A brief overview is provided about the plastic deformation mechanisms in nanotwinned metals. The unique two-dementional nanoscale twin lamellae lead to different dislocation slip systems activated during plastic deformation. It has been revealed that there are three distinctly different dislocation-mediated deformation mechanisms in nanotwinned metals, namely dislocation pile-up against and slip transfer across twin boundaries, Shockley partials gliding on twin boundaries leading to twin boundary migration, and threading dislocations slip confined by neighboring twin boundaries. It is further demonstrated that these three dislocation-mediated mechanisms are switchable upon changing in the loading direction with respect to twin boundaries.
The grain size effect and the specimen size effect on the strength of metals are briefly reviewed with respect to their history and current status of research. It is revealed that the fundamental strengthening mechanisms responsible for these two types of size effect are to increase the resistance to dislocation motion and to dislocation generation, respectively. It is shown that both strengthening mechanisms take place in some nanostructured metals, which leads to a suggestion to use these two mechanisms for optimizing the strength and ductility of nanostructured metals. This suggestion is verified by some results obtained in nanostructured pure aluminum.
This work summarized the deformation techniques of preparing the nanostructured metallic materials, including large-strain deformation techniques (clod rolling, accumulative cold-bonding, equal channel angular pressing, high pressure torsion), high-strain-rate deformation technique (dynamic plastic deformation), and high-strain-gradient deformation techniques (surface mechanical attrition treatment and surface mechanical grinding treatment). The effects of deformation modes and deformation parameters on grain refinement are analyzed. Future trends and challenges of the deformation techniques for preparing nanostructured metallic materials are discussed.
In this paper, recent investigations on strengthening ability and effects of length scale and interface, plastic deformation behavior and stability of nanolayered metallic materials are reviewed systematically. The basic mechanisms on the abilities in strengthening and toughening for nanolayered metallic materials were discussed. Finally, several key issues on improving the strengthening and toughening abilities of the nanolayered materials and the potential investigations in the future are stressed.
Severe plastic deformation techniques including high-pressure torsion and equal channel angular pressing have been widely used to refine coarse-grained materials to produce nanocrystalline and ultrafine-grained materials, or manipulate the microstructure of nanocystalline materials for superior mechanical properties. This paper overviews severe plastic deformation induced structural and mechanical property evolutions on bulk nanocrystalline metals, mainly in a nanocrystalline Ni-20%Fe (mass fraction) alloy with a face-centred cubic (fcc) structure processed by high-pressure torsion to different strain values. The structural evolution and mechanical property evolution at different strain values were studied. Comprehensive characterizations on structural evolution during deformation indicate that: (1) grain growth occurred via grain rotation, and is accompanied with changes in dislocation density and twin density; (2) there is a significant grain size effect on deformation induced twinning and de-twinning. There exists an optimum grain size range for the formation of deformation twins. Outside of this grain size range the de-twinning process will dominate to annihilate existing twins; (3) different types of dislocation-twin boundary (TB) interactions occurred during deformation. Dislocation density plays an important role in dislocation-TB interactions. In a twinned grain with a low dislocation density, a dislocation may react with a TB to fully or partially penetrate the TB or to be absorbed by the TB via different dislocation reactions. In a twinned grain with a high dislocation density, dislocations tangle with each other and are pinned at the TBs, leading to the accumulation of dislocations at the TBs and raising the local strain energy. In order to release the stress concentration, stacking faults and secondary twins formed by partial dislocation emissions from the other side of the TB; (4) atom probe tomography investigation reveals that C and S atoms, which are the major impurities in the Ni-Fe alloy and segregated at grain boundaries (GBs) of the as-deposited material, migrated from disappearing GBs to the remaining GBs during high-pressure torsion. Investigation on structure-hardness relationship of the Ni-Fe alloy reveals that: strain hardening and strain softening occurred at different deformation stages. Dislocation density evolution plays a major role in the hardness evolution, while other structural evolutions, including twin density and grain size evolutions, play minor roles in the hardness evolution.
How to defeat the conflict of strength vs toughness and achieve unprecedented levels of damage tolerance within structural materials is a great challenge for designing microstructure-sensitive materials. The nanostructured metallic multilayers (NMMs) are widely used as essential components of high performance microelectronics and interconnect structures owing to their smart, tunable internal features and their outstanding mechanical properties. The deformation and fracture of NMMs during their service processes has been identified as an important factor influencing their reliability. The present authors had systematically investigated the size and interface effects on the mechanical properties, such as hardness/strength, tensile ductility, fracture toughness, deformation and fracture mechanisms of Cu/X (X=Cr, Nb, Zr) nanolayered films/micropillars, in addition to their microstructure evolution. In this paper, based on these experimental results achieved by the present authors, as well as the progresses at home and abroad made in the deformation and fracture behavior of NMMs, the correlation of microstructure-size constraint-mechanical performance in NMMs (and nanolayered micropillars) is reviewed, and the universities in their deformation and fracture modes and the related mechanisms are revealed. Finally, a brief prospect on the studies of NMMs in future in the light of manipulation of the internal features, origin and dynamics of dislocations and the high performance of NMMs at extreme is discussed.
When grain sizes of crystals are down to nano-scale, the so-called nanocrystalline materials exhibit distinct physical properties in contrast to their conventional counterparts. The strength and plastic deformation mechanisms were among the most broadly investigated properties from mechanical society. Since deformation and pre-mature failure in interfaces (including grain boundaries, twin boundaries, and interfaces between different media) could be the origin of low ductility in nanocrystalline materials, the effort to evade the strength-ductility trade-off dilemma in nanocrystalline materials, by tuning their interfacial structures/properties, is usually called as interfacial engineering. Twin boundaries stand out among all possible boundary structures for their capability to enhance strength and retain ductility of crystalline metals. In this paper, current understanding about the mechanical behavior associated with interfaces in nanostructured metals is reviewed, with a focus on the strengthening mechanisms played by twin/grain boundaries and current physical models to shed light on the size-effect induced by grain sizes and twin thicknesses.
Influences of stacking fault energy (SFE) on the microstructures, tensile properties and fatigue behaviors of nanostructured (NS) Cu-Al alloys prepared by severe plastic deformation (SPD) were systematically summerized. With the reduction of SFE, it is found that the dominant formation mechanism of nanostructures gradually transformed from the dislocation subdivision to the twin fragmentation and the grain sizes also decrease; while microstructural homogeneity is achieved more readily in the materials with either high or low SFE than in the materials with medium SFE. The strength of NS Cu-Al significantly increases with decreasing the SFE, while there is an optimal SFE for the ductility of these materials. More significantly, the strength-ductility synergy of Cu-Al alloys is prominently enhanced with reducing the SFE. Finally, simultaneous improvements of low-cycle fatigue and high-cycle fatigue properties of NS Cu-Al alloys were achieved with decreasing the SFE. This can be attributed to the enhanced microstructure stability and the reduced strain localization in shear bands. With the reduction of SFE, the fatigue damage micro-mechanism was also transformed from grain boundary (GB) migration to other GB activities such as, atom shuffling, GB sliding and GB rotation.
Some structural metallic materials, with only necessary to?be?“nanocrystallized” rather than to be increased with the contents of Cr and Al, have the ability to thermally grow a protective scale of Cr2O3 and Al2O3 at high temperature environments. Nanocrystallizationof alloys containing Cr or/and Al is an alternative to conventionally coating them with a high-Cr or/and high-Al material. High temperature corrosion behaviors of nanocrystalline metallic materials have been extensively reported in the past 20 years. In this paper, characteristics of high temperature corrosion of nanocrystalline metals, together with the fundamental reasons and questions desired to be clarified for the improvement of the corrosion resistance of alloys by nanocrystallization, were briefly reviewed.
Compared with the traditional coarse-grained materials, the electrochemical corrosion behavior of nanocrystalline materials has changed obviously. Nanocrystallization influences the properties of passive film on passive materials. In this paper, the current understanding of the growth of passive film and local pitting corrosion on nanocrystalline materials by dynamic research techniques were reviewed. The results indicate that nanocrystallization changed the nucleation mechanism of the passive film from progressive to instantaneous, and which promotes the growth rate of the passive film, both of which promote the compact properties of the passive film. The effects of nanocrystallization on local pitting corrosion behavior are concluded: (1) more frequent occurrence of metastable pits, but with lower probability of transition to stable pits, which is attributable to differences in morphologies of sulfur and manganese as well as outstanding repassivation ability of nanocrystalline thin film; (2) nanocrystallization decreases stable pit generation rate and its propensity to form larger pit cavities, and modifies the morphology of stable pit cavity.
The early Bauschinger effect in nanocrystalline Al thin films with different thicknesses and microstructural orientations was investigated using large-scale atomistic simulations. The simulation results indicate that the microstructural orientation heterogeneity has a significant influence on the early Bauschinger effect and the associated plastic deformation mechanisms. The (110)-textured thin films show less Bauschinger effect compared to non-textured films despite having the same grain size, shape and thickness. The atomistic simulations reveal that the early Bauschinger effect originates from the reverse motion of dislocations and the reduction in dislocation density due to dislocation reactions during unloading, which are driven by the internal residual stress caused by the inhomogeneous deformation during loading.
Strengthening by twin boundaries at nanoscale and gradient surface nanocrystallization are two important strengthening approaches recently drawing considerable attention in the field of metallic material research. In the present work, a novel nanostructure, i.e., gradient nanoscale twin boundaries, is proposed. To reveal their unique deformation mechanism, uniaxial tension simulations of gradient nanotwinned copper are investigated by molecular dynamics simulations. The results show that partial dislocations govern the deformation of relatively thicker twins while full dislocations control the deformation of relatively thinner twin layers. Nanoindentation processes of gradient nanotwinned copper are also performed, providing insights on the strengthening and hardening effects of nanoscale twin boundaries.
Microstructure of as-deformed and annealed Cu-Nb composite wires were investigated by SEM and TEM. Hardness of the as-deformed and annealed samples were measured. The results showed that both the interface density and the rate of interface-increasing increased with increasing strain. When the microstructure reached nano-scale (strain=24.8), the interface density showed a sharp increase which induced a rapid increase in hardness, accompanied by formation of stacking faults and rotation grain boundaries in Cu. During the annealing, the size effect impacted the evolution of microstructure of the multi-scale Cu matrix. The evolution can be classified in to three stages with respect to annealing temperatures: recovery and recrystallization of large Cu, while that of the nano Cu were restrained; recovery and recrystallization of nano Cu; spheroidization and coarsening of Nb.
Grain refinement induced increase in hardness is of interest from a tribological point of view. Most of nanostructured metals show an enhanced wear resistance in comparison with their coarse-grained counterparts. To understand the related wear mechanism, the tribological properties and worn subsurface structure of nanostructured Cu were investigated under both dry and oil-lubricated sliding conditions, respectively. The wear resistance and worn subsurface structure of nanostructured Cu were compared under different conditions. The results indicate that nanostructured Cu exhibits a dynamic recrystallization (DRX) dominated wear mechanism under both conditions. A pronounced correlation is identified that wear rate increases significantly with an increasing grain size or decreasing hardness of DRX structure.
Ultrafine-grained (UFG) and nanostructured (NS) materials have attracted considerable interest due to their special microstructure and mechanical properties. Severe plastic deformation is one of the optimum approaches to fabricate bulk, dense and contamination-free UFG and NS metallic materials. However, high density of dislocations and unstable microstructure were usually induced in these UFG and NS metallic materials, resulting in poor tensile plasticity and fatigue properties. In this study, bulk UFG and NS Cu-Al alloys were successfully prepared via friction stir processing (FSP) with additional forced water cooling. FSP Cu-Al alloys exhibited uniform recrystallized microstructure with equiaxed ultrafine grains, and the grain sizes reduced gradually as the stacking fault energy (SFE) decreased. Abundant nano-twin layers formed in the ultrafine grains of FSP Cu-Al alloys with low SFEs, which further refined the ultrafine grains and NS microstructure was achieved. The strength of the FSP Cu-Al alloys increased clearly with decreasing the SFEs due to the gradually refined microstructure, but the uniform elongation increased initially and then decreased in the Cu-Al alloy with the lowest SFE.
The electrochemical actuation performance of nanoporous gold samples deformed by compression was investigated. Although the porosity and specific surface area decrease with increasing compression strain, the strain amplitude of actuation which were measured along the compression direction, increases and then decreases with increasing compression strain. The compression also greatly increases the strain energy density of nanoporous gold actuator. The improvement of actuation performance is attributed to the morphology change of nanoporous structure during compression. The understanding of the underlying mechanism requires quantitative characterization of morphology and morphological evolution of nanoporous structure during compression.
