Volume 100, Issue 12

The processing of bulk solids through the application of severe plastic deformation has now become a well-established procedure for achieving grain refinement to the submicrometer or even the nanometer level. Several basic techniques are currently in place but most attention has been devoted to the procedures of equal-channel angular pressing, high-pressure torsion and accumulative roll-bonding. This paper presents an introductory overview of processing by severe plastic deformation including historical developments and recent new advances in the processing technology. Special emphasis is placed on the move towards the commercialization of the severe plastic deformation approach.

Severe plastic deformation techniques have been widely used to produce bulk ultrafine-grained and nanocrystalline materials. Severe plastic deformation of materials is a complex process that includes both grain refinement via the activities of dislocations and twinning, and grain growth. In this paper, we present a short review on the mechanisms of grain refinement and grain growth induced by severe plastic deformation.

Equal-channel angular pressing is an effective tool for producing exceptional grain refinement in bulk polycrystalline metals. Typically, processing in this way refines the grains to the submicrometer level in a wide range of metals but recent experiments have established that the mechanism of grain refinement is different in fcc and hcp metals. Specifically, the refining of grains in aluminum involves the introduction of elongated bands of cells or subgrains and the subsequent evolution of this structure into an array of ultrafine grains whereas in magnesium the limited number of slip systems leads to the formation of new grains along the existing grain boundaries. Because of these limitations, magnesium alloys are especially susceptible to the production of materials having bimodal grain distributions.

In a previous study (R.Z. Valiev et al.: J. Mater. Res. 17 (2002) 5), unusual combinations of yield strength and ductility in ultrafine grained copper processed by equal-channel angular pressing of 360 MPa 23 % (by Bc route two passes) and 380 MPa 55 % (by Bc route 16 passes) were reported. However, results from recent work suggest that the tensile strain, when measured using a non-standard method and miniature dog-bone tensile specimens, is likely to contain significant errors. In this study, by implementing a standard strain measurement and tensile specimens with geometries that meet the ASTM requirements, we report yield strength and ductility combinations of 370 MPa 9 % (Bc 2 passes) and 370 MPa 16 % (Bc 16 passes) in the ultrafine grained copper. The higher ductility of the ultrafine grained copper processed by route Bc 16 passes was rationalized on the basis of several factors including the presence of equiaxed grains, lower dislocation density and higher fraction of high-angle grain boundaries (which result in larger strain rate sensitivity and strain hardening).

High pressure torsion is a well known and widespread processing technique for severe plastic deformation. The aim of high pressure torsion and other comparable techniques is to obtain ultrafine-grained or even nanocrystalline materials with enhanced mechanical and physical properties compared with their coarse-grained counterparts. Generally this refinement process is strongly influenced by processing parameters such as temperature or accumulated strain, but can also simply be affected by the entire experimental setup. Therefore, the benefits and limitations of the process with regard to grain refinement, homogeneity and specimen size, underlined with experimental results using different tools, will be discussed.

Processing by high-pressure torsion at room temperature was employed to achieve a uniform nanostructure in a metastable Ti 67.4 Nb 24.6 Zr 5 Sn 3 bcc β alloy with grains of ∼20 − 60 nm after five turns. By comparison, there was an ultrafine grain structure after equal-channel angular pressing at 903 K. The majority of refined grains were identified as bcc β-titanium with the presence of a small fraction of orthorhombic α″ martensite. It is proposed that the pronounced grain refinement is facilitated by a stress-/strain-induced β → α″ martensitic transformation. However, the presence of a large fraction of β phase in the high-pressure torsion processed samples is attributed to a reverse α″ → β martensitic transformation stimulated by the applied shear stress and the inhibition effect of the nanoscale grains on martensite formation.

High purity Al (99.99 wt.%) and high purity Cu (99.99 wt.%) were processed using high pressure torsion. The mechanism for unusual softening in Al after the resultant intense plastic straining was investigated. The pure Cu was used to simulate a thermal effect on the hardness behavior and microstructural evolution after processing. This simulation consists of processing and subsequently keeping the pure Cu at the temperature equivalent to room temperature for pure Al. It is shown that the softening of the processed Cu occurs by static recrystallization. It is then suggested that the softening in Al should be due to recovery and/or recrystallization. Whereas the softening in Cu occurs rather slowly, it is very quick in Al and it may even be dynamic during processing by high pressure torsion.

The segregation of solute elements at the grain boundaries of an Al–Zn–Mg–Cu alloy processed by equal-channel angular pressing was characterised using three-dimensional atom probe tomography. The results show that Mg and Cu segregate strongly to the grain boundaries but Zn shows no clear segregation and even becomes depleted near the boundaries. Trace elements such as Zr, Cr, Si and Mn show no clear segregation at the grain boundaries. An increase in the number of passes leads to a decrease in the grain size but there is no clear effect on the levels of solute segregation at the boundaries. The significant segregation of certain major alloying element at the boundaries of ultrafine-grained alloys implies that the less super-saturation solutes in the matrix will be available for precipitation with a decrease in the average grain size.

This work focuses on the deformation behavior of an ultra-fine grained Al-Mg-Si alloy processed by equal channel angular pressing over a wide range of temperatures and strain rates. The effect of temperature and strain rate on the homogeneity of plastic deformation, the evolution of microstructure, the strain rate sensitivity and the underlying deformation mechanisms are investigated. It is demonstrated that the localization of plastic deformation at the micro scale is triggered by grain boundary sliding due to grain boundary diffusion. The contributions of different deformation mechanisms during the plastic deformation of the material are discussed.

The strength and ductility in an Mg-3 wt.% Al-1 wt.% Zn alloy were both increased after a single pass of equal channel angular pressing with back pressure (BP-ECAP). The strong soft texture in the case of ECAP without backpressure is changed to a much weak texture with some transformed to hard orientation, and together with the stronger grain refinement strengthening effect, contributes to the higher strength and ductility in samples ECAP processed with backpressure. Dynamic recrystallization and the activation of new deformation modes other than slip on the base plane during ECAP with backpressure lead to more effective grain refining and less soft texture development.

This work is related to the enhancement of the fatigue properties in ultrafine-grained Ti alloys produced by severe plastic deformation techniques. To process commercially pure Ti Grade 4 and Ti-6Al-4V alloys, combined severe plastic deformation techniques that include equal channel angular pressing and additional thermal and deformation treatments were used. As a result we could produce ultrafine-grained Ti materials with a similar grain size of less than 300–400 nm but different in their shape and grain boundary structure (both low- and high-angle, equilibrium and non-equilibrium grain boundaries). It is shown that tailoring grain boundaries by severe plastic deformation techniques makes it possible to considerably enhance the strength of Ti materials while preserving high ductility. In turn, ultrafine-grained materials with enhanced strength and ductility demonstrate superior fatigue endurance and life.

The effect of equal channel angular pressing (ECAP) on the structure and mechanical properties of Al-4 % Mg-1.5 % Mn-0.4 % Zr and Al-4 % Mg-1.5 % Mn-0.4 % Zr-0.4 % Sc (wt.%) alloys in the initial as-cast and heat treated states was studied. The ECAP processing was shown to lead to the formation of a predominantly submicrocrystalline structure with an average grain size of 850 nm for initial as cast state and 860 nm for initial heat treated state in the Al–Mg–Mn–Zr–Sc alloy and 1060 nm for initial as cast state and 1240 nm for initial heat treated state in the Al–Mg–Mn–Zr alloy. It is remarkable that both strength and ductility of the two alloys in the initial conditions were enhanced by ECAP. The highest strength was observed in the Al–Mg–Mn–Zr–Sc alloy (UTS = 425 MPa), in combination with elongation to failure of EL = 17 %. The characteristics of strength and ductility after ECAP for the Al–Mg–Mn–Zr alloy as well as for the Al–Mg–Mn–Zr–Sc alloy were similar whether the initial state was as-cast or as-heat treated.

A commercial purity Al alloy and an Al-0.3Sc (wt.%) alloy, the latter in either the supersaturated or artificially aged condition, were accumulative roll bonded at either 200 or 350°C to high strain to generate sheet materials consisting of 32 or 64 alternating layers of Al and Al(Sc). The microstructure and texture of the processed materials were investigated mainly using electron backscattered diffraction scanning electron microscopy and transmission electron microscopy. The deformation microstructure and texture of these two alloy combinations were strongly influenced by both the initial heat treatment condition of the Al(Sc) alloy whereby large-scale shear bands were generated during rolling when a dispersion of fine Al 3 Sc particles is present in the Al(Sc) layers. The effect of initial microstructure and processing temperature affected the subsequent recrystallization microstructure and texture of the Al/Al(Sc) composite during annealing at 350°C. Here, the Al(Sc) layers remain unrecrystallized in all materials with the Al layers undergoing continuous and discontinuous recrystallization after low and high temperature ARB, respectively. The lack of recrystallization in the Al(Sc) layers generated an alternating recrystallized/recovered microstructure in all materials.

Finite element analysis for the recently developed plastic deformation process, accumulative back extrusion, is carried out to understand the plastic deformation behavior during the processing in terms of geometry, equivalent strain, principal stress, and load history. The simulated results show that the deformation is highly nonuniform, and that it is almost impossible to achieve strain homogeneity by repeating the accumulative back extrusion process. In addition, the folding defects generated in the workpiece with cycles of accumulative back extrusion can cause surface fracture.

This paper assesses the possibility of synthesizing bulk nanostructured or ultrafine structured metallic materials by consolidation of powders using high strain powder compact forging. In the work, inter-particle boundaries were categorized into three types: Type I-fresh surface/fresh surface boundaries, Type II-fresh surface/oxide covered surface boundaries and Type III-oxide covered surface/oxide covered surface boundaries. The possibility of turning each of these types of inter-particle boundaries into grain boundaries or interphase boundaries without causing significant grain/particle growth through plastic deformation, and the amount of plastic deformation needed were discussed. The related experimental findings were also reviewed. It was concluded that by using high strain powder compact forging the possibility of producing bulk nanostructured or ultrafine structured metallic materials is very high.

The present work investigates the optimal level of residual hydrogen in partially de-hydrogenated powder to produce CP-Ti plate compacts using ECAP with back pressure which are subsequently rolled at low temperature. A comparative study of the compaction of two TiH 2 powders and a CP-Ti powder, with particle sizes 150 μm, 50 μm and 45 μm respectively, has been carried out. The hydride powders have also been compacted in a partially de-hydrogenated state. The optimal level of residual hydrogen with respect to the density of the resulting compact and the associated mechanical properties has been defined. ECAP at 300°C produced compacts from these partially de-hydrogenated powders of 99.5 % theoretical density, while CP-Ti was compacted to almost full theoretical density under the same ECAP conditions. Therefore, the compaction of powder by ECAP does not benefit from temporary hydrogen alloying. These compacts then were rolled at temperatures ranging from room temperature to 500°C with an 80 % reduction in a single pass. Heat treatment after the rolling can modify the microstructure to improve the resulting mechanical properties and in this regard the temporary alloying with hydrogen has been observed to offer some significant benefits. It is shown that ECAP followed by low temperature rolling is a promising route to the batch production of fully dense CP-Ti wrought product from powder feedstock that avoids the need to subject the material to temperatures greater than 500°C. This low temperature route is expected to be efficient from an energy point of view and it also avoids the danger of interstitial contamination that accompanies most high temperature powder processing.

Mg-based nanocomposites are promising candidates for hydrogen storage applications exhibiting fast H-sorption kinetics at reasonably low temperatures when processed by high-energy ball milling techniques. However, since compaction of the highly reactive nanometric powder is desirable before application, the search for other effective processing routes for the preparation of Mg-based nanocomposites is relevant. In this work, we have used a combination of equal channel angular pressing, cold rolling and high-energy ball milling in the processing of the commercial AZ31 extruded alloy to evaluate its use as a hydrogen storage material. Severe plastic deformation carried out at different temperatures, combined with further mechanical processing resulted in a controlled texture and signifiant grain refinement, which are desirable microstructural characteristics for hydrogen storage applications.