Interface controlled plasticity in metals: dispersion hardening and thin film deformation

Plastic deformation and strengthening of metals, a classic subject of physical metallurgy, is still a central theme of present-day materials research. This review focusses on two modern aspects of fundamental and practical interest: the mechanism of dispersion hardening at high temperatures, which allows the design of alloys operating close to their melting point; and the constraints on dislocation and diffusional deformation processes in metallic thin films, a potential reliability problem for micro-systems subjected to high internal stresses. The commonality lies in the importance of interfacial effects: the interaction of lattice dislocations with interfaces — between particle and matrix, or between film and substrate — controls the strengthening effect in both instances; diffusional creep occurs in both cases, but is again limited by interface effects. An attempt is made to summarize the current understanding of these phenomena with special emphasis on modelling and transmission electron microscopy results.     

Annealing‐Induced Hardening in Ultrafine‐Grained Ni–Mo Alloys

The influence of Mo alloying on annealing‐induced hardening in ultrafine‐grained (UFG) Ni is studied. The hardening observed after low temperature annealing is explained by the annihilation of mobile dislocations and a concomitant clustering of the remaining dislocations into low energy configurations. This study reveals that, with increasing Mo concentration, the hardening effect decreases as the Mo solute atoms hinder the annihilation and rearrangement of dislocations. This trend is the opposite to that observed in electrodeposited Ni–Mo alloys where the larger alloying element concentration yields a higher annealing‐induced strengthening effect. The difference is attributed to the different deformation mechanisms in UFG and nanocrystalline Ni–Mo alloys.

     

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。     

A new life extension method for high cycle fatigue using micro-martensitic transformation in an austenitic stainless steel

Most of the conventional strengthening methods for metals and alloys such as work hardening, precipitation hardening, cause a decrease in ductility and are not very effective for cyclic loading. In this study, a new strengthening method, which is effective for high cycle fatigue, has been developed. The intersections of dislocations in a stainless steel are freezed by very fine martensite particles, which are supposed to suppress dislocation motion at low stress amplitudes. Fatigue life in a high cycle regime increased >60 times, and no decrease in ductility was observed in tensile tests, as compared to a work-hardened stainless steel.     

A new life extension method for high cycle fatigue using micro-martensitic transformation in an austenitic stainless steel

Most of the conventional strengthening methods for metals and alloys such as work hardening, precipitation hardening, cause a decrease in ductility and are not very effective for cyclic loading. In this study, a new strengthening method, which is effective for high cycle fatigue, has been developed. The intersections of dislocations in a stainless steel are freezed by very fine martensite particles, which are supposed to suppress dislocation motion at low stress amplitudes. Fatigue life in a high cycle regime increased >60 times, and no decrease in ductility was observed in tensile tests, as compared to a work-hardened stainless steel.     

On dislocation-based artificial neural network modeling of flow stress

Computational design of materials processes has received great interests during the past few decades. Successful designs require accurate assessment of material properties, which can be influenced by the internal microstructure of materials. This work aim to develop a novel computational model based on dislocation structures to predict the flow stress properties of metallic materials. To create sufficient training data for the model, the flow stress of a precipitation–hardening aluminum alloy was measured by characterizing the dislocation structure of specimens from interrupted mechanical tests using a high resolution electron backscatter diffraction technique. The density of geometrically necessary dislocations was calculated based on analysis of the local lattice curvature evolution in the crystalline lattice. For three essential features of dislocation microstructures – substructure cell size, cell wall thickness, and density of geometrically necessary dislocations – statistical parameters of their distributions were used as the input variables of the predictive model. An artificial neural network (ANN) model was used to back-calculate the in situ non-linear material parameters for different dislocation microstructures. The model was able to accurately predict the flow stress of aluminum alloy 6022 as a function of its dislocation structure content. In addition, a sensitivity analysis was performed to establish the relative contribution of individual dislocation parameters in predicting the flow stress. The success of this approach motivates further use of ANNs and related methods to calibrate and predict inelastic material properties that are often too cumbersome to model with rigorous dislocation-based plasticity models.     

Analysis of strengthening mechanisms in a severely-plastically-deformed Al–Mg–Si alloy with submicron grain size

Methods of severe plastic deformation of ductile metals and alloys offer the possibility of processing engineering materials to very high strength with good ductility. After typical amounts of processing strain, a submicrocrystalline material is obtained, with boundaries of rather low misorientation angles and grains containing a high density of dislocations. In the present study, an Al–Mg–Si alloy was severely plastically deformed by equal channel angular pressing (ECAP) to produce such a material. The material was subsequently annealed for dislocation recovery and grain growth. The strength of materials in various deformed and annealed states is examined and the respective contributions of loosely-arranged dislocations, many grain boundaries, as well as dispersed particles are deduced. It is shown that dislocation strengthening is significant in as-deformed, as well as lightly annealed materials, with grain boundary strengthening providing the major contribution thereafter.     

A mechanism for the deformation of disordered states of matter

As the frontier in advanced materials development has shifted into highly disordered systems, concepts of deformation based on crystal lattice dislocations often become too coarse to be of relevance. Therefore, a new deformation process, localized to dimensions smaller than those involved in dislocation mechanisms, was proposed sometime ago. Some of its important features are discussed here to suggest that this mechanism is likely to be of use in understanding the superplastic deformation of metals and alloys, ceramics, metal-matrix- and ceramic-matrix-composites, dispersion hardened materials, intermetallics, geological materials, metallic glasses and poly-glasses of grain sizes in the μm-, sub-μm- or nm-range – a much wider area of application than originally anticipated. This will allow one to define “superplasticity” as due to a unique physical mechanism, rather than by the extreme elongations obtainable in tensile testing or the strain rate sensitivity index being more than ∼0.30.     

Pipe diffusion along isolated dislocations

The complete solution of dislocation pipe diffusion is applied to experimental data for different metals. For f.c.c. metals the mechanisms of vacancies bound to the dislocation and of diffusion in the stacking fault ribbon between dissociated dislocations are discussed. Cationic diffusion along dislocations will be obscured in the alkali halides by aliovalent impurities, but it may be observed in some metallic oxides. Anionic diffusion along dislocations may be found in all NaCl-type ionic crystals.     

球状颗粒强化复合材料Al-6061/Al2O3的强化作用

研究了6061/Al₂O₃复合材料中球形Al₂O₃颗粒的强化作用。该强化颗粒的加入大大提高了材料在固溶处理后未时效状态的强度。对组织的观察和位错密度的测量表明,该强化作用与位错强化模型的计算结果一致。强化颗粒的加入还显著提高了材料在各种时效状态的加工硬化率。将Ashby提出的含异质颗粒复合材料的几何必须位错模型与位错强化模型结合,可以很好地解释加工硬化率的提高。     

Role of work hardening characteristics of matrix alloys in the strengthening of metal matrix composites

The strengthening of particulate reinforced metal-matrix composites is associated with a high dislocation density in the matrix due to the difference in coefficient of thermal expansion between the reinforcement and the matrix. While this is valid, the role of work hardening characteristics of the matrix alloys in strengthening of these composites is addressed in the present paper. It is found that commercial purity aluminium which has the lowest work hardening rate exhibits the highest strength increment. This effect is due to increased prismatic punching of dislocations. This relationship of decreasing work hardening rate associated with increasing prismatic punching of dislocations in the order 7075, 2014, 7010, 2024, 6061 and commercial purity aluminium leading to increased strength increments is noted.     

A theoretical model for the electromagnetic radiation emission during plastic deformation and crack propagation in metallic materials

This paper presents a theoretical model for the electromagnetic radiation (EMR) emissions during plastic deformation and crack propagation in metallic materials. It is shown that under an externally applied stress, edge dislocations within the plastic zone ahead of a crack tip form accelerated electric line dipoles which give rise to the EMR emissions. The dynamic motion of these dislocations becomes overdamped, underdamped or critically damped, depending upon the material/microstructural properties such as mass per unit length of dislocation, line tension, damping coefficient, and distance between the dislocation pinning points. The nature of the EMR signals, viz. exponential decay or damped sinusoidal, is decided essentially by these damping characteristics. The EMR emissions are followed by crack propagation in metallic materials. The EMR has a continuous frequency spectrum with a frequency bandwidth ranging from 10⁸ to 10¹² radians s⁻¹, depending upon the properties of the metals. Screw dislocations do not contribute to the EMR emissions. The paper also presents some experimental results on the EMR emissions in ASTM B265 grade 2 titanium sheets. The nature (damped sinusoidal and exponential decay), amplitude and frequency of the observed EMR emissions are in conformity with the predictions of the theoretical model.     

Strength and ductility in ultrafine-grained wrought aluminum alloys

Strength and ductility are two of the most important mechanical properties of engineering materials. In this work, a 6061 aluminum alloy was subjected to multi-directional forging (MF) and aging treatment. The samples possess high strength and high ductility after processing. The strength of samples was enhanced by dispersing ultrafine precipitate particles within the grains, reducing grain-size and increasing dislocation density after MF and aging. The ductility was improved due to reducing the forging stress during aging. Moreover, a mass of dispersing ultrafine precipitate particles widespread within the grains after aging, which helps to accumulate dislocations, increase the dislocation storage capability and resist dislocation slip that lead up to increasing work hardening, the ductility was also enhanced. A linear strengthening elastic–plastic model was developed by simplifying the stress–strain curves. On this basis, the strength and ductility of ultrafine-grained (UFG) materials were discussed. This also provides fundamental insight into the mechanisms that govern the strength and ductility of UFG materials.     

Fundamental factors on formation mechanism of dislocation arrangements in cyclically deformed fcc single crystals

This paper systematically summarizes the cyclic deformation behaviors of different kinds of face-centered cubic (fcc) single crystals, including copper, nickel, silver, as well as copper-aluminium, copper-zinc alloys in attempt to provide a historical perspective of the developments over the last several decades. Combined with plenty of previous research results, the influencing factors on cyclic deformation behaviors can be listed as follows: orientations, stacking fault energy (SFE), short-range order (SRO) and friction stress, or more generally, the ease of cross slip. Among them, the effect of orientations mainly reflects in the formation of the complex dislocation patterns, which depends on the activating secondary slip system. According to the effect of slip mode, the materials can be divided into two types: pure metals and alloys. For pure fcc metals, the effect of SFE is decisive. Due to the easy cross slip of screw dislocations, regular dislocation arrangements, e.g. veins, persistent slip bands (PSBs), labyrinth and cell patterns, are always to form. With increase in alloying element, antiphase boundary energy gradually replaces SFE to become a new decisive factor affecting the cyclic deformation behaviors of fcc alloy single crystals. The corresponding dislocation arrangements consist of dipole array and stacking faults (SFs) under the influence of planar slip. The relationship among several factors is well explained, which will help us better understand the nature of the fatigue damage of metallic materials and then improve the performance of the related materials.     

TEM study of the recovery process of cyclically deformed MgO single crystals

Single crystals of MgO were subjected to plastic strain-controlled push-pull cyclic deformation at elevated temperatures. Below 400° C the crystals were very brittle and failed with a few fatigue cycles. At 470° C a large number of cycles could be obtained before failure, and the cyclic stress-strain response showed a period of rapid hardening followed by a period of decreasing hardening rate. TEM investigations of the lower temperature samples show structures of isolated dislocation dipoles, multipoles and debris. At 470° C dense bundles of dislocations were observed aligned perpendicular to the Burgers vector direction. The regions between the bundles were relatively dislocation free, but they contained a high density of debris. Bowed out screw dislocations are observed between the edge dislocation bundles, suggesting that screw dislocations were largely mobile. Comparisons are made with the cyclic deformation and structure of fcc metals and other NaCl structure single crystals.     

多级结构的金属材料强韧化机理研究进展

强度和塑韧性是金属结构材料主要的性能指标,然而通常会出现强度与塑韧性倒置的现象,即传统的固溶强化、纳米晶强化、弥散强化和加工硬化在追求强度的同时会不可避免地牺牲金属材料的塑韧性.根据多级多尺度仿生结构可协同提高强度和韧性的思路,系统介绍了两级Ti-TiBw/Ti复合材料、不锈钢复合板、多层复合钢、层/网耦合结构钢和超细...     

Continuous dynamic recrystallization during severe plastic deformation

Severe plastic deformation (strains > 100%) has been shown to create significant grain refinement in polycrystalline materials, leading to a nanometric equiaxed crystalline structure for such metals as aluminum, copper and nickel alloys. This process, termed continuous dynamic recrystallization, is governed by evolution of the dislocation structure, which creates new grain boundaries from dislocation walls. In the proposed model, plasticity occurs which firstly involves dislocation multiplication, leading to strain hardening limited by dynamic recovery. After a critical dislocation density is reached new grain boundaries are formed by condensation of walls of dislocations, creating a new stable configuration that is favored due to a reduction of the system free energy. This evolution of the microstructure continues to develop, with a consequent progressive decrease in the average grain diameter. The proposed model provides a quantitative prediction of the evolution of the average grain size, as well as the dislocation density, during continued plastic strain. The model can be calibrated by use of results from any experiment that involves large plastic deformation of metals, subject to negligible annealing effects. In this paper, the model has been calibrated, and consequently validated, through experiments on machining of Al 6061-T6.     

Strengthening effects of coherent interfaces in nanoscale metallic bilayers

Dislocation nucleation and propagation in a Cu–Ni bilayer with coherent (1 1 1) interface are examined using atomistic simulations. Nanoindentation model is applied to generate dislocations at and near the surface. Mechanisms of interactions between gliding dislocations and coherent interface are investigated. It is found that the interface acts as a strong barrier to dislocation propagation, which results in considerable strengthening of the bilayer. The results are compared to indentation of pure Cu and pure Ni single crystals. It is found that the obtained maximum load for indentation of the Cu–Ni bilayer is higher than for any of the two pure materials. Strain hardening of the bilayer system due to the presence of interface is investigated by analyzing the indentation load–displacement curves.     

Development of a Heterogeneous Lamellar Structure Based on Layer Instabilities during Accumulative Roll Bonding

The strategy of designing heterogeneous structures has emerged as a new tool in avoiding the prominent strength–ductility trade-off dilemma in metallic materials. However, methods to reproducibly synthesize heterostructured materials with effective microstructural control are still elusive. Herein, a highly reproducible method based on conventional accumulative roll bonding (ARB) and postannealing is proposed for designing heterostructured Cu samples with precisely defined heterogeneous lamellar structure (HLS) wherein the size, volume fraction, and spatial distribution of coarse and fine grains can be accurately controlled. An optimal heterogeneous lamellar microstructure is found which gives rise to a roughly 3 times higher yield strength than that of coarse-grained Cu and a good tensile ductility of ≈34.7% total elongation. Experimental observations reveal that the unique HLS induces strong heterodeformation-induced hardening and promotes dislocation accumulation, leading to the observed exceptional strength–ductility synergy. This novel fabrication method is applicable to a considerably larger number of metal systems and can be utilized to further tailor HLS to optimize the mechanical properties of metals.