钛合金中原子迁移性质的第一原理计算

钛合金作为一种重要的航空材料,具有比强度高、耐蚀性好等优异性能,并具有一定的高温蠕变抗力。但是在更高温度下,有限的蠕变抗力限制了钛合金的进一步应用。研究表明,钛合金稳态蠕变过程蠕变激活能与合金的表观扩散激活能非常接近,说明原子的扩散与蠕变过程密切相关。采用第一性原理方法,计算了钛合金中常见的杂质及合金原子的迁移能垒。结果表明,对以空位机制扩散的合金原子,其在基面内迁移的能垒从高到低为Al、V、Ti、Sn、Ta、Mo、Nb、Zr,面间原子迁移由难到易为Al、Sn、V、Ti、Ta、Mo、Nb、Zr。以间隙机制迁移的Co、Fe、Ni迁移能垒较低,与实验观测到的这些原子为快扩散原子相符。     

Ta原子在TaN(001)表面上绕岛迁移的第一原理计算

纳米薄膜的各方面性能和薄膜自身的微观结构是密切相关的,研究原子的迁移行为可以充分了解纳米材料的形成机理以及基本结构.笔者运用第一性原理VASP软件包对粒子的迁移行为进行模拟计算,内容包括模型的建立、原子吸附能的计算以及NEB方法计算迁移激活能.计算结果表明:Ta原子在2Ta2N岛附近的P3和P5位置为两个最稳定吸附,分...     

金属Zr热力学性质的第一原理研究

应用基于密度泛函与密度泛函微扰理论的平面波赝势方法计算了一组不同晶格常数下hcp结构金属Zr的声子谱及相应的静态总能,由此得到不同晶格常数下的自由能,由准谐近似及自由能极小判据得到自由能与温度的关系,进而计算了金属Zr热膨胀系数、体弹性模量、定容及定压摩尔热容与温度的关系,计算结果在较宽的温度范围内与实验相符.对热膨胀系数,同时应用Debye-Grüneisen模型进行了计算,并与第一原理方法的结果进行了比较.对摩尔热容的计算考虑了电子的贡献,表明在高温区电子热容的贡献不能忽略.     

Ti-V合金hcp结构弹性性质的第一原理研究

为研究V元素对Ti合金弹性模量的影响,采用平面波展开的第一原理赝势法,对固溶状态下hcp结构的Ti-V合金的结构、弹性模量进行研究,结果表明V含量小于5.6 at%时合金呈稳定的hcp结构,且随V含量的增加,合金的体弹模量、剪切模量和杨氏模量单调减小,泊松比单调增大。     

Ag表面性质的第一原理计算

采用第一原理赝势平面波方法,计算了Fcc-Ag晶体及其Ag(111)、Ag(110)与Ag(100)自由表面的能量、几何与电子结构.根据表面能的计算,预测了Ag表面的结构稳定性,结果表明密排Ag(111)面结构稳定性最好,低指数奇异面Ag(100)面次之,Ag(110)面的结构稳定性最差.通过对不同表面几何与电子结构的比较,初步分析了其结构稳定性差异的产生原因.表面原子驰豫不仅引起表面几何结构的变化,而且使表面层的电子结构与键合特性发生改变.驰豫后表面层原子的部分价电子跑到了表面层以上的真空区,使表面层原子的电子态密度峰形发生变化,还新形成了表面态,这是表面能产生的主要原因,而Ag(110)表面相对于Ag(111)与Ag(110)表面具有高表面活性的主要原因则源于其表面层原子显著的结构驰豫.     

锂离子电池正极材料第一原理计算研究

利用第一原理计算方法可分析和预测锂-金属氧化物电池正极材料在Li离子嵌入脱出过程中的电势和稳定性等性能。本文详细介绍了第一原理计算方法的理论背景以及目前LiCoO2正极材料计算研究的现状。利用此方法对LiNiO2及多组分材料掺杂进行研究是今后工作的重点。     

元素掺杂对Bi电迁移影响的第一原理计算

应用第一原理方法研究通过元素掺杂来抑制SnBi无铅焊料中Bi的电迁移问题.在SnBi体系中掺杂zn和Sb元素,通过用近弹性带方法计算掺杂体系中Bi元素的扩散能垒.结果表明:加入Sb之后,Bi的扩散能垒由原来的0.32 eV升高到0.46 eV,扩散激活能由原来的1.14 eV升高到1.18 eV;加入Zn后,Bi的扩散能垒由原来的0.32 eV升高到0.48 eV,扩散激活能由原来的1.14 eV升高到1.22 eV.由此可得,Zn和Sb的加入都能够提高Bi的扩散激活能,起到抑制扩散的作用.通过分析态密度可知:加入Zn和Sb后,体系中Sb与Bi的p态曲线几乎完全重合,比Sn与Bi的p态曲线重合度高很多,说明sb和Bi的共价键作用很强,且比Sn-Bi的共价键作用强,从而增加Bi的扩散能垒.同样,Zn和Bi的p态曲线重合度也比Sn和Bi的曲线重合度高很多,表明Zn-Bi的共价键同样比sn-Bi的共价键强,所以Zn的加入同样增加Bi的扩散能垒.总结说来,Sb和zn的掺杂能够抑制SnBi焊料中Bi的电迁移.     

氧化锆同质异性体的力学性能第一原理研究

采用第一原理计算立方(c)、四方(t)、单斜(m)结构的氧化锆同质异性体能量,对能量体积变化关系计算发现,在0K下m-ZrO2能量最低,并且单斜相保持到较大温度范围,而t-ZrO2和c-ZrO2则通过热膨胀、应力等体积转变方式而导致能量不同,从而引起结构转变。采用应力应变关系描述其弹性系数,通过Voigt-Reuss-Hill近似表征氧化锆同质异性体的力学性能,计算得到的结果与实验符合较好,研究表明单斜氧化锆的模量明显低于四方和立方结构,是由于其密度降低,导致原子堆垒不紧密和化学键变弱,造成力学性能下降。     

ZrV2体系的第一原理研究

基于局域密度泛函理论，结合赝势方法，研究了ZrV2-H体系的结构及其能量性质，研究结果表明，H原子填入ZrV2晶胞中的2Zr2V四面体间隙所形成的氢化物最稳定，填入1Zr3V四面体间隙所开成氢化物的稳定性次之，填入4V四面体间隙所形成氢化物的稳定性最差，吸H前后ZrV2合金晶体结构类型不变，随含H量增大，晶格常数增大，当H／ZrV2小于2．0时，ZrV2的氢化物应具有较强的抗粉化能力，H在ZrV2合金中的固溶度小，p-C坪台度，室温平衡压低，有利于贮存氢同位素。     

Mg-La和Mg-Nd二元合金相稳定性的第一原理研究

La和Nd是镁合金中常用的稀土添加元素,为了帮助理解它们在Mg合金中的强化机制,应用第一原理计算方法,研究了Mg-La和Mg-Nd二元合金的相稳定性.计算结果表明,Mg-La和Mg-Nd合金在Mg₁₂RE和Mg₃RE之间的平衡相分别为Mg₁₇La₂和Mg₄₁Nd₅;La和Nd在Mg中的溶解度大小差别较大,表明者在镁合金中产生强化的机理不同;Mg₃Nd具有比Mg₁₂La更大的弹性模量,因而具有更好的强化效果.     

DD6单晶合金筏化过程中元素定向迁移行为

对[001]取向DD6单晶合金进行1000°C、300MPa条件下的拉伸蠕变实验,以研究合金的筏化现象以及元素的定向迁移行为,SEM对合金显微组织的观察表明,γ′相在温度和应力的作用下发生了筏化,且筏化方向与应力轴垂直,同时平行于应力轴的基体通道变宽,垂直方向的基体通道逐渐消失。TEM对合金的成分统计分析表明,合金中Al、Re、W元素发生了定向迁移,其中Al向与应力轴平行的基体通道扩散,而Re、W向与应力轴垂直的基体通道扩散,并主要富集于相互垂直的基体通道的交叉区域。     

Nucleation of Y–Si–O Nano-clusters in Multi-microalloyed Nano-structured Ferritic Alloys: a First-principles Study

First-principles energetics analyses were performed to investigate the nucleation of(Y-Si-O) nano-clusters(NCs) in nanostructured ferritic alloys(NFAs).It was predicted that the nucleation of(Y-Si-O) NCs follows the same general pathway as previously found for(Y-Ti/Al/Zr-O) NCs in NFAs:they all tend to begin with the(O-O) pairs and grow to(O-Y)-cores and further to larger NCs by attracting Y and other solute elements nearby.Nucleation of a hexa-atomic-[2Y-Si-3O]-cluster can reduce the total energy by 4.71 eV.Among various microalloying-induced NCs,the nucleation preference ordering was predicted as(Y-Zr-O)(Y-Ti-O)(Y-Al-O)(Y-Si-O).The number densities and chemical compositions of NCs in multi-microalloyed NFAs would largely depend on the number availability of(O,Y)-cores,as well as the relative abundance and atomic diffusivities of microalloying solute elements nearby.     

First-principles theory on electronic structure and floatability of spodumene

The band structure, density of states, Mulliken populations, and frontier orbital of spodumene crystal were calculated using the first-principles method based on the density functional theory(DFT) and further analyzed in detail. The calculation results reveal that the O in spodumene is the most active and easily links with H+in the water, but the active Li is very low, so it is better to add activator to increase the concentrate grade and recovery rate of spodumene in the flotation process. Si–O bonds in spodumene crystal are mainly covalent, since the covalency of Al–O bonds is stronger than that of Li–O bonds,and minerals dissociate along the weakest Li–O bonds. In addition, the study of the frontier orbital indicates that both O and Si atoms have large contribution to the frontier orbital in the spodumene crystal. Oleate and dodecylamine are used as the collectors of spodumene. The results contribute to the understanding of crystal structures of spodumene, and can be used in guiding related practical applications.     

Fe-RE(Y、La和Ce)合金中相稳定性和固溶度的第一性原理研究

合金元素的固溶度对于新型合金的设计和合金动力学过程的研究具有重要的作用。本文利用密度泛函理论,计算了Fe-RE二元化合物的基态,确定了Fe-Y和Fe-Ce体系的稳态和亚稳态结构。计算结果表明,Fe-Y的稳定结构为Fe₁₂Y.tI26、Fe₁₇Y₂.hP38和Fe₂Y.cF24,Fe-Ce的稳定结构为Fe₁₇Ce₂.hP38、Fe₁₉Ce₅.hR24和Fe₂Ce.cF24。基于稀格子气统计热力学理论,利用第一性原理计算得到了稀土元素在α-Fe中随温度变化的固溶度曲线。结果显示,稀土Y、La和Ce元素在α-Fe中的固溶度为S_La>S_Ce>S_Y,这一趋势对应于三者在α-Fe中的溶解形成焓关系H_sol(La)< H_sol(Ce)< H_sol(Y).     

Zr-Ti合金相稳定性与弹性性质的第一性原理研究

利用特殊准随机结构(specialquasi-randomstructure,SQS)超晶胞模型及第一性原理计算研究了Zr_(1-x)Tix合金中Ti含量及相结构(hcp与bcc结构,α相与β相)对合金相稳定性及弹性性质的影响。研究表明:从弹性性质上看,Ti含量增加会使Zr-Ti合金的结构更加稳定,Zr-Ti合金α相的结构稳定性普遍高于β相;Zr-Ti合金普遍具有延展性,Zr-Ti合金的α相具有更高的硬度并且其硬度和韧性都与Ti元素浓度成正比,Zr-Ti合金的β相具有更高的韧性;从电子结构上看,Ti不会显著改变hcp结构和bcc结构Zr-Ti合金系统的态密度(DOS),Zr-Ti合金系统的DOS分布中的费米能级与纯Zr系统的相似,Ti含量对Zr-Ti合金的相稳定性影响不大。     

First-principles analysis method for the multiplet structures of rare-earth ions in solids

A first-principles fully relativistic multielectron method based on molecular orbital (MO) theory is applied to the analysis for the 4f5d configurations of Pr³⁺-doped LaF₃, LiYF₄ (YLF), and CaF₂ crystals. The formation of MOs between the Pr 5d orbitals and the ligand fluorine 2p orbitals is considered using a (PrF₁₁)⁸⁻ and (PrF₈)⁵⁻ molecular models. The energy of the lowest level of 4f5d configuration decreases in the order LaF₃, LiYF₄, and CaF₂ (O_h symmetry). The high energy in Pr³⁺:LaF₃ is ascribed to the smaller ligand-field splitting due to the large coordination number. The analysis of many-electron wave functions shows the mixtures between the spin-orbit splitting of the 4f-level MOs and the ligand-field splitting of the 5d-level MOs. Configuration interaction with one-electron excitations gives transition probability to the originally forbidden two-electron excitations, which produce satellite structures in spectra.     

第一性原理研究压力下Ni-Mo二元化合物的力学性能和电子结构

采用基于密度泛函理论的第一性原理方法研究压力对Ni-Mo二元化合物Ni4Mo、Ni3Mo(DOa)、Ni3Mo(DO22)、Ni2Mo力学性能和电子结构的影响。研究表明：0~40GPa压力范围内,随着压力的增大,相对体积V/V0不断减小且趋势减缓;形成热均为负值,且随着压力的增大形成热减小,说明增大压力可提高化合物的合金化能力;体积模量B、剪切模量G、杨氏模量E、拉梅常数λ、硬度H的计算结果表明压力可提高四种化合物的抗变形、抗压缩能力及硬度,另外,B/G和泊松比ν表明所有化合物均为延性和塑性的;进行态密度的分析,阐明增大压力可提高四种化合物的稳定性及硬度。     

First-principles study of pressure-induced magnetic transition in siderite FeCO3

The crystal structure, electronic configuration, spin state and electronic structure of siderite FeCO₃ under pressure have been studied by first-principles calculations in the framework of density functional theory (DFT). The real antiferromagnetic (AFM) spin ordering state has been considered and the hydrostatic pressure condition is simulated. The calculated geometric structural data (unit-cell volume V, lattice constant a, cell angle α and atomic internal coordinate u) of FeCO₃ at ambient condition in good agreement with available data from the literature. FeCO₃ transforms from high spin (HS) AFM state to low spin (LS) nonmagnetic (NM) state between 40 and 50 GPa, concomitant with a volume collapse of 11%. The unit cell volume V₀, bulk moduli B₀, and its pressure derivative B′₀ of the HS and LS state are fit with a 3rd-order Birch-Murnaghan equation of state, which consist well with available experimental results. The insulating nature of FeCO₃ is remained after the magnetic transition. The 3d electrons of Fe²⁺ ions for the LS NM state are more localized than those of the HS AFM state, which leading to the magnetic transition.     

Influence of high-temperature creep stress on growth of thermally grown oxide in thermal barrier coatings

Recently, a large local stress has been found, caused by the change of both the diffusion rate of oxygen through an existing oxide and the rate of chemical reaction at the oxide/oxidized material interface. Since high thermal stress occurs in the thermal barrier coating (TBC) system, the volume expansion of the newly grown oxide, and centrifugal force, the growth rate of the thermally grown oxide (TGO) may change depending on the temperature, the exposure time, and the stress. The aim of this study is to make clear the influence of stress on the growth rate of the TGO thickness under static oxidation. The results show that TGO thickening was affected by the increase of not only the exposure temperature but also the applied stress. The tensile stress in a longitudinal direction in a TBC system due to the applied load makes the oxidant transport and oxide flow more easy, i.e., the tensile stress promotes a volume expansion of newly formed oxide. The increase rate of the TGO thickness was approximately 34% when the applied stress increased from 0 to 205 MPa at 900 °C for 325 h, and approximately 25% when the stress increased from 0 to 150 MPa at 950 °C for 125 h.     

ZrCr2 Laves相弹性性质和堆垛层错能的第一性原理计算

采用缀加平面波加局域轨道方法和广义梯度近似对立方C15结构的ZrCr2Laves相金属间化合物的弹性性质，包括弹性常数和弹性模量，以及层错能进行理论计算。结果表明：计算得到的ZrCr2Laves相的弹性性质与实验结果相近，其泊松比和弹性各向异性系数大小说明ZrCr2中原子键合的方向性并不强烈；ZrCr2Laves相的内禀和外禀层错能分别为112mJ／m^2和98mJ／m^2。并计算了层错与位错的弹性交互作用。对ZrCr2Laves相的力学特性和变形机制进行了讨论。