Papers by Keyword: Severe Plastic Deformation (SPD)

Abstract: The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed.

Abstract: This report presents main achievements of international R&D activities of the Institute of Physics of Advanced Materials of Ufa State Aviation Technical University (Ufa, Russia) with a special attention to the innovative potential of nanostructured metals and alloys produced by severe plastic deformation techniques. Several examples of the first promising applications of bulk nanostructured materials as well as potential competing technologies are considered and discussed.

Abstract: High-resolution transmission electron microscopy investigations revealed different types of deformation structures in a nanostructured commercial Al–Mg alloy processed by high pressure torsion at room temperature. Microtwins and stacking faults were detected within both nanocrystalline grains and ultrafine grains. Full dislocations in the form of dipoles were observed within grains and near the grain boundaries. Two twinning mechanisms previously predicted by molecular-dynamics simulations were directly verified including the heterogeneous twins nucleated by the successive emission of Shockley partials from grain boundaries and homogeneous twins formed in the grain interiors by the dynamic overlapping of stacking faults. Hence, the formation of full dislocations, stacking faults and twins in the present aluminum alloy subjected to severe plastic deformation may be interpreted in terms of molecular-dynamics simulations based on generalized planar fault energy curves for pure metal systems.

Abstract: Repeated cold rolling with intermediate folding (RCR) represents a technique to obtain severe plastic deformation that avoids excessive heating at the internal interfaces and that proceeds without the simultaneous action of a high pressure in the range of several GPa. Aside from the opportunity to obtain amorphous bulk samples, the processing pathway also allows for synthesizing dense, bulk nanocrystalline materials. The sequential combination of different processing routes that drive a material to a different extent -, with different rates - and by different means from thermodynamic equilibrium present new and attractive processing opportunities to obtain bulk nanocrystalline or massive ultrafine grained materials that are widely unexplored. Here, an overview is presented concerning the sequential application of different deformation methods with largely different strain and pressure levels. The basic underlying mechanisms that can lead to ultrafine grained or nanocrystalline microstructures for pure metals or to two-phase nanocomposites or bulk metallic glasses for alloys are discussed and the current state of nanostructure control is highlighted by selected examples.

Abstract: Severe plastic deformation (SPD) has been demonstrated to be the most efficient method to produce bulk metals with ultrafine grained (UFG, 100 nm < grain size d < 500 nm) and nanocrystalline (NC, d<100 nm) microstructures. Such metals exhibit some unique properties owing to their unusual microstructures such as high-energy, non-equilibrium grain boundaries. Efforts in the past two decades have focused on metals with face-centered cubic (fcc) structures. Recent experimental results have shown that UFG/NC metals with body-centered cubic (bcc) structures have some properties that are distinct from their fcc counterparts. Further, the majority of the fcc metals are very ductile and have relatively low melting points, making them easier to process using SPD. On the contrary, many bcc metals are refractory, and are very sensitive to interstitial impurities, rendering them difficult to work via SPD. In this article, we attempt to summarize the state-of-the-art of UFG/NC refractory metals processed by SPD, with focus on the microstructure and mechanical properties. Comparisons with UFG/NC fcc metals are made where appropriate. Outstanding issues and future directions are also addressed.

Abstract: The progress in bulk ultrafine and nanostructured materials through consolidation of particles by severe plastic deformation (SPD) is reviewed. The focus is on the processes of high pressure torsion (HPT) and equal channel angular pressing (ECAP) with or without the application of a back pressure. Various materials consolidated are described in terms of their densities, microstructures and mechanical properties. The important processing parameters and their effects on the resulting materials are discussed. It is shown that SPD consolidation of particles is an effective way of producing bulk nanostructured materials although much work is needed to understand the consolidation behaviour and to design the optimum compositions and microstructures.

Abstract: The present work examines the reversal response of a face-centered cubic (fcc) polycrystalline metal after large pre-strains. While reversal responses among different fcc metals are similar after small pre-strains, they can vary widely after large pre-strains depending on material and microstructure. In this article, these characteristics are considered to be governed by three distinct mechanisms: (1) reverse glide of dislocations previously held by backstresses, (2) reverse glide of dislocations previously held by barriers, and (3) ‘reverse hardening’ by reverse glide over stable dislocation barriers formed in pre-straining. These small-scale mechanisms are incorporated into a polycrystal code to investigate their influence on the macroscopic reversal response and to interpret large strain reversal tests in the literature. It is shown that mechanism (2) is responsible for worksoftening and reductions in hardening rate and mechanism (3) for the overshoot seen in α- brass and other low stacking fault energy alloys. Mechanism (1) is responsible for the Bauschinger effect and occurs in all metals. A large fraction of second phases leads to a strong Bauschinger effect that can either reduce or postpone the effects of mechanisms (2) and (3).

Abstract: Since the mid-1990’s the fabrication of bulk nanostructured metals and alloys using severe plastic deformation (SPD) has been evolving as a rapidly advancing direction of modern nano-materials science that is aimed at developing materials with new mechanical and functional properties for advanced applications. This paper highlights and considers two new trends in SPD processing, which are recently being developed for fabrication of bulk nanostructured materials (BNM). One of these recent developments is associated with nanostructuring of metals and alloys by SPD processing for advanced properties. The new strategies and approaches to produce nanometals with enhanced and often unique properties are discussed. Another new direction is the progress in the processing of BNM not only at laboratory scale but also at the level semi-products (sheets, wires, rods, etc.) suitable for production implementation. The paper considers these developments together with the examples performed at our laboratory in Ufa (Russia), which lay a firm foundation for the BNM use in advanced structural and functional applications.

Abstract: Junction disclinations are important elements of the structure of nanostructured metals produced by severe plastic deformation (SPD). Effect of these defects on the formation energy of vacancies in grain boundaries (GBs) is studied by means of atomistic computer simulations. Estimates based on the calculations of vacancy formation energies suggest that at least two orders of magnitude increase of the GB diffusion coefficient can be expected due to junction disclinations in nanostructured metals.

Abstract: Microstructure of machined copper chips at very low velocity was characterized by transmission electron microscopy. The structure of the machined chip produced by reasonable combinations of machining parameters is virtually entirely occupied by isolated equiaxed submicron grains of 100~300nm in size with high-angle boundaries. A finite element model was developed to study large plastic deformation in plain orthogonal machining copper. The numerical results show most of the grain refinement associated with the formation of ultra-fine grained chip may be attributed to the large shear strain imposed in the deformation zone. It is feasible to take machining process as a method of preparing ultra-fine grained materials. But the optimal design of the machining process requires a precise and quantitative understanding of the mechanics of deformation-induced subgrain microstructure.