Materials Science Forum Vol. 579

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: This paper reviews a method, “in situ consolidation ball milling” that provides artifactfree bulk nanocrystalline samples for several ductile metals such as Zn, Al and Al alloys, and Cu and Cu alloys. The preparation method is described in this paper and examples of the mechanical behavior of nanocrystalline materials made by this technique are given. It is found that in such artifact-free metals, combinations of both high strength and good ductility are possible.

Abstract: Equal-channel angular pressing (ECAP) is a convenient processing method for refining the grain size of bulk materials to the submicrometer level. Metallic alloys processed by ECAP often exhibit excellent superplastic characteristics including superplasticity at high strain rates. This paper summarizes recent experiments designed to evaluate the occurrence of superplasticity in representative aluminum and magnesium alloys and in the Zn-22% Al eutectoid alloy.

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: 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: 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: Based on strain-induced grain refinement, a novel surface mechanical attrition treatment (SMAT) technique has been developed to synthesize a nanostructured surface layer on metallic materials in order to upgrade their overall properties and performance without changing their chemical compositions. In recent several years, the microstructures and properties of surface layer were systematically investigated in various SMAT metals and alloys, including b.c.c., f.c.c. and h.c.p. crystal structures. Different grain refinement approaches and nanocrystalline formation mechanisms were identified in these deformed materials, involving dislocation activities, mechanical twinning and interaction of dislocations with mechanical twins. The properties of the surface layer were measured by means of hardness, tensile, fatigue and wear tests. The enhanced properties of the surface layer are mainly attributed to the strain-induced grain refinement. In this work, we reviewed the microstructures and properties of surface layer in the SMAT materials.

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: Deformation structures produced by high pressure torsion (HPT) and accumulative roll-bonding (ARB) were characterized by transmission electron microscopy and electron backscatter diffraction, and the mechanical properties of the ARB samples were determined by uniaxial tensile testing. The structural evolution during HPT in high purity nickel has been examined and an extended lamellar boundary structure was observed at high strains. For ARB samples deformed to high strains, an almost similar structural morphology has been observed in both interstitial free steel and in commercial purity aluminum, whereas a relatively equiaxed structural morphology was observed in high purity aluminum samples. In all samples, both deformed by HPT and ARB, the deformation structures were composed of a large fraction of high-angle boundaries, together with low-angle boundaries and isolated dislocations between the boundaries. Common characteristics have been identified in the mechanical behavior of the ARB samples, namely a very high strength, a small uniform elongation and a relatively large post-uniform elongation after necking. For HPT and ARB the structural morphology and structural parameters are compared, and for the ARB samples structure-property relationships are also discussed.