Channel formation and multiplication in irradiated FCC metals: a 3D dislocation dynamics investigation

Our goal in this work is to investigate post-irradiation tensile deformation of FCC grains using 3D dislocation dynamics (DD) simulations. We focus on irradiation dose conditions where plastic strain is expected to localize into defect-depleted channels. Two DD simulation types are used for treating distinct space and time scale effects. Type-I simulations describe the formation of single dislocation channels at a high resolution (nm). Here, the irradiation-induced defects are described explicitly, in the form of prismatic dislocation loops. Type-II simulations are used to describe the channel multiplication process itself, i.e. at the grain scale (μm). This time, the irradiation-induced defects are treated in a simplified way, taking advantage of Type-I simulation results. Simulated channel spacing is found to depend on three main input parameters: the dose-dependent stress level, grain size and critical cross-slip stress. The results are rationalized in terms of a micro-model based on simple, finite-sized dislocation arrangements. The model is further validated by comparison with available experimental evidence.     

A new micromechanics-based scale transition model for the strain-rate sensitive behavior of nanocrystalline materials

An original two-step “three phase” elastic–viscoplastic scale transition model is developed based on the combined self-consistent and Mori–Tanaka schemes. A coated inclusion is embedded within a matrix, wherein the inclusion represents grain interiors and the coating of the inclusion mimics the effects of grain boundaries and triple junctions. The predominant behavior within the grain interiors is captured through dislocation glide, whereas grain boundary (GB) dislocation emission and absorption, as well as thermally assisted GB sliding, describe the deformation processes within the coating describing the GB affected zone. Furthermore, an imperfect interface is assumed between the inclusion and the coating to account for viscoplastic grain boundary sliding along a stick-slip mechanism. Results and discussion focus on the competitive roles of GB sliding, GB dislocation emission/absorption, dislocation sweep in grain cores and collective dislocation plasticity, and the origins of the pronounced strain rate sensitivity of fcc NC materials.     

强流脉冲电子束作用下金属纯Cu的微观结构状态——变形结构

为了研究金属的超快变形机理,利用强流脉冲电子束(HCPEB)技术对多晶纯Cu进行了辐照处理,并利用透射电子显微镜对HCPEB诱发的表面微结构进行了表征.实验结果表明,HCPEB轰击多晶纯Cu后,在轰击表层诱发了幅值极大的应力和极高的应变速率.1次HCPEB轰击材料表层的变形结构以交滑移形成的位错胞和位错缠结结构为主;多次轰击后平行的位错墙和孪晶是该区域的主要变形结构特征;原子面的扩散乃至位错攀移可在晶界和孪晶界上形成台阶结构.根据各自区域的变形结构特征,对相应的变形机理进行了探讨. 关键词： 强流脉冲电子束 多晶Cu 变形结构 孪晶     

Microstructure evolution of T91 steel after heavy ion irradiation at 550 ℃

Fe-Cr ferritic/martensitic (F/M) steels have been proposed as one of the candidate materials for the Generation IV nuclear technologies. In this study, a widely-used ferritic/martensitic steel, T91 steel, was irradiated by 196-MeV Kr⁺ ions at 550 ℃. To reveal the irradiation mechanism, the microstructure evolution of irradiated T91 steel was studied in details by transmission electron microscope (TEM). With increasing dose, the defects gradually changed from black dots to dislocation loops, and further to form dislocation walls near grain boundaries due to the production of a large number of dislocations. When many dislocation loops of primary a₀/2<111> type with high migration interacted with other defects or carbon atoms, it led to the production of dislocation segments and other dislocation loops of a₀<100> type. Lots of defects accumulated near grain boundaries in the irradiated area, especially in the high-dose area. The grain boundaries of martensite laths acted as important sinks of irradiation defects in T91. Elevated temperature facilitated the migration of defects, leading to the accumulation of defects near the grain boundaries of martensite laths.     

In-situ transmission electron microscopy observations and molecular dynamics simulations of dislocation-defect interactions in ion-irradiated copper

An in-situ transmission electron microscopy straining technique has been used to investigate the dynamics of dislocation-defect interactions in ion-irradiated copper and the subsequent formation of defect-free channels. Defect removal frequently required interaction with multiple dislocations, although screw dislocations were more efficient at annihilating defects than edge dislocations were. The defect pinning strength was determined from the dislocation curvature prior to breakaway and exhibited values ranging from 15 to 175 MPa. Pre-existing dislocations percolated through the defect field but did not show long-range motion, indicating that they are not responsible for creating the defect-free channels and have a limited contribution to the total plasticity. Defect-free channels were associated with the movement of many dislocations, which originated from grain boundaries or regions of high stress concentration such as at a crack tip. These experimental results are compared with atomistic simulations of the interaction of partial dislocations with defects in copper and a dispersed-barrier-hardening crystal plasticity model to correlate the observations to bulk mechanical properties.     

Precipitation of γ′ phase and helium bubbles during proton and α-particle irradiation of nickel–silicon

Segregation of silicon was induced by light-ion irradiation at elevated temperatures in Ni–8Si specimens. Its occurrence at external surfaces, helium-induced cavities, dislocation loops, coherent twin boundaries, grain boundaries, and precipitate-matrix interfaces has been investigated by transmission electron microscopy. Layers of ordered γ^′ (Ni₃Si) phase were formed at most of these point defect sinks. The behaviour of grain boundary precipitation was found to be exceptional in various respects. In particular, a high rate of precipitation distinguishes grain boundaries from all other kinds of point defect sinks investigated here. This phenomenon of rapid precipitation was found to be adjoined to precipitation-driven grain boundary migration and is attributed to a radiation-induced “discontinuous” precipitate reaction. Observations of helium bubble distributions created during α-particle irradiations at growing dislocation loops and at migrating grain boundaries are also briefly discussed.     

Electron microscope studies of the influence of electrons with subthreshold energies on the formation and annealing of structural defects in ZnTe and CdZnTe

The influence of subthreshold-energy electrons on the formation and annealing of structural defects in ZnTe and on the annealing of dislocation loops preinjected in the sample by bombardment with argon ions is investigated by transmission electron microscopy. The final form and density of structural defects formed after electron irradiation is observed to depend on the total electron flux, whereas the rate of formation of defects and annealing of dislocation loops increases linearly with the intensity of electron irradiation. A comparison is drawn with CdZnTe, Si, and GaAs irradiated under similar conditions. The results are attributed to subthreshold defect nucleation mechanisms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 111–115, January, 1996.     

铁冷轧形变以及热处理对形变缺陷的影响

采用常用冷轧设备对铁进行冷轧引入形变缺陷。研究形变量和温度对形变缺陷的影响。形变样品中的微观缺陷、物相结构和形貌分别使用正电子湮没技术（PAT）、X射线衍射仪（XRD）和透射电子显微镜（TEM）进行表征分析。对经过673 K热处理的形变样品前后进行XRD测试,结果显示,随着形变量的增加,样品中晶面方向（200）具有择优生长趋势,673 K热处理后,择优趋势更加明显,同时晶粒的尺寸也增大。利用正电子湮没寿命谱和多普勒展宽能谱对样品中形变缺陷的热力学稳定性进行研究,发现形变引入的空位型缺陷约在673 K回复完毕,723 K后位错缺陷开始回复。The pure iron was cold rolled with the thickness reduction from 0% to 75%. The microstructure defects, crystallographic structure and morphology of deformed specimens were characterized by positron annihilation technique (PAT), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD results show that the intensity of (200) increased with increasing deformation, 673 K heat-treatment promote the preference of (200) and the grain size of (200) was increased. The PAT results show that the vacancy type defect was annihilated at 673 K and the dislocation type defects start to annihilate at 723 K.     

Effects of excess point defects flow and interface orientation on grain boundary migration under irradiation

Fe-Cr-Ni, Ni-Al and Ni-Si alloys, and 316L stainless steels as reference were electron-irradiated using a high voltage electron microscope (1MV), and in-situ observations of structural evolution and micro-chemical analysis were carried out. From the compositional analysis it was found that nickel was enriched and chromium depleted near grain boundary in Fe-Cr-Ni alloys including 316L stainless steels, and that simultaneously grain boundary migration was caused during irradiation, even if no grain boundary migration occurred in the un-irradiated area at the same irradiation temperature. The occurrence of boundary migration strongly depended upon orientation relationship between boundary interfaces. It is suggested that grain boundary migration under irradiation remarkably occurs in the alloys in which solute enrichment is taken place at the grain boundary as a result of the flow of radiation-introduced point defects into grain boundary and that their magnitude depend upon net flow of point defect, especially that of under-sized interstitial atoms.     

强流脉冲电子束诱发纯钛表面的微观结构及应力状态

利用强流脉冲电子束(HCPEB)装置对金属纯钛进行轰击,采用X射线衍射,扫描电子显微镜及透射电子显微镜技术详细分析了轰击样品表层的结构和缺陷. X射线衍射分析表明, HCPEB能够在材料表层诱发幅值为 GPa量级的压应力,并在(100), (102)和(103)晶面出现择优取向.表层微观结构的观察表明: HCPEB轰击后材料表层发生了马氏体相变,形成了大量的片状马氏体组织; 此外, HCPEB轰击还在辐照表面诱发了强烈的塑性变形,一次轰击后,晶粒内部的塑性变形以(100)晶面的位错滑移为主,位错密度显著提高;多次轰击后,样品变形结构发生变化,变形孪晶的数量明显增多. 这些变形微结构不仅影响表层的织构演化行为,而且还能细化晶粒,进而提高材料表面硬度, 为HCPEB技术进行纯钛表面强化提供了一条有效的途径.     

强流脉冲电子束作用下金属锆的微观结构与应力状态

利用强流脉冲电子束 (HCPEB) 技术对金属纯锆进行表面处理, 采用X射线衍射, 扫描电子显微镜及透射电子显微镜详细分析了辐照诱发的表层微观结构和缺陷. X射线分析结果表明, HCPEB辐照后在材料表层诱发幅值为GPa量级的压应力, 并形成{0002}, {1012}, {1120}及{1013}织构. 表层微观结构观察表明, 与其他金属材料不同, HCPEB辐照在材料表层诱发的熔坑数量极少, 多次轰击甚至几乎没有表面熔坑的形成. 此外, 在快速的加热和冷却状态下, 在表面熔化层形成大量的超细晶粒结构, 同时诱发马氏体相变和强烈的塑性变形. 1次HCPEB辐照后表层内形成的变形微结构以位错为主, 孪晶数量较少; 5 次辐照样品的位错密度迅速增高, 孪晶数量也显著增加; 10次辐照后样品中的变形微结构以变形孪晶为主, 且出现二次孪晶现象. 表层晶粒内部变形的晶体学特征不仅决定了表层的织构演化行为, 而且还起到细化晶粒的作用, 为纯锆及锆合金表面强化提供了一条有效的途径. 关键词： 强流脉冲电子束 纯锆 微观结构 应力状态     

Deformation of micron-sized aluminium bi-crystal pillars

The deformation of micron-sized single-crystals is jumpy and stochastic, and this may pose potential formability and reliability problems if components for future micro-machines are to be made from small metal volumes. In this work, micron-sized bi-crystal pillars were fabricated by focussed ion-beam milling from grain-boundary regions in coarse-grained polycrystalline aluminium. Each bi-crystal pillar contained a grain boundary intersecting its top surface, and was subjected to compression using a flat-ended nanoindenter tip. Their deformation was found to have smaller strain bursts, fewer periods of strain hardening at elastic-like rates, as well as greater work-hardening rate and flow stress, than single-crystal pillars of similar sizes. Transmission electron microscopy revealed severe dislocation accumulation in the deformed bi-crystal pillars, whereas the residual dislocation density remained low in single-crystal micro-pillars of similar dimensions after deformation to comparable strains. The results suggest that a grain boundary inside a micro-specimen can trap dislocations inside the specimen, leading to a significant rise in the strain-hardening rate as well as to smoother deformation.     

Dislocation vs. production bias revisited with account of radiation-induced emission bias. I. Void swelling under electron and light ion irradiation

Early experimental data on void swelling in electron-irradiated materials disagree with the dislocation bias models based on the dislocation-point defect elastic interactions. Later, this became one of the factors that prompted the development of models based on production bias (PBM) as the main driver for swelling, which assumed that the dislocation bias was much lower than that predicted by theoretical analyses of dislocation bias. However, the PBM in its present form fails to account for important and common observations, namely, the indefinite void growth often observed under cascade irradiation and the swelling saturation observed under high-dose irradiation and in void lattices. In this paper, we show that these contradictions can be naturally resolved in the framework of the rate theory that accounts for the radiation-induced vacancy emission from extended defects, such as voids, dislocations and grain boundaries. This modification introduces a new bias type in the theory, namely, the emission bias. This modified rate theory agrees well with the experimental data and demonstrates that the original dislocation bias should be used in rate theory models along with the emission bias in different irradiation environments. The modified theory predictions include, but are not limited to, the radiation-induced annealing of voids, swelling saturation under high-dose irradiation, generally, and in void lattices, in particular.     

Evolution pattern of collision cascades in bcc V with different grain boundary structures: an atomic scale study

Abstract

In this paper, by means of atomic-scale simulations, we investigate modifications of the evolution pattern of collision cascades in bcc vanadium (V) with different grain boundary (GB) structures on picosecond (ps) timescale. In primary damage state, in agreement with previous results of bcc and fcc bi-crystals, we find that the GBs in V are biased towards interstitials. The biased absorption of interstitials over vacancies reduces the in-cascade annihilation of vacancy-interstitial pairs and leads to aggregation of more number of vacancies in the grains interiors. The sessile vacancies accumulate in the bulk and form immobile vacancy clusters; in contrast, the glissile interstitials disperse in the damage zone and mostly diffuse in the form of single self-interstitial atoms (SIAs)/di-interstitials towards the GB region. Moreover, meanwhile, as we discuss the mechanisms that reduce (or increase) the concentration of defects in bi-crystal structures on picosecond timescale, we study the energetics of defects in close vicinity of pristine GBs, as an alternative driving force that facilitates formation and accumulation of defects in the GB regions. Finally, in a prolonged irradiation, we examine stability and sink properties of the damaged GBs. The results reveal that, irrespective of GB structure, the presence of grain boundaries leads to aggregation of more number of vacancies in the grain interiors in continuous bombardment. Overall, based on the results obtained in the primary damage event and the prolonged irradiation, we conclude that the GBs in bcc V act as efficient defect sinks on the simulated time frame.     

Microstructure and hardening effect of pure tungsten and ZrO2 strengthened tungsten under carbon ion irradiation at 700℃

Microstructure evolution and hardening effect of pure tungsten and W-1.5%ZrO₂ alloy under carbon ion irradiation are investigated by using transmission electron microscopy and nano-indentation. Carbon ion irradiation is performed at 700 ℃ with irradiation damages ranging from 0.25 dpa to 2.0 dpa. The results show that the irradiation defect clusters are mainly in the form of dislocation loop. The size and density of dislocation loops increase with irradiation damages intensifying. The W-1.5%ZrO₂ alloy has a smaller dislocation loop size than that of pure tungsten. It is proposed that the phase boundaries have the ability to absorb and annihilate defects and the addition of ZrO₂ phase improves the sink strength for irradiation defects. It is confirmed that the W-1.5%ZrO₂ alloy shows a smaller change in hardness than the pure tungsten after being irradiated. From the above results, we conclude that the addition of ZrO₂ into tungsten can significantly reduce the accumulation of irradiated defects and improve the irradiation resistance behaviors of the tungsten materials.     

Case studies on the application of high-resolution electron channelling contrast imaging – investigation of defects and defect arrangements in metallic materials

In 1967, Coates discovered the electron channelling contrast of backscattered electrons (BSEs) in scanning electron microscopy, and by this the possibility to investigate arrangements of lattice defects in deformed microstructures of materials. Since that time, a straightforward development of the scanning electron microscopes as well as of the electron channelling contrast technique took place. Nowadays, the performance of scanning electron microscopes is high enough that the resolution of electron channelling contrast imaging (ECCI) micrographs is comparable with conventional bright field transmission electron microscopy (TEM) micrographs. In the first part of the present paper, a historical review on the development of the ECCI technique starting from its discovery more than 45 years ago up to the combination with other advanced methods of scanning electron microscopy like electron backscatter diffraction or high-resolution selected area channelling patterning in the last few years is given. Major important investigations using this technique for the visualization of individual lattice defects like stacking faults (SFs) and dislocations or dislocation arrangements are chronologically summarized. The second part demonstrates that nowadays, ECCI micrographs taken in high-resolution scanning electron microscopes can be called high-resolution ECCI (HR-ECCI). It is shown that the resolution of individual SFs and dislocations in the HR-ECCI micrographs is comparable to that of conventional TEM (about 15 nm defect image width). Furthermore, the paper is demonstrating that HR-ECCI micrographs can be obtained for various types of materials after different mechanical loadings and different grain sizes ranging from large grain size of 500 μm (cast steel) down to less than 2 μm (γ-TiAl).     

高温退火后非掺杂磷化铟材料的电子辐照缺陷

对不同气氛下高温退火非掺杂磷化铟(InP)材料的电子辐照缺陷进行了研究. 除铁受主外，磷化铁(FeP₂)气氛下退火后的InP中辐照前没有深能级缺陷，而辐照后样品的热激电流谱(TSC)中出现了5个较为明显的缺陷峰，对应的激活能分别为0.23 eV, 0.26 eV, 0.31 eV, 0.37 eV和0.46 eV. 磷(P)气氛下退火后InP中的热生缺陷较多，电子辐照后形成的缺陷具有复合体特征. 与辐照前相比，辐照后样品的载流子浓度和迁移率产生显著变化. 在同样的条件下，经FeP₂ 气氛下高温退火后的InP样品的辐照缺陷恢复速度较快. 根据这些现象分析了缺陷的属性、快速恢复机理和缺陷对材料电学性质的影响. 关键词： 磷化铟 电子辐照 缺陷     

Microstructure of high-pressure-synthesized diamonds under rapid quenching and electron irradiation

A type of synthetic diamond single crystal about 0.4–0.5 mm in dimension prepared under high pressure–high temperature (HPHT) in the presence of a FeNi molten catalyst was quenched from HPHT and irradiated with 300 keV electrons at room temperature. Transmission electron microscopy was employed to examine the microstructure of the diamond single crystal before and after electron irradiation. It was found that there exists a large amount of cellular interfaces in the quenched diamond sample, which indicates the growth condition of the diamond under HPHT. Hexagonal dislocation loops about several tens of nanometers in dimension were observed in the high-pressure-synthesized diamond single crystal before electron irradiation, which strongly suggests that a number of vacancies were quenched-in due to rapid quenching from high temperature at the end of diamond synthesis, and were aggregated in the synthetic diamond to form vacancy disks on the (111) plane, the collapse of such vacancy disks forming vacancy-type dislocation loops. After electron irradiation, it was found that defect clusters present as interstitial-character dislocation loops were formed in the electron-irradiated region of the diamond. The interstitial dislocation loops grow with increase of the irradiation time. The present study, in comparison to previous work on ion implantation on diamond, indicates that electron irradiation does not induce a phase transformation but produces interstitial dislocation loops due to the migration of interstitial atoms and vacancies. The result of the study directly indicates that interstitials and vacancies in diamond are mobile at room temperature under electron irradiation. Nitrogen, as the most important kind of impurity contained in the HPHT as-grown diamond, probably acts as nucleation of the interstitial loops. Received: 16 November 2001 / Accepted: 12 June 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. E-mail: yinlw@sdu.edu.cn     

高温电子辐照硅中缺陷的研究

本文对高温电子辐照硅中产生的缺陷进行了研究,发现缺陷的引进率随电子辐照温度的增加而增加,在达到极值温度T_m后,缺陷的引进率将随之而下降,T_m值与缺陷的退火激活能有关。E₃缺陷(E_c—0.36eV)浓度在高温电子辐照中显著增加,在330℃高温电子辐照时,E₃缺陷浓度为室温电子辐照的6倍。研究结果表明,E₃缺陷的可能结构为与多空位和氧有关的复合体。 关键词：     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.