铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为.结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错.在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位.间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚.     

氦泡在bcc钨中晶界处成核长大的分子动力学模拟

钨(W)是潜在的聚变堆面向等离子体材料.聚变反应中产生的氦(He)不溶于金属W,并在其中易聚集形成He泡,使W基体发生脆化,从而导致W基体的性能发生退化.在前人工作的基础上,本文采用分子动力学研究了He泡在单晶bcc-W中以及bcc-W中∑3[211](110)和∑9[110](411)晶界处He泡形核长大初期的演化过程.结果发现,晶界处He泡的长大机制和单晶W中有所不同.单晶W中He泡通过挤出位错环促进长大.而He泡在∑3[211](110)晶界处的长大机制为:首先挤出并发射少量自间隙W原子,而后挤出1/2⟨111⟩位错线,随后,该位错线会沿晶界面上[111]方向迁移出去;在∑9[110](411)晶界处,He泡在我们的模拟时间尺度范围内没有观察到W自间隙子的发射和位错的挤出.     

bcc Fe的刃型位错中氦-空位团的稳定性

 采用周期性原子阵列方法建立bcc Fe中的刃型位错,利用分子动力学计算了0 K时bcc Fe的位错芯里氦-空位团的稳定性,并与理想Fe晶体里氦-空位团的稳定性进行比较发现,位错的作用导致氦-空位团不稳定。点缺陷（He、空位与自间隙Fe原子）与氦-空位团的结合能与团中氦-空位比例密切相关,当氦与空位数之比在3~6时,结合能趋于稳定。     

充氚不锈钢中氦行为的实验研究

对充氚和未充氚的抗氢-2不锈钢(HR-2)样品进行退火处理,利用正电子湮没寿命谱技术以及金相检验技术探讨不锈钢中氦和微缺陷的相互作用行为.未充氚样品中,影响正电子寿命值的主要因素为杂质元素在晶界的析出.充氚样品实验中,退火温度小于300℃时,正电子寿命值的增加说明了氦泡的形成过程为非热形成,通过“冲出位错环”机制形成及长大;退火温度在300~600℃之间,充氚样品正电子寿命值的降低以及He的跃迁概率的计算结果,说明He原子通过热迁移至晶界;退火温度大于600℃时,热平衡空位浓度的计算结果以及正电子寿命值的增加说明热平衡空位开始发挥作用.     

氦、氘对纯铁辐照缺陷的影响

在核聚变堆的辐照环境中, 核嬗变产物氢、氦对结构材料的抗辐照性能将产生很大的影响. 本实验采用离子注入和电子辐照模拟研究了氦和氘对具有体心立方结构的纯铁的影响. 采用离子加速器在室温分别对纯铁注入氦离子和氘离子, 经500℃时效1 h后在高压电镜下进行电子辐照.结果表明: 室温注氦和室温注氘的纯铁在500℃时效后分别形成间隙型位错环和空位型位错环. 在电子辐照下, 间隙型位错环吸收间隙原子而不断长大, 而空位型位错环吸收间隙原子不断缩小. 通过计算位错环的变化速率发现, 空位型位错环比间隙型位错环吸收了更多的间隙原子, 即室温注氘纯铁的位错偏压比室温注氦纯铁的偏压参量大, 这意味着相同实验条件下空位型位错环对辐照肿胀的贡献大于间隙型位错环对辐照肿胀的贡献. 利用氦-空位复合体和氘-空位复合体的结构, 分析了注氦和注氘后在纯铁中形成不同类型位错环的原因. 关键词： 氦 氘 辐照损伤 位错环     

Al辐照损伤初期的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法,对Al辐照损伤初期产生的本征点缺陷和He缺陷进行了研究.通过晶体结构、缺陷形成能和结合能,分析比较了缺陷形成的难易程度及对晶体稳定性的影响,并从态密度、差分电荷密度和电荷布居的角度,分析了其电子机理.结果表明:对于同类型的缺陷,其造成的晶格畸变越大,体系稳定性越低,缺陷形成的难度越大.同类型缺陷形成的难易程度由易到难依次为空位(置换位原子)、八面体间隙原子和四面体间隙原子,但相同位置的本征缺陷的形成难度小于He缺陷.间隙原子容易与空位结合,且Al原子与空位结合的能力强于He原子.间隙Al原子和He原子主要存在于八面体,且缺陷原子引起部分电子向更高能级转移,并导致与其最邻近的Al原子之间的共价作用减弱,从而降低了体系稳定性.间隙Al原子与最邻近的Al原子之间产生了强烈的共价作用,而He原子和最邻近Al原子之间主要为范德瓦耳斯力和较弱的离子键,这是含He缺陷的体系稳定性更低的重要原因.     

氦团簇及氦间隙原子在钨中的稳定性研究

首先采用分子动力学方法研究了在钨中预存氦-空位团簇（He_nV₂₂）后氦原子结合能与氦-空位比的关系。研究发现:当氦-空位比小于4.5时,氦原子结合能随氦-空位比呈线性减小趋势;当氦-空位比大于4.5时,氦原子的结合能随氦-空位比出现剧烈振荡的现象,这种现象是由于钨中预存氦-空位团簇随机挤出位错环使体系能量骤降所导致的。与此同时,氦-空位团簇周围出现了一些处于亚稳态的fcc结构和hcp结构的钨。为了研究氦团簇周围压强对钨基体相变的影响,本文利用第一性原理对钨的三种结构进行了高压相变计算,发现静水压力不能使钨的三种结构互相转变。另外,通过对bcc钨和fcc钨中四面体间隙氦原子和八面体间隙氦原子电荷密度差的计算,发现bcc钨中四面体间隙氦原子的稳定性高于八面体间隙氦原子的稳定性,而在fcc钨中四面体间隙氦原子的稳定性弱于八面体间隙氦原子的稳定性。     

注氢纯铝中间隙型位错环一维迁移现象的原位观察

核聚变堆材料在高能粒子辐照过程中会产生大量点缺陷,导致辐照脆性和辐照肿胀等现象.因而,研究点缺陷在辐照过程中的演变过程至关重要.点缺陷团簇的一维迁移现象是这种演变过程的主要研究内容之一.本文采用普通低压(200 kV)透射电镜,在室温条件下对注氢纯铝中的间隙型位错环在电子辐照下的一维迁移现象进行了观察和分析.在200 keV电子辐照下,注氢纯铝中的位错环可多个、同时发生一维迁移运动,也可单个、独立进行一维迁移运动.位错环沿柏氏矢量1/3<111>的方向可进行微米尺度的一维长程迁移,沿柏氏矢量1/2<110>的方向一维迁移也可达数百纳米.电子束辐照时产生的间隙原子浓度梯度是引起位错环一维迁移并决定其迁移方向的原因.位错环发生快速一维迁移时,其后会留下一条运动轨迹;位错环一维迁移的速率越快,运动的轨迹则越长,在完成迁移过后的几十秒内这些运动轨迹会逐渐消失.     

氢对铁中缺陷行为的影响

本文研究了铁中氢与辐照缺陷的相互作用。氢与这些缺陷结合,形成不同类型的氢与缺陷的复合体,在从低温到高温的时效过程中,可以清楚地观察到间隙型位错环(>300℃)、空位型位错环(>450℃)和空洞(520℃)的形成。研究表明,这三种缺陷分别是氢与间隙原子、氢与单个空位、氢与复数个空位等三种复合体聚集而成。 关键词：     

He原子掺杂铝材料的第一性原理研究

采用基于第一性原理的平面波超软赝势方法,结合广义梯度近似(GGA),计算了铝及铝晶胞间隙位置掺入He原子后体系的几何结构、电子结构、总体能量和电荷布居值.计算结果表明:随着氦在金属铝中逐渐形成,铝晶胞体系会发生晶格畸变,但总的趋势是He在铝体系的八面体位置的晶格畸变小于其在四面体位置的晶格畸变.He在铝晶胞八面体和四面体间隙的杂质形成能分别为1.3367 eV和2.4411 eV.由此可知,He在铝晶胞中最稳定位置是八面体间隙位置.同时,文中还从原子尺度层面分析了He原子在铝晶胞中的占位及其键合性质,讨论 关键词： 铝材料 第一性原理 形成能     

On the generation of vacancies by moving dislocations

New experiments of Molenaar and Aarts, Blewitt and others seem to confirm the view of the author, previously based only on the experiments of Gyulai and Hartly and Stepanow on sodium chloride, that vacant lattice sites, and possibly interstitial atoms, are generated during plastic flow in ductile crystals, particularly in metals. It is pointed out that the average temperatures near a moving dislocation are probably not sufficiently high to evaporate vacant lattice sites or interstitial atoms as a result of thermal effects alone. Instead, one apparently must conclude that the imperfections are generated either by purely geometrical means during the looping of dislocations about appropriate obstacles, as the result of dynamical instability in the motion of a dislocation, possibly near a jog, or in the very high thermal pulses or ‘spikes’ which are generated either in the zone where two dislocations of opposite sign annihilate one another or near impediments where dislocations are strongly curved. It is pointed out that a pair of vacancies is probably stable near room temperature and may diffuse more rapidly than a single vacancy. It is also proposed that vacancies retained during quenching of Al-Cu alloys and those generated by cold-work play an important role in the precipitation process. The origin of work hardening in single crystals is discussed and several alternative interpretations, which involve the impediment of Frank-Read generators either directly or indirectly as a consequence of the generation of vacancies, are presented. The importance of prismatic dislocations formed by condensation of vacancies is restated. The role that vacancies formed by cold-work may play in determining the stored energy and decrease in density and in affecting processes such as creep and the hardening of latent slip planes is also discussed. Finally a few experiments are proposed, typical of those which could prove decisive in isolating the influence of vacancies.     

Formation and migration properties of the rare gases He,Ne, Ar,Kr, and Xe in nickel

The energies of formation and migration of various rare gas-point defect complexes in an f.c.c. nickel lattice have been calculated for He, Ne, Ar, Kr, and Xe. Formation energies of rare gas atoms at interstitial sites are compared with those in substitutional sites. Binding energies are presented for self-interstitials and vacancies trapped to the various rare gas substitutionals. We also present migration energies and migration paths for various rare gas interstitials and substitutionals with and without trapped vacancies and self-interstitials. The migration energies are compared with the breakup energies for the corresponding complexes. We find that divacancy-rare gas complexes are rather stable and will migrate at relatively low energies compared to other substitutional rare gas migration processes.     

Effect of grain boundary structures on the behavior of He defects in Ni: An atomistic study

We investigated the effect of grain boundary structures on the trapping strength of He_N(N is the number of helium atoms) defects in the grain boundaries of nickel. The results suggest that the binding energy of an interstitial helium atom to the grain boundary plane is the strongest among all sites around the plane. The He_N defect is much more stable in nickel bulk than in the grain boundary plane. Besides, the binding energy of an interstitial helium atom to a vacancy is stronger than that to a grain boundary plane. The binding strength between the grain boundary and the He _N defect increases with the defect size. Moreover, the binding strength of the He_N defect to the Σ3(12)[110] grain boundary becomes much weaker than that to other grain boundaries as the defect size increases.     

氦在铝表面辐照和积聚的团簇动力学模拟

我们利用团簇动力学模型(IRadMat)研究了keV-He离子辐照金属铝的缺陷动力学和氦的聚集行为.通过对不同俘获类型(团簇、晶界和位错)俘获He浓度的定量分析,我们发现大多数He原子被晶界吸收,这成为铝在低辐照通量下发生脆化的主要原因.随着辐照能量的增加,He滞留峰的位置会变得更深.然而,随着辐照通量的增加,He在体内的滞留量会变得更多,但滞留深度的峰值位置不变.我们的结果表明,晶界的影响在He的滞留分布以及铝的脆化行为中起着关键作用,这也有助于我们理解He在金属中的动力学行为和损伤的分布.     

嵌入原子法计算金属钚中点缺陷的能量

钚因放射性衰变而出现老化效应.钚中点缺陷的性质和行为是理解钚老化效应的一个基础和前提.运用分子动力学模拟技术,计算了金属钚中点缺陷和点缺陷团簇的形成能和结合能.其中钚-钚、钚-氦和氦-氦相互作用势分别采用嵌入原子多体势、Morse对势和Lennard-Jones对势.计算结果表明,单个自间隙原子易以〈100〉哑铃状形态存在;间隙氦原子在理想晶格的八面体间隙位置相对较为稳定;氦原子与空位的结合能较大,在钚的自辐照过程中两者易于结合并形成氦-空位团簇;氦-空位团簇的形成能随氦原子数的增加而增大,当氦与空位的数     

Point defect interactions with crystal interfaces

The structures of a small closed system of grain boundaries and the interactions of vacancies with these boundaries has been investigated using computer simulation techniques based on empirical interatomic potentials. The boundaries chosen are the {;111}; and {;112}; twins in both body centred cubic and face centred cubic metals, the potentials used being matched to the physical properties of iron and copper. Two stable structures arise for the {;112};_bcc twin so that effectively five boundaries have been studied. The structures and energies of these are extremely varied, the {;112};_fcc twin in particular being very broad. This influences the binding of vacancies to the boundaries and the migration of vacancies along the boundaries. Near the {;111};_bcc twin a split-vacancy consisting of a divacancy and an interstitial is the most stable configuration. This has a very high binding energy and an exceptionally high migration energy. Near the other boundaries the vacancy migration energies are less than in the bulk. The implications of the results are discussed.     

Plasma-stimulated penetrability of hydrogen as a tool for studying phase transitions at grain boundaries

It is well known that the diffusion of hydrogen atoms through the intrinsic defects of a crystal lattice has characteristics different from those of bulk diffusion and, at certain parameters for some polycrystalline metals, ensures the determining contribution to the transfer of hydrogen atoms through the material. Grain boundaries (and dislocations) are the most important and shortest paths, the diffusion through which is much faster than bulk diffusion through a crystal lattice. It is particularly important to take into account this diffusion in materials with grains having sizes of about several nanometers. The possibility of using the method of the plasma-stimulated penetrability of hydrogen to analyze phase transitions at the grain boundaries is demonstrated on the example of polycrystalline niobium foils. In contrast to the existing methods, this method proposed for studying grain-boundary diffusion and phase transitions is simple and ensures control over the surface. The temperature characteristics of the diffusion of hydrogen atoms through niobium grain boundaries have been measured.     

Changes in the fine structure of grain boundaries, induced by the absorption of helium, and helium embrittlement

The changes in the structure of grain boundaries in tungsten due to the absorption of helium atoms are investigated experimentally and theoretically. Intergranular dilatation localized in a plane layer of subatomic thickness is observed. It is established that dilatation is accompanied by splitting of the cores of grain-boundary dislocations and a decrease in the grain-boundary stacking fault energy. The relationship of intergranular damage to the changes induced in the parameters of grain-boundary dislocations by the absorption of helium is discussed. Zh. Tekh. Fiz. 68, 64–69 (July 1998)