mixed effect due to solute segregation and due to excess vacancies. Furthermore, excess-vacancy-inhibited grain coarsening can be attained, which plays a minor role in holding the thermal stability of nanocrystalline alloys, as compared to the effect of solute segregation.