Deformation twinning in nanocrystalline materials

Nanocrystalline (nc) materials can be defined as solids with grain sizes in the range of 1-100 nm. Contrary to coarse-grained metals, which become more difficult to twin with decreasing grain size, nanocrystalline face-centered-cubic (fcc) metals become easier to twin with decreasing grain size, reaching a maximum twinning probability, and then become more difficult to twin when the grain size decreases further, i.e. exhibiting an inverse grain-size effect on twinning. Molecular dynamics simulations and experimental observations have revealed that the mechanisms of deformation twinning in nanocrystalline metals are different from those in their coarse-grained counterparts. Consequently, there are several types of deformation twins that are observed in nanocrystalline materials, but not in coarse-grained metals. It has also been reported that deformation twinning can be utilized to enhance the strength and ductility of nanocrystalline materials. This paper reviews all aspects of deformation twinning in nanocrystalline metals, including deformation twins observed by molecular dynamics simulations and experiments, twinning mechanisms, factors affecting the twinning, analytical models on the nucleation and growth of deformation twins, interactions between twins and dislocations, and the effects of twins on mechanical and other properties. It is the authors’ intention for this review paper to serve not only as a valuable reference for researchers in the field of nanocrystalline metals and alloys, but also as a textbook for the education of graduate students.     

基于分子动力学模拟的铜锆晶体/非晶双相纳米复合材料力学行为

金属玻璃因其较差的室温塑性限制了其广泛应用,因此提升金属玻璃的力学性能、探明金属玻璃的变形机制已经成为当前材料领域的研究热点。采用分子动力学方法研究了晶粒尺寸和分布对晶体/非晶B2-CuZr/CuZr双相复合材料力学行为的影响。研究结果表明,随着纳米晶粒的尺寸增大,复合材料变形模式发生了从相对均匀变形到单一剪切带的局部变形的转变。研究指出,增大纳米晶粒尺寸/体积分数能有效提高复合材料的峰值应力,但除了较小尺寸纳米晶粒模型外,双相复合材料的塑性没有明显增强。此外,相对于交叉排列,纳米晶粒的对齐排列导致了更严重的塑性应变局部化。本文的研究结果对于设计和制备高性能的金属玻璃材料具有重要的参考价值和指导意义。      

高强度超细晶金属材料塑性行为及增塑研究进展

强度和塑性是金属结构材料最重要的力学性能指标,金属高性能化的关键是在高强度水平下保证良好的塑性,然而两者往往不能兼顾。在众多强化方法中,晶粒细化长期以来被认为是强化金属最理想的手段,在传统晶粒尺寸范围,细化晶粒既可以显著提高材料的强度,又能改善材料的塑韧性。因此,近几十年来超细晶/纳米晶金属得到了广泛研究和发展,出现了以大塑性变形(SPD)、先进形变热处理(ATMP)技术为代表的超细晶制备方法,所得晶粒可以细化到亚微米或纳米尺度,金属性能大大提高。然而,大量研究证实当晶粒细化到亚微米或纳米尺度时金属强度提高但塑性显著下降,与传统的细晶强化规律不符。对此,国内外学者进行了很多研究,试图阐明其机理、揭示晶粒超细化导致塑性降低的物理本质。此外,由于细化晶粒方法受到塑性的限制,新的高强度水平下增强塑性的方法成为钢铁材料高性能化的研究热点。针对塑性下降的事实,为了进一步提高超细晶金属材料性能,研究者开展了许多增强塑性的工作,获得了较好的效果,但仍存在一些不足。关于金属晶粒超细化导致塑性降低的普遍共性现象,目前广泛认可的理论主要有晶界捕获(吸收)位错的动态回复理论、位错运动湮灭理论、高初始位错密度以及位错源缺失机制等。前三者都主要关注超细晶金属材料低(无)加工硬化能力,并将其归结为延伸率降低所致。主要是因为低(无)加工硬化使材料在变形早期发生塑性失稳或局部变形从而表现出低塑性。超细晶金属增塑研究主要体现在增塑方法和机理方面,目前,增塑方法主要有(1)形成纳米孪晶;(2)获得粗晶-细晶双峰组织;(3)利用相变诱发塑性/孪生诱发塑性(TRIP/TWIP)效应;(4)引入铁素体软相;(5)利用纳米第二相粒子等。这些增塑方法的主要机理是利用组织结构的改变提高超细晶金属的加工硬化能力以维持良好的均匀塑性变形以及利用组织相变提高塑性。本文归纳了常用的超细晶金属制备方法,综述了超细晶金属材料塑性降低的研究进展,总结了超细晶金属增塑的研究结果,分析了目前研究中存在的不足,探讨了超细晶金属增强增塑的发展趋势,以期为超细晶金属塑性降低理论及增强增塑研究提供参考。     

Deforming Nanocrystalline Metals: New Insights,New Puzzles

Recent experiments have brought new insights into the mechanisms which govern the plasticity of nanocrystalline metals. In particular, new opportunities have arisen from the finding that bulk nanocrystalline samples with extremely small grain size, prepared by the inert gas condensation technique, can be deformed to large true strain. The findings elucidate the roles of creep, partial dislocation activity along with its consequences, faulting and twinning, as well as grain boundary sliding and grain rotation. However, they also rise intriguing new questions, specifically with respect to the mechanisms of dislocation nucleation at grain boundaries, and with respect to slip system selection and alignment in twinned grains. An emerging insight is that there is not ‘the’ deformation mechanism at small grain size; instead, deformation mechanism maps in, for instance, the parameter space spanned by the strain rate and the grain size, are more appropriate representations of the various processes that control the materials behavior.     

Research and development of nanocrystalline W/W-based materials: novel preparation approaches,formation mechanisms,and unprecedented excellent properties

Tungsten (W) has become the most promising plasma-facing material (PFM) in fusion reactor, and W still faces performance degradation caused by low-temperature brittleness, low recrystallization temperature, neutron irradiation effects, and plasma irradiation effects. The modification of W/W-based materials in terms of microstructure manipulation is needed, and such techniques to improve the performance of materials are the topics of hot research. Researchers have found that refining the grain can significantly improve the strength and the irradiation resistance of W/W-based materials. In this paper, novel approaches and technique routes, including the “bottom-up” powder metallurgy method and “top-down” severe plastic deformation method, are introduced to the fabrication of nanocrystalline W/W-based materials. The formation mechanisms of nanocrystalline W/W-based materials were revealed, and the nanostructure stabilization mechanisms were introduced. The mechanical properties of nanocrystalline W/W-based materials were tested, and the irradiation behaviors and performances were studied. The mechanisms of their high mechanical properties and excellent irradiation-damage resistance were illustrated. This article may provide an experimental and theoretical basis for the design and development of high-performance novel nanocrystalline W/W-based materials.     

Balancing benefits of strength,plasticity and glass-forming ability in Co-based metallic glasses

Development of advanced metals materials with ultrahigh strength,large plasticity and high thermosta-bility is one of the most attractive aims for materials researchers.Co-based bulk metallic glasses(BMGs)with the highest strength(up to 6 GPa)and special strength(up to 650 Nm/g)among all of metals mate-rials so far we known have received extensive attentions.In this paper,a family of Co-Ta-B-Si BMGs with high glass-transition temperature(above 870 K),large compressive plasticity(up to 6.4％)and high strength(above 5.5 GPa),and high glass-forming ability(the critical diameter,Dc:up to 4 mm)was devel-oped by accurately tuning metalloid element contents of Si and B in the parental alloy of Co55Ta10B35.The changes of glass formation and plasticity caused by the adjustment of the constituent metalloid elements were evaluated by the combination of experimental and calculated results.The reason for the significant improvement of plastic deformation is revealed by the analysis of the self-organization behaviors of high-density shear bands.     

纳米材料微阵列超塑微成形机理与尺度效应

微成形技术是未来批量制造高精密微小零件的关键技术,但是,微小尺度下材料的塑性变形行为不仅表现出明显的尺度效应,而且零件尺度已经接近常规材料的晶粒尺寸,每个晶粒的形状、取向、变形特征对整体变形产生复杂的影响,难以保证微成形的工艺稳定性。本项目采用纳米材料进行微成形,制造微阵列,零件内部包含大量的晶粒,可以排除晶粒复杂性的影响,而且纳米材料具有超塑性,在超塑状态下,变形抗力和摩擦力都明显降低,从而显著降低微成形工艺对模具性能的苛刻要求,提高工艺稳定性和成形精度。目前,纳米材料超塑性微成形技术方面的研究极少,变形时纳米材料的力学行为、变形机理、尺度效应、位错演化、力学模型等关键问题还有待研究。采用电沉积技术制备晶粒尺寸可控的纳米材料,将工艺实验研究、性能测试、组织分析、力学性能表征、数值模拟相结合,深入探究了纳米材料微阵列超塑性微成形机理和成形规律,以促进该技术的广泛应用。     

Strain rate effects in the ultrafine grain and nanocrystalline regimes—influence on some constitutive responses

Mounting evidence is pointing to some emerging novel behaviors of metals with ultrafine-grain (UFG) and/or nanocrystalline (NC) microstructures. One such novel behavior is related to the thermodynamic and kinetic aspects of plastic response in the UFG/NC regime. Two inter-related parameters, viz., the strain rate sensitivity (SRS) and the activation volumes of plastic deformation, are used as fingerprints for the thermodynamics and kinetics of plastic deformation. Changes of these parameters with grain size may indicate transition of plastic deformation mechanisms. Therefore, investigations of these phenomena may bring out new strategies for ingenious design and synthesis of UFG/NC materials with desirable properties. In this article, we present a critical review on the experimental results and theories associated with the SRS of UFG/NC metals with different lattice structures, and the influences on some constitutive responses.     

Diffusion in Nanocrystalline Metals and Alloys—A Status Report

Diffusion is a key property determining the suitability of nanocrystalline materials for use in numerous applications, and it is crucial to the assessment of the extent to which the interfaces in nanocrystalline samples differ from conventional grain boundaries. The present article offers an overview of diffusion in nanocrystalline metals and alloys. Emphasis is placed on the interfacial characteristics that affect diffusion in nanocrystalline materials, such as structural relaxation, grain growth, porosity, and the specific type of interface. In addition, the influence of intergranular amorphous phases and intergranular melting on diffusion is addressed, and the atomistic simulation of GB structures and diffusion is briefly summarized. On the basis of the available diffusion data, the diffusion‐mediated processes of deformation and induced magnetic anisotropy are discussed.     

Stretchable and Soft Electronics using Liquid Metals

The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e‐skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self‐healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.     

Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.     

Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites

The extraordinary mechanical, thermal and electrical properties of carbon nanotubes have prompted intense research into a wide range of applications in structural materials, electronics, chemical processing and energy management. Attempts have been made to develop advanced engineering materials with improved or novel properties through the incorporation of carbon nanotubes in selected matrices (polymers, metals and ceramics). But the use of carbon nanotubes to reinforce ceramic composites has not been very successful; for example, in alumina-based systems only a 24% increase in toughness has been obtained so far. Here we demonstrate their potential use in reinforcing nanocrystalline ceramics. We have fabricated fully dense nanocomposites of single-wall carbon nanotubes with nanocrystalline alumina (Al2O3) matrix at sintering temperatures as low as 1,150 degrees C by spark-plasma sintering. A fracture toughness of 9.7 MPa m 1/2, nearly three times that of pure nanocrystalline alumina, can be achieved.     

Externally constrained plastic flow in miniaturized metallic structures: A continuum-based approach to thin films, lines, and joints

Mechanical behavior of materials at small length scales has received significant attention in recent years, due mainly to the development of devices and components having micro- and nano-scale feature sizes. In miniaturized structures, deformation is heavily influenced by the physical confinement imposed on the material. The present article is devoted to this type of constrained plastic deformation in metals. Continuum plasticity is used as a primary tool to describe the deformation features, for the purpose of establishing an overall mechanistic view which is often missing in the materials science community. We discuss recent progresses in understanding the externally influenced plastic flow behavior in selected metallic structures, including thin continuous films attached to stiff or compliant substrates, various forms of metal lines in modern semiconductor devices, and metallic joints in electronic packages. Special emphasis is on the evolution of local deformation pattern and overall mechanical response, and their implications in the interpretation of experimental results and the structural integrity for real-life applications. Aside from the review of current status of knowledge, we also address common misconceptions, remaining challenges as well as directions of future research in constrained small-scale plasticity.     

纳米晶金属块体材料制备技术与力学性能研究进展

主要总结了纳米晶金属块体材料的制备技术与力学性能的研究进展.讨论了各种制备技术的特点与现存的问题,以及纳米晶金属块体材料的强度、塑性、应变速率敏感性、超塑性、变形机制与断裂机制等力学性能问题.     

Nanostructured interfaces for enhancing mechanical properties of composites: Computational micromechanical studies

Computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials and the potential of interface nanostructuring to enhance the materials properties are reviewed. Several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) are considered with view on the effect of nanostructured interfaces on their properties. The structures of various nanostructured interfaces (protein structures and mineral bridges in biopolymers in nacre and microfibrils in wood; pores, interphases and nanoparticles in fiber/matrix interfaces of polymer fiber reinforced composites and nanocomposites; dislocations and precipitates in grain boundaries of nanocrystalline metals) and the methods of their modeling are discussed. It is concluded that nanostructuring of interfaces and phase boundaries is a powerful tool for controlling the material deformation and strength behavior, and allows to enhance the mechanical properties and strength of the materials. Heterogeneous interfaces, with low stiffness leading to the localization of deformation, and nanoreinforcements oriented normally to the main reinforcing elements can ensure the highest damage resistance of materials.     

Plasticity Enhancement Processes in NiAl and MoSi_2

This paper demonstrates that plasticity enhancement processes previously observed in bcc metalsand associated with the presence of surface films and precipitates or dispersoids are operative in or-dered intermetallic compounds.' Some specific results are given for NiAl-and MoSi_2-basedmaterials.The plasticity of NiAl and related B2 ordered alloys at room temperature during monotonicdeformation can be enhanced by application of surface films such as electroless nickel platings.Sig-nificant effects are also observed during cyclic deformation.The presence of ductile second phasessuch as fcc γ or the ordered phase γ' introduced by alloying also enhances the plasticity of NiAl.MoSi_2 exhibits similar effects of surface films and second phases,but primarily at elevated tempera-tures,T>900℃.In this paper,we illustrate the effects of ZrO_2 surface films deposited near roomtemperature and TiC particulate additions introduced by powder processing techniques.In all cases,the plasticity enhancement is associated with enhanced dislocation generation processes under theconditions of constrained deformation at the film-substrate or precipitate/dispersoid-matrix inter-face of the composite system.     

Mechanical properties of nanocrystalline materials

The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume is already low in the conventional polycrystalline regime. In fatigue, it seems that the S-N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than that of conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization.The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation after crossing them.     

Structural nanocrystalline materials: an overview

This paper presents a brief overview of the field of structural nanocrystalline materials. These are materials in either bulk, coating, or thin film form whose function is for structural applications. The major processing methods for production of bulk nanocrystalline materials are reviewed. These methods include inert gas condensation, chemical reaction methods, electrodeposition, mechanical attrition, and severe plastic deformation. The stability of the nanocrystalline microstructure is discussed in terms of strategies for retardation of grain growth. Selected mechanical properties of nanocrystalline materials are described; specifically strength and ductility. Corrosion resistance is briefly addressed. Examples of present or potential applications for structural nanocrystalline materials are given.     

Failure surfaces of high-strength materials predicted by a universal failure criterion

Advanced high-strength materials including metallic glasses (MGs) and nanocrystalline (NC) metals have attracted great attentions due to their promising mechanical properties. However, since they usually exhibit relatively brittle fracture behavior, a proper failure criterion is urgently required for the safety design of structural components and devices using these high-strength materials. It has been found that the conventional failure criteria may not soundly explain the fracture behaviors of MGs, while the ellipse criterion shows promising universality for well describing the fracture behaviors of several kinds of high-strength materials under various loading conditions. In this study, the principal stress form of the ellipse criterion was derived for the first time, and the two-dimensional failure surfaces according to this criterion were plotted and compared with the other predictions, as well as the experimental and simulation results of MGs and NC metals. The results confirm the applicability of the ellipse criterion for predicting the failure of high-strength materials, and may also improve the understanding of strength and fracture of various materials.     

Delta-doped electron-multiplied CCD with absolute quantum efficiency over 50% in the near to far ultraviolet range for single photon counting applications

We have used molecular beam epitaxy (MBE) based delta-doping technology to demonstrate nearly 100% internal quantum efficiency (QE) on silicon electron-multiplied charge-coupled devices (EMCCDs) for single photon counting detection applications. We used atomic layer deposition (ALD) for antireflection (AR) coatings and achieved atomic-scale control over the interfaces and thin film materials parameters. By combining the precision control of MBE and ALD, we have demonstrated more than 50% external QE in the far and near ultraviolet in megapixel arrays. We have demonstrated that other important device performance parameters such as dark current are unchanged after these processes. In this paper, we briefly review ultraviolet detection, report on these results, and briefly discuss the techniques and processes employed.