The new trends in fabrication of bulk nanostructured materials by SPD processing

During the past decade, fabrication of bulk nanostructured metals and alloys using severe plastic deformation (SPD) has been evolving as a rapidly advancing direction of nanomaterials science and technology aimed at developing materials with new mechanical and functional properties for advanced applications. The principle of these developments is based on grain refinement down to the nanoscale level via various SPD techniques. This paper is focused on investigation and development of new SPD processing routes enabling fabrication of fully dense bulk nanostructured metals and alloys with a grain size of 40–50 nm and smaller, namely, SPD-consolidation of powders, including nanostructured ones, as well as SPD-induced nanocrystallization of amorphous alloys. We also consider microstructural features of SPD-processed materials that are responsible for enhancement of their properties.     

制备块体纳米/超细晶材料的大塑性变形技术

综述了采用SPD技术制备块体超细晶(UFG)和纳米晶(NC)材料的几种新方法，如等通道角挤压、高压扭转、多向锻造、多向压缩、板条马氏体冷轧法、累积轧焊法、冷拔、反复弯曲平直法等，分析了采用这些工艺制备的块体纳米材料所共有的微观组织特点。着重阐述了SPD技术的研究进展。     

Unique mechanical properties of nanostructured metals

Recently, it becomes possible to fabricate bulk metals having ultrafine grained or nanocrystalline structures of which grain size is in nano-meter dimensions. One of the promising ways to realize bulk nanostructured metals is severe plastic deformation (SPD) above logarithmic equivalent strain of 4. We have developed an original SPD process, named Accumulative Roll Bonding (ARB) using rolling deformation in principle, and have succeeded in fabricating bulk nanostructured sheets of various kinds of metals and alloys. The ARB process and the nanostructured metals fabricated by the ARB are introduced in this paper. The nanostructured metals sometimes perform quite unique mechanical properties, that is rather surprising compared with conventionally coarse grained materials. The unique properties seem to be attributed to the characteristic structures of the nano-metals full of grain boundaries.     

Dilatometry: a powerful tool for the study of defects in ultrafine-grained metals

Vacancies, dislocations, and interfaces are structural defects that are deliberately introduced into solids during grain refinement processes based on severe plastic deformation (SPD). Specific combinations of these defects determine the improved mechanical properties of the obtained ultrafine-grained materials. High-precision, non-equilibrium dilatometry, i.e., measurement of the irreversible macroscopic length change upon defect annealing, provides a powerful technique for the characterization and the study of the kinetics of these defects. It is applied to determine absolute concentrations of vacancies, to characterize dislocation processes, and to assess grain boundary excess volume in pure, FCC and BCC ultrafine-grained metals processed by SPD.     

Strength and fatigue properties enhancement in ultrafine-grained Ti produced by severe plastic deformation

Severe plastic deformation (SPD) of titanium creates an ultrafine-grained (UFG) microstructure which results in significantly enhanced mechanical properties, including increasing the high cycle fatigue strength. This work addresses the challenge of maintaining the high level of properties as SPD processing techniques are evolved from methods suitable for producing laboratory scale samples to methods suitable for commercial scale production of titanium semi-products. Various ways to optimize the strength and fatigue endurance limit in long-length Grade 4 titanium rod processed by equal channel angular pressing (ECAP) with subsequent thermal mechanical treatments are considered in this paper. Low-temperature annealing of rods is found to increase the fatigue limit, simultaneously enhancing UFG titanium strength and ductility. The UFG structure in titanium provides an optimum combination of properties when its microstructure includes mostly equiaxed grains with high-angle boundaries, the volume fraction of which is no less than 50%.     

Nanostructuring of metals by severe plastic deformation for advanced properties

Despite rosy prospects, the use of nanostructured metals and alloys as advanced structural and functional materials has remained controversial until recently. Only in recent years has a breakthrough been outlined in this area, associated both with development of new routes for the fabrication of bulk nanostructured materials and with investigation of the fundamental mechanisms that lead to the new properties of these materials. Although a deep understanding of these mechanisms is still a topic of basic research, pilot commercial products for medicine and microdevices are coming within reach of the market. This progress article discusses new concepts and principles of using severe plastic deformation (SPD) to fabricate bulk nanostructured metals with advanced properties. Special emphasis is laid on the relationship between microstructural features and properties, as well as the first applications of SPD-produced nanomaterials.     

Interface sheet-constrained groove pressing as a modified severe plastic deformation process

In this paper, a new severe plastic deformation (SPD) process entitled interface sheet-constrained groove pressing (ISCGP) as a new variant of conventional CGP has been developed for producing ultrafine-grained metallic materials. In this process, repetitive shear deformation is imposed into the sheet material by utilising symmetrically grooved die along with two interface sheet on both sides. To study the applicability, mild steel sheets were processed by both ISCGP and CGP processes, and mechanical and microstructural properties of the processed samples were investigated. The results show a considerable improvement in mechanical properties including hardness, yield strength, and ultimate tensile strength, though the ductility sacrifice was reduced. Comparing ISCGP and conventional CGP revealed interesting results, which are shown that ISCGP can result in better surface quality and ductility.     

Modelling of equal channel angular pressing using a mesh-free method

Severe plastic deformation (SPD) processes are widely recognised as efficient techniques to produce bulk ultrafine-grained materials. As a complement to experiments, computational modelling is extensively used to understand the deformation mechanisms of grain refinement induced by large strain loading conditions. Although considerable research has been undertaken in the modelling of SPD processes, most of the studies have been accomplished using mesh-based methods, such as the finite element method (FEM). Mesh-based methods have inherent difficulties in modelling high-deformation processes because of the distortions in the mesh and the resultant inaccuracies and instabilities. As an alternative, a mesh-free method called smoothed particle hydrodynamics (SPH) is used. The effectiveness of this technique is highlighted for modelling of one of the most popular SPD techniques, equal channel angular pressing. A benchmark between SPH and FE calculation is performed. Furthermore, a number of simulations under different processing conditions are compared to existing literature data. A satisfactory agreement is found, which indicates that SPD processes can be approached by mesh-free methods, such as SPH.     

Tensile behavior of bulk nanostructured and ultrafine grained aluminum alloys

In the present study, data on tensile behavior of bulk nanostructured aluminum alloys processed via consolidation of mechanically milled powders and severe plastic deformation are analyzed. High strength and low strain hardening were observed in bulk nanostructured and ultrafine-grained Al alloys. The ductility of aluminum alloys decreases with decreasing grain size. The high amount of intercrystalline components may have an influence on tensile properties of bulk nanostructured materials when grain sizes are less than 100 nm. The high strength in bulk nanostructured Al-Mg alloy may be attributed to contributions arising from grain size strengthening, the presence of high dislocation densities, Orowan strengthening, precipitation hardening and solid-solution hardening. The large and sudden stress drops in the stress-strain curves of cryomilled Al alloys are most probably indicative of the dislocation annihilation in the vicinity of or breakaway from the strong pinning role of dispersoids.     

Architecturing of Metal‐Based Composites with Concurrent Nanostructuring: A New Paradigm of Materials Design

Over the past decades severe plastic deformation (SPD) techniques advanced to one of the most potent ways of producing bulk ultrafine grained materials. The main advantage of SPD is the ultra‐high strength they can deliver. While the improvement of the strength there is a limit to what is achievable SPD when they are applied to individual materials especially for strain‐hardening and ductility. Therefore a new application of SPD processes in manufacturing of architecture hybrid materials with simultaneous nanostructuring is investigated experimentally and theoretically.     

大塑性变形制备细晶材料的研究、开发与展望

系统介绍了大塑性变形(SPD)细化晶粒的条件和目的,综述了4种主要的大塑性变形工艺的基本原理、特点和应用,剖析了细晶材料的强度和超塑性特征,展示了大塑性变形制备细晶材料的诱人前景和发展方向.     

纳米和超细晶金属材料的退火强化

本文综述了纳米和超细晶金属材料的退火强化研究现状和发展趋势。本文关注致密纳米和超细晶材料的研究,首先介绍了电沉积纳米Ni、强塑性变形制得的超细晶金属钛和纯铝的退火强化的实验现象,随后综述了这一强化现象的微观机理,最后探讨了进一步的实验及理论分析的途径。     

Efficient fabrication of ultrafine-grained 316L stainless steel surfaces for orthopaedic applications

Commonly used severe plastic deformation (SPD) methods are suitable for fabrication of bulk nano and ultrafine-grained metals. Drawbacks of these methods include durability of dies, geometrical restrictions and reduced ductility of the products. In this study, two common machining techniques used in manufacturing of orthopaedic components, turning and milling, were applied on 316L stainless steel as surface SPD to refine the surface microstructures of the workpiece. Machining with optimised parameters resulted in substantial grain refinement down to 98?nm on the surfaces. Biological experiments showed up to ～70% and ～280% increased bone cell density on milled and turned samples compared to conventionally machined 316L stainless steel at 5 days, which was correlated with nanocrystallisation and nanoroughness of the samples.     

Nanoindentation analysis for local properties of ultrafine grained copper processed by high pressure torsion

Severe plastic deformation (SPD) techniques have recently been developed for producing bulk ultrafine grained metallic materials. High pressure torsion (HPT) produces finer microstructures than those achieved by other SPD processes because of the higher imposed strain and hydrostatic pressure. It is known that HPT-processed metals show a highly heterogeneous microstructure not only along the radius due to the nature of torsional deformation but also through the thickness. Since the sample size for HPT is small, the local properties of HPT-processed specimens have not been investigated yet. In this paper, we propose a method to obtain stress–strain curves from nanoindenting curves by combining the finite element method and the recursion method. The nanoindentation technique was employed to elucidate the local mechanical properties, especially the stress–strain behavior. The method to extract the stress–strain curves from the load–displacement curves obtained by nanoindentation tests was applied to the edge region of the HPT-processed sample. The extracted properties correlated well with experimental results qualitatively.     

Deformation Behavior Study of Multi-Pass ECAE Process for Fabrication of Ultrafine or Nanostructured Bulk Materials

Severe plastic deformation (SPD) is an efficient approach for producing ultrafine or nanostructured bulk materials. Equal channel angular extrusion (ECAE) is the most effective SPD solution for material nanostructuring, as material billet undergoes severe and large deformation and the grains are efficiently broken up in the process. To improve material nanostructuring, the ECAE die design and process configuration are critical. The deformation behavior study through FE simulation in ECAE process provides basic and useful information for optimizing die design and process determination. In this research, the deformation behavior for three different die design scenarios is studied and the related deformation mechanisms and nanostructuring performance are investigated via FE simulation. Through multi-pass simulation, the optimal design scenario is then identified. The simulation results reveal deformation phenomena, and nanostructuring performance of the designs and the corresponding process can be recommended accordingly for improving die and process performance.     

Synthesis of nanostructured materials using supercritical CO<Subscript>2</Subscript>: Part I. Physical transformations

Nanostructured materials have been attracting increased attention for a wide variety of applications due to their superior properties compared to their bulk counterparts. Current methods to synthesize nanostructured materials have various drawbacks such as difficulties in control of the nanostructure and morphology, excessive use of solvents, abundant energy consumption, and costly purification steps. Supercritical fluids especially supercritical carbon dioxide (scCO₂) is an attractive medium for the synthesis of nanostructured materials due to its favorable properties such as being abundant, inexpensive, non-flammable, non-toxic, and environmentally benign. Furthermore, the thermophysical properties of scCO₂ can be adjusted by changing the processing temperature and pressure. The synthesis of nanostructured materials in scCO₂ can be classified as physical and chemical transformations. In this article, Part I of our review series, synthesis of nanostructured materials using physical transformations is described where scCO₂ functions as a solvent, an anti-solvent or as a solute. The nanostructured materials, which can be synthesized by these techniques include nanoparticles, nanowires, nanofibers, foams, aerogels, and polymer nanocomposites. scCO₂ based processes can also be utilized in the intensification of the conventional processes by elimination of some of the costly purification or separation steps. The fundamental aspects of the processes, which would be beneficial for further development of the technologies, are also reviewed.     

Some aspects of synthesis of nanocrystalline materials

Abstract

This paper reviews research work at the Institute for Materials and Advanced Processes, University of Idaho, on the synthesis of nanocrystalline materials and their consolidation. Nanocrystalline materials have been synthesised by a number of ‘far from equilibrium’ processes including mechanical alloying (MA), mechanochemical processing (MCP), supercritical fluid processing (SCFP), and severe plastic deformation (SPD). Examples of the materials include the TiAl based intermetallic compounds and composites produced by MA and SPD, Ti base alloys and metal carbides synthesised by MCP, thin film Cu produced by SCFP, and Al–Fe alloys produced by SPD. Details of the processes used and the enhancement of properties owing to the nanoscale structures in consolidated material will be presented. The potential of these processes to substitute for conventional methods of production will also be discussed.     

Recycling of titanium machining chips by severe plastic deformation consolidation

It has been demonstrated that severe plastic deformation (SPD) can be used to consolidate particles of a wide range of sizes from nano to micro into fully dense bulk material with good mechanical properties. SPD consolidation allows processing to be conducted at much lower temperatures and is therefore suitable for particles with highly metastable structures such as nanocrystalline. It is especially useful in the fabrication of multiphase materials including metal matrix nanocomposites. In this investigation, SPD consolidation was applied to recycle Ti machining chips. In particular, the as-received chips were consolidated by equal channel angular pressing at temperatures between 400 and 600 °C with the application of a back pressure from 50 to 200 MPa. Fully dense bulk Ti with fine grain sizes was produced, possessing strength comparable or higher than that of commercially pure wrought Ti. It is concluded that SPD consolidation is a promising method for recycling and value-adding of Ti chips.     

Nanostructured materials -mind over matter-

Considerable interest is being exhibited in the novel and enhanced properties of nanostructured materials. These materials, with their constituent phase or grain structures modulated on a length scale less than 100 nm, are now artificially synthesized by a wide variety of physical, chemical, and mechanical methods. Nanostructured materials with modulation dimensionalities of zero (clusters), one (multilayers), two (ultrafine-grained overlayers), and three (nanophase materials) are considered. The basic principles involved in the synthesis of these new materials are discussed in terms of the special properties sought using selected examples from particular synthesis and processing methodologies. Some examples of the property changes that can result from one of these methods, cluster assembly of nanophase materials, are presented.     

Electrodeposition of nanostructured coatings and their characterization—A review

Nanostructured materials have gained importance in recent years due to their significantly enhanced properties. In particular, electrochemistry has a special role in producing a variety of nanostructured materials. In the current review, we discuss the superiority of electrochemical deposition techniques in synthesizing various nanomaterials that exhibit improved characteristics compared with materials produced by conventional techniques, as well as their classification, synthesis routes, properties and applications. The superior properties of a nanostructured nickel coating produced by electrochemical deposition are outlined. The properties of various nanostructured coating materials produced by electrochemical techniques are also described. Finally, the importance of nanostructured coatings in industrial applications as well as their potential in future technologies is emphasized.