Modeling of irradiation hardening of polycrystalline materials

High energy particle irradiation of structural polycrystalline materials usually produces irradiation hardening and embrittlement. The development of predictive capability for the influence of irradiation on mechanical behavior is very important in materials design for next-generation reactors. A multiscale approach was implemented in this work to predict irradiation hardening of iron based structural materials. In the microscale, dislocation dynamics models were used to predict the critical resolved shear stress from the evolution of local dislocation and defects. In the macroscale, a viscoplastic self-consistent model was applied to predict the irradiation hardening in samples with changes in texture. The effects of defect density and texture were investigated. Simulated evolution of yield strength with irradiation agrees well with the experimental data of irradiation strengthening of stainless steel 304L, 316L and T91. This multiscale modeling can provide a guidance tool in performance evaluation of structural materials for next-generation nuclear reactors.     

Strength and Ductility of Bi‐Modal Cu

Engineering a microstructure with multiple length scales has been proposed as a strategy to enhance the plasticity of nanostructured materials which otherwise lack extensive dislocation activity, and therefore low ductility. To that effect, various research groups have implemented this concept by promoting the formation of so called bi‐modal microstructures (e.g., consisting of a mixture of ultrafine and micro‐grains) with balanced combinations of strength and ductility. Despite encouraging results, fundamental, information on important questions remained unanswered. For example, what is the relationship between the volume fraction of the coarse grained phase and the overall ductility? What is the influence of size of the coarse grained phase, and how does its distribution influence ductility? To provide insight into these, and other related questions, in this work, we prepared bimodal Cu with homogeneous distribution of different‐volume‐fraction micro‐grains via isothermal recrystallization of an ultrafine grained Cu matrix at 200 °C. Analysis of the tensile results and microstructural characterization suggest that both strength and ductility of the bi‐modal Cu follows the rule‐of‐mixtures, with interesting results related to volume fraction. Our work provides a pathway for optimizing the mechanical properties of multiscale materials.     

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。     

Multi-length scale micromorphic process zone model

The prediction of fracture toughness for hierarchical materials remains a challenging research issue because it involves different physical phenomena at multiple length scales. In this work, we propose a multiscale process zone model based on linear elastic fracture mechanics and a multiscale micromorphic theory. By computing the stress intensity factor in a K-dominant region while maintaining the mechanism of failure in the process zone, this model allows the evaluation of the fracture toughness of hierarchical materials as a function of their microstructural properties. After introducing a multi-length scale finite element formulation, an application is presented for high strength alloys, whose microstructure typically contains two populations of particles at different length scales. For this material, the design parameters comprise of the strength of the matrix–particle interface, the particle volume fraction and the strain-hardening of the matrix. Using the proposed framework, trends in the fracture toughness are computed as a function of design parameters, showing potential applications in computational materials design.     

Computational design of nanostructured steels employing irreversible thermodynamics

Abstract

Recent theory demonstrates that the Kocks–Mecking formulation of plasticity has a foundation in multiscale irreversible thermodynamics. The key terms in the formulation can be obtained form experiments and from fundamental calculations. This offers two advantages to materials scientists and alloy designers: the Kocks–Mecking approach goes beyond being a phenomenological approach, gaining a solid physical foundation in multiscale computational physics; the new formulation can be employed to conceive new alloys displaying complex synergistic interactions at several scales and among several phases. This approach is ideal for designing and modelling nanostructured steels. This work incorporates four concomitant strengthening effects: solid solution, Hall–Petch, dislocation forest and precipitation. The new formulation is applied to nanostructured martensitic, dual phase and twinning induced plasticity steels, describing with excellent accuracy of their stress–strain behaviour.     

Alloy design by dislocation engineering

Ultra-high strength alloys with good ductility are ideal materials for lightweight structural application in various industries. However, improving the strength of alloys frequently results in a reduction in ductility, which is known as the strength-ductility trade-off in metallic materials. Current alloy design strategies for improving the ductility of ultra-high strength alloys mainly focus on the selection of alloy composition (atomic length scale) or manipulating ultra-fine and nano-grained microstructure (grain length scale). The intermediate length scale between atomic and grain scales is the dislocation length scale. A new alloy design concept based on such dislocation length scale, namely dislocation engineering, is illustrated in the present work. This dislocation engineering concept has been successfully substantiated by the design and fabrication of a deformed and partitioned (D&P) steel with a yield strength of 2.2 GPa and an uniform elongation of 16%. In this D&P steel, high dislocation density can not only increase strength but also improve ductility. High dislocation density is mainly responsible for the improved yield strength through dislocation forest hardening, whilst the improved ductility is achieved by the glide of intensive mobile dislocations and well-controlled transformation-induced plasticity (TRIP) effect, both of which are governed by the high dislocation density resulting from warm rolling and martensitic transformation during cold rolling. In addition, the present work proposes for the first time to apply such dislocation engineering concept to the quenching and partitioning (Q&P) steel by incorporating a warm rolling process prior to the quenching step, with an aim to improve simultaneously the strength and ductility of the Q&P steel. It is believed that dislocation engineering provides a new promising alloy design strategy for producing novel strong and ductile alloys.     

A Modeling Approach Across Length Scales for Progressive Failure Analysis of Woven Composites

This paper presents a multiscale modeling approach for the progressive failure analysis of carbon-fiber-reinforced woven composite materials. Hierarchical models of woven composites at three different length scales (micro, meso, and macro) were developed according to their unique geometrical and material characteristics. A novel strategy of two-way information transfer is developed for the multiscale analysis of woven composites. In this strategy, the macroscopic effective material properties are obtained from property homogenizations at micro and meso scales and the stresses at three length scales are computed with stress amplification method from macroscale to microscale. By means of the two-way information transfer, the micro, meso and macro structural characterizations of composites are carried out so that the micromechanisms of damage and their interactions are successfully investigated in a single macro model. In addition, both the nucleation and growth of damages are tracked during the progressive failure analysis. A continuum damage mechanics (CDM) method is used for post-failure modeling. The material stiffness, tensile strength and damage patterns of an open-hole woven composite laminate are predicted with the proposed multiscale method. The predictions are in good agreement with the experimental results.     

Using lifetime of point defects for dislocation bias in bcc Fe

The interaction between dislocations and point defects is key to deformation processes and microstructural evolution of structural materials. In this work, we compute the lifetime of point defects to describe their interaction with dislocations. This approach can accurately account for the effects of the dislocation core and anisotropic defect dynamics to accumulatively determine the capture efficiency, sink strength, and dislocation bias at different temperatures and dislocation densities. Particularly, the absorption of point defects by straight screw and edge dislocations in a model bcc iron system is studied. The maximum swelling rates based on the obtained bias factors are in close agreement with a variety of experimental measurements, including both neutron and ion-irradiation data, especially when considering the survival fraction for point defects from displacement cascades. This approach applies to many other processes and sinks, such as dislocation loops and interfaces, providing a powerful means to develop fundamental insights critical for improving radiation resistance and mechanical properties of structural materials through controlling defect interaction and evolution.     

冷压焊界面结合机理与结合强度研究现状EI北大核心CSCD

固相焊接因其突出的技术优势在先进金属结构材料焊接中获得越来越多的应用。冷压焊作为一种特殊固相焊接方法,其界面结合机理和结合强度预测是长期以来困扰研究者的两大难题。本文综述了冷压焊界面结合机理理论研究现状,重点讨论了冷压焊接头界面结合强度影响因素及其预测模型,并指出了今后冷压焊界面结合机理需综合考虑扩散、再结晶、位错等因素,同时结合强度预测模型应考虑变形程度、变形速率、变形温度等方面。     

Boundary Strengthening over Five Length Scales

The strengthening effect of grain boundaries and dislocation boundaries is a traditional metallurgical theme, which at the present time is being revived. The reason is the scientific and industrial interest in strength obtained through a structural refinement. Such a refinement is found in nanocrystalline metals subdivided by grain boundaries and in heavily deformed metals subdivided by a mixture of dislocation boundaries and high angle boundaries. The structural dimension of these materials covers a scale form the nanometre range to the hundreds micrometers level i.e. five length scales. Consequently, a multiscale analysis has been applied to the relationship between strength and the boundary parameters. As an example this analysis suggests that the yield stress/grain size relationship follows the Hall‐Petch relationship down to a grain size of about 15–20 nm showing large potential for strengthening of a metal through a structural refinement.     

Coupling atomistics and continuum in solids: status, prospects, and challenges

The recent focus of the scientific community on multiscale computer modeling techniques of nano-engineered materials stems from the desire to develop more realistic methodologies that are capable of accurately describing the varied time and length scales associated with this class of materials. Of importance is the ability to model the atomistic region using the appropriate techniques such as quantum mechanics/molecular dynamics, and the continuum region using homogenized properties. The continuity of atomistic and continuum regions in a solid necessitates a seamless coupling between these two regions. This is carried out using a transition region. In view of the large discrepancy between length and time scales in atomistic and continuum regions, the development of the transition region has been the main concern of the research community. It is the purpose of this review to critically discuss the issues concerning the transition region and the efforts made by the scientific community in treating them. In particular, this review addresses issues concerning the coupling of molecular dynamics to finite element modeling techniques. Three aspects of this review are accordingly considered. The first is concerned with the current state of atomistic–continuum coupling techniques in computational mechanics. The second is concerned with present the research conducted in the Engineering Mechanics and Design Laboratory at the University of Toronto in the field of nano-reinforced interfaces. Finally, we present the limitations of the current techniques and suggestions for improvements.     

Mechanical Behaviour and Structure Instability of Al_3Ti Alloy

Trialuminide alloys of elements such as Ti. Nb or Zr are of particular interest as materials for high temperature usage because their density is very low and specific strength and elastic rnoduli are then very high. This report concentrates on recent work on Al3Ti alloys which have been alloyed with ternary elements such that the higher symmetry ordered cubic structure is obtained, leading to somewhat easier operation of deformation mechan isms and hence improved ductility and toughness.Fine details of the crystal structure of cubic trialuminides are considered here and it is shown that the materials generally possess some remnant tetragonal chemical ordering which can affect their me chanical behaviour. In addition the compositional range over which a stable single phase is retained is shown to be extremely small, such that in most cases the materials examined show some form of microstructural instability. These instabilities affect the mechanical behaviour of the materials, for exarnple producing general strengthening. leading to precipitation hardening du ring hig h temperature testing, and causing age hardening instabilities during high temperature static or dynamic testing.Such structural instabifity feads to significant modifications at superdislocations, affecting both the dislocation cores and their associated APB's. Failure for these cubic materials still occurs at very small plastic strains and seems to be determined by difficulties of superdislocation creation near a propagating crack rather than by problems of suitable dislocation configuration and mobility. Possible ways to enhance ductility and toughness by alloying and microstructural modification will be discussed.     

Enrichment based multiscale modeling for thermo‐stress analysis of heterogeneous material

This paper details a novel new multiscale technique for modeling heterogeneous materials undergoing substantial thermal stresses. The technique is based on an enriched partition of unity approach that incorporates the thermal effects occurring on the microstructure into the global model. We demonstrate the effectiveness of this technique by implementing it into both the standard finite element method and the octree partition of unity method (OctPUM). The results demonstrate that the technique has uniquely improved accuracy over the homogenization method conditional to the method into which it is implemented in. The multiscale technique, when implemented into either the standard finite element method or OctPUM, increases the accuracy of the strain energy calculation. When the multiscale technique is implemented into OctPUM, it also is able to capture the unique stress fields on the microstructure of the model. Published 2012. This article is a US Government work and is in the public domain in the USA.     

Hydriding of titanium: Recent trends and perspectives in advanced characterization and multiscale modeling

Titanium (Ti) and its alloys are attractive for a wide variety of structural and functional applications owing to excellent specific strength, toughness and stiffness, and corrosion resistance. However, if exposed to hydrogen sources, these alloys are susceptible to hydride formation in the form of TiH_x (0 < x ≤ 2), leading to crack initiation and mechanical failure due to lattice deformation and stress accumulation. The kinetics of the hydriding process depends on several factors, including the critical saturation threshold for hydrogen within Ti, the specific interaction of hydrogen with protective surface oxide, the rates of mass transport, and the kinetics of nucleation and phase transformation. Unfortunately, key knowledge gaps and challenges remain regarding the details of these coupled processes, which take place across vast ranges of time and length scales and are often difficult to probe directly. This work reviews recent advances in multiscale characterization and modeling efforts in Ti hydriding. We identify unanswered questions and key challenges, propose new perspectives on how to solve these remaining issues, and close knowledge gaps by discussing and demonstrating specific opportunities for integrating advanced characterization and multiscale modeling to elucidate chemistry and composition, microstructure phenomena, and macroscale performance and testing.     

Electrochemomechanical degradation of high-capacity battery electrode materials

Enormous efforts have been undertaken to develop rechargeable batteries with new electrode materials that not only have superior energy and power densities, but also are resistant to electrochemomechanical degradation despite huge volume changes. This review surveys recent progress in the experimental and modeling studies on the electrochemomechanical phenomena in high-capacity electrode materials for lithium-ion batteries. We highlight the integration of electrochemical and mechanical characterizations, in-situ transmission electron microscopy, multiscale modeling, and other techniques in understanding the strong mechanics-electrochemistry coupling during charge-discharge cycling. While anode materials for lithium ion batteries (LIBs) are the primary focus of this review, high-capacity electrode materials for sodium ion batteries (NIBs) are also briefly reviewed for comparison. Following the mechanistic studies, design strategies including nanostructuring, nanoporosity, surface coating, and compositing for mitigation of the electrochemomechanical degradation and promotion of self-healing of high-capacity electrodes are discussed.     

Mechanics at the interfaces of 2D materials: Challenges and opportunities

At the interfaces of 2D materials, the solid-state condensed matter physics is closely intertwined with the mechanics in terms of adhesion/separation and friction as well as deformation of 2D materials. With atomically thin 2D layers in atomically close proximity, the chemical, physical, and mechanical interactions simultaneously evolve and influence each other, leading to a wide range of topological structures and properties across nano- and micro-scales. Can the study on the mechanics of interfaces help to understand the physics and chemistry at the interfaces of 2D materials or vice versa? This Opinion aims to highlight the recent mechanics research on such material interfaces, where a multiscale, multidisciplinary effort is most effective moving forward with plenty of challenges and opportunities.     

Dislocation viscoplasticity aspects of material fracturing

Dislocation viscoplasticity, whether occurring before, during, or after the appearance of cracking, and with energy requirement additional to needed crack surface energy, has proved to be the bane of simplistic strength/energy evaluations applied to the fracturing properties of solid materials. George Irwin, in later years, turned his attention to such crack-effected viscoplasticity consideration in relation to the complex microstructural aspects of fracturing behaviors obtained for nuclear pressure vessel steels. The work is reviewed here with a broader research focus on dislocation plasticity aspects of fracturing behaviors, at ever smaller crack sizes, for a wider range of materials, especially, of relatively greater hardness.