Investigations and Applications of Severe Plastic Deformation

Equal Channel Angular Extrusion (ECAE) was invented in the former Soviet Union by Vladimir Segal [1]. It is an innovative process capable of producing relatively uniform intensive plastic deformation in a variety of material systems, without causing substantial change in geometric shape or cross section. Multiple extrusions of billets by ECAE permits severe plastic deformation in bulk materials. More importantly, by changing the orientation of the billet between successive extrusions, sophisticated microstructures and textures can be developed. According to the orientation chosen after each pass, four fundamental equal-channel angular extrusion routes are defined and used for obtaining different textures and microstructures [2], Researchers in the Texas A&M University’s (TAMU) Deformation Processing Laboratory in the Department of Mechanical Engineering have been conducting research on the ECAE process since 1992. The inventor of the process, Dr. V. Segal, was a research associate in the Lab from 1992 to 1995. Research has concentrated in two areas: (1) development of theoretical and practical knowledge of the mechanics of ECAE and the further expansion of that knowledge leading to the capability of effectively mathematically modeling the process, and (2) investigation of the various technological advantages of ECAE and possible industrial applications for a variety of materials. Process modeling has involved both geometric, analytical (upper-bound analysis, slip-line theory) methods and non-linear finite element analysis [2].

This article focuses on the results from recent studies of severe plastic deformation of various types of steels, mainly, austenitic, ferritic and ferritic-pearlitic, in temperature ranges of hot, warm and cold deformation. The principles of formation of various types of structures are considered: 1) dynamically recrystallized and dynamically recovered structures during severe hot deformation. 2) dynamically recovered and submicrostructures during severe warm deformation. 3) submicro- and nanostructures during severe cold deformation. The favorable effect of phase transformations and microalloying in steels on the formation of nanostructures is discussed. The methods and techniques of severe plastic deformation for processing of steels and favorable factors for attaining large plastic strains are considered.

Conditions for producing nano- and microcrystalline structures (NMC) by ECAP technology are described. The concept of “clear ECAP” for describing uniform ECAP deformation is introduced. A model of grain refinement and temperature-strain rate conditions of effective refinement is described.

Significant interest exists within the materials science community to create nanocrystalline microstructures in metallic alloys in an effort to obtain improved strength and processability of these alloys. Repeated deformation methods, such as 3-axis forging, ECAE (Equal Channel Angular Extrusion), pressurized torsion etc. have been used to create bulk materials with 50–500 nm size microstructures. Stability of these microstructures to further deformation and to thermal exposure is however a major problem which needs to be addressed in the future years. Another issue for process scale-up is die-wall sticking with the moving workpiece, which make scale-up of billet size a nontrivial problem. To minimize these structure-altering aspects of rolling deformation, a surface-shear based deformation technique has been developed and applied to plates and sheets of aluminum alloys to create desired fine-scale microstructures. Initial studies using this and other processes on several Al alloys with and without dispersoid particles have been analyzed by optical, and electron microscopy. Preliminary results on mechanical behavior and texture are reported.

The continuous kinematically admissible velocity fields for domains with arbitrary angles between two channels are constructed by the K. Shvarts-A. Christoffel’s (S—C) integral method. In complex form such important kinematical parameters as vector velocity, strain rate and shear strain degree are obtained. These parameters are used as a basis for determining corrected parameters by the standard procedure of simulating the actual metal motion in the domains.

Gamma TiAl based alloys and a TiAl/Ti₅Si₃ composite with nanocrystalline and submicrocrystalline structures were produced by mechanical alloying (MA’ing) and hot isostatic pressing (HIP’ing). MA’ing in a high-energy mixer mill led to severe plastic deformation and amorphization of the powder particles. The γ-TiAl alloys produced from the amorphous powders had a nanocrystalline structure. The grain size increased when the HIP temperature and time increased. Grain growth in the TiAl alloys during annealing for up to 800 hours in the temperature range of 725°C to 1200°C was studied. No effect of the Ti₅Si₃ phase particles on grain growth kinetics was found.

A method based on initiation of dynamic recrystallization (DRX) during hot working has been developed to produce a submicrocrystalline (SMC) structure (d < 1 µm) in massive work-pieces of hard-to-deform materials, like titanium aluminides, The method involves continuous grain refinement due to dynamic recrystallization at a decreasing temperature. A microstructure with a grain size of 0.1 to 0.4 µm and no porosity was produced in different TiAl and Ti₃Al based alloys. Partial disordering was detected in a Ti₃Al alloy with the SMC structure. The grain refinement hardened the intermetallic alloys at room temperature (RT). In a fully ordered Ti₃Al alloy RT ductility increased when the grain size decreased, while the ductility of a partially disordered SMC Ti₃Al and TiAl alloys was close to zero.

Research efforts to model severe plastic deformation (SPD) processes and to quantify the workability of materials during such operations are summarized. The majority of research on the modeling of the equal channel angular extrusion (ECAE) process invented by Segal has focused on the kinematics of metal flow under nominally frictionless conditions. Physical (visioplasticity) models suggest that such assumptions yield reasonable estimates of imposed strain over the bulk of the sample. Recent work on the stress state developed during ECAE has shown that a high degree of compression exists in the deformation zone except near the inner corner of the tooling. At this location, tensile stresses which may lead to fracture are generated. Thus, ductile metals can often be deformed through multiple passes in ECAE unless they exhibit flow softening which can lead to flow localization in the form of shear bands. Research on the modeling of the second major SPD process, that of torsion under superimposed compression, a method developed by Bridgman, has been investigated to a less extent primarily because of the mixed displacement-load boundary conditions and the statically indeterminate nature of the operation.

There is currently a great deal of interest in the use of severe deformation for producing alloys with submicron grain structures. High-resolution electron back scattered diffraction (EBSD) analysis has been employed to measure the boundary misorientations within the deformation structures of Al-alloys processed by equal channel angular extrusion (ECAE), using different strain paths. The strain path was varied by rotating the billet through 0, 90 and 180 degrees, between each extrusion cycle. This has highlighted great differences in the deformed state, as a function of the processing route. It has been found that the most effective method of forming a submicron grain structure is to maintain a constant strain path. Cyclic redundant strains lead to a far lower density of new high angle grain boundaries being formed.

This paper deals with thermomechanical conditions used for preparation of microcrystalline structures in metallic materials on the basis of the analysis of well known deformation methods. A new method for formation of microcrystalline structures and a device for realization of this method are proposed.

Since the early eigthies, large strain deformation has developed into a main field of research within the plasticity community. In the meantime the occurrence of stage IV and stage V has been revealed in all usual metals [1–5] (Fig. 1) and many alloys [6], for all deformation temperatures ([7, 8]), and for many different deformation modes including iterative ones [1]. Both stages are defined by a significant change in work hardening coefficient Θ = dτ/dγ (τ, γ being the resolved shear stress/resolved shear strain) as compared with previous stage III where Θ strongly decreases: In stage IV a constant or even increasing hardening occurs whereas in stage V the work hardening coefficient Θ re-decreases as long as the macroscopic strength finally gets constant in a steady state deformation. Detailed thermal activation analyses (TAA) yielding the strain rate sensitivity and the density of thermally activatable obstacles as a function of deformation suggested that hardening in stage IV is governed by athermal storage of dislocations while hardening in stage V is characterized by thermally driven annihilation of dislocations [1]. TAA data in connection with measurements of total dislocation density also suggest to assume the storage & annihilation of edge dislocations in low temperature stage IV & V, and of screw dislocations in high temperature stage IV & V [3].

Anisotropic strain broadening in X-ray line profile analysis means that the breadth or the Fourier coefficients of diffraction profiles are not a monotonous function of the diffraction angle. The lack of a physically sound model makes the interpretation of line broadening difficult or even impossible. Dislocations are anisotropic lattice imperfections with anisotropic contrast effects in diffraction. It has been suggested recently that anisotropic X-ray line broadening is caused by dislocations. The classical procedures of Williamson and Hall, and Warren and Averbach are suggested to be replaced by the modified Williamson-Hall plot and the modified Warren-Averbach method in which the modulus of the diffraction vector or its square, g or g2, are replaced by g$$ {\bar{C}^{{1/2}}} $$ or g2$$ \bar{C} $$, respectively, where $$ \bar{C} $$ are the average dislocation contrast factors. A straightforward procedure has been elaborated to separate size and strain broadening which enables to determine particle size and size distribution and the structure of dislocations in terms of dislocation densities and the character of dislocations in polycrystalline or submicron grain size materials.

The results of investigations of structure peculiarities in metals subjected to severe plastic deformation (SPD) are presented. Special attention is paid to X-ray investigations of microstructure evolution in bulk nanostructured samples of pure Cu obtained by SPD processing by torsion under high applied pressure and equal channel angular (ECA) pressing. Computer simulation is used to analyze the X-ray results.

Synthesis of the intermetallics from pure metal powders was performed in a spherical capsule of conservation, with explosions of charges symmetrically distributed over its surface. X-ray diffraction, metallography, and scanning and transmission electron microscopy, were used to study typical structures. These were found to be dendrite, martensite and swirl-like antiphase domain structures. Phases were found to be enriched with iron. The microstructures thus revealed can be used as indications of the phenomena that occur in the capsule of conservation under explosive loading.

The amorphous alloys Fe₈₁Si₇B₁₂, Fe₈₁Si₄B₁₃C₂, Fe₆₄Co₂₁B₁₅, Fe_73.5Cu₁Nb₃Si_13.5B₉, and Fe₅Co₇₀Si₁₅B₁₀ prepared as strips by fast melt quenching on the rotating Cu disc were examined. The strips were 6–12 mm wide and 25–40 μn thick. The structure, coercivity and friction coefficient of these amorphous alloys after plastic deformation by tension, rolling, shear pressure, and dry sliding friction were analyzed. The samples underwent mechanical loading at 293 K in air at rates excluding heating of the material. Coercivity was shown to increase by a factor of ten or more at all types of deformation. The electron microscopic study of the structure of deformed strips revealed the presence of crystalline precipitates 2 to 50 nm in size having a relatively equiaxed shape in the amorphous matrix of the Fe-based alloys and a nearly plate shape in the Co-based alloys. The phases were identified from electron microdiffraction patterns of the alloys.

The prospect of obtaining large volumes of materials with submicron grain size has become increasingly real. The equal channel angular (ECA) pressing appears to be among the most successful methods for producing such materials [1]. The structure of the materials obtained by this method has been the subject of intensive investigation [2–5]. In most cases the description of the structure of such materials has been qualitative. Recently detailed quantitative investigations of materials produced by ECA pressing, were reported [6, 7]. At the same time investigations of the structure of materials deformed by tension, compression, rolling and torsion up to great degrees of deformation have also become more quantitative as well (see, for example, [8–10]). For practical application of ultrafine-grained materials data on their structural stability is rather important. The later is closely connected, firstly, with the internal stress fields and, secondly, with the size distribution function for the substructure parameters [11]. Consequently, in such investigations the measurement of such parameters should be carried out statistically.

Much evidence [1–3] suggests that the unusual properties of ultrafine-grained (UFG) metallic materials are determined not only by the small grain size, but also by the high defective concentration in the grain boundaries. The grain-boundary defects in this case may be not only dislocations, but partial disclinations as well [4]. Direct structural analysis [5] has shown a high density of these defects in the grain boundaries of UFG nickel produced by equal-channel angular (ECA) pressing. In the grain bulk, high-energy structural states with high continuous misorientations (high curvature of the lattice) have been detected as well. This paper deals with revealing via comparative electron microscopy the above structural states in distinct UFG materials (copper, nickel, and the complexly doped intermetallic compound Ni–18at.% Al–8at.% Cr–1at.% Zr–0.15at.% B) produced by various methods of severe plastic deformation (SPD).

The size-distribution of grain size and subgrain size and the dislocation density were determined in submicron grain size copper by a new procedure of x-ray line profile analysis based on the dislocation model of strain anisotropy. Taking into account the anisotropic dislocation contrast factors in the modified Williamson-Hall plots of the FWHM and the integral breadths and in the modified Warren-Averbach procedure enables to determine three stable size parameters, D, d and L ₀ . The width and the mean size, σ and m, of a log-normal size distribution is obtained from these size parameters in a simple least squares procedure. The grain or subgrain size of the copper specimen was reduced well below 100 nm by equal channel angular pressing (ECA) in the bulk. It has been found that with increasing the number of ECA passes the width of the size distribution of grains or subgrains becomes narrower, however, the average dislocation density decreases. The data indicate as if 40 to 50 nm would be a lower size barrier which cannot be overcome by applying ECA at room temperature alone.

Mechanical alloying (MA) is one of the novel processes of alloy formation by severe plastic deformation of metallic components in high-energy ball mills. Analysis of the energy intensity and average temperature of the milling process in planetary ball mill was carried out by computer simulation. The dependences of energy dissipation and average temperature in a vial on the fill fraction of the vial by balls, on the elasticity of ball’s collision and their friction coefficient are determined. The results of computer simulation were compared with ones calculated using the analytical formula. The obtained results allow one to choose optimal modes of ball milling with regard to the specific character of concrete tasks.

The procedure of equal-channel angular (ECA) pressing may be used to subject a material to severe plastic deformation by pressing repetitively through a special die without any concomitant change in the cross-sectional dimensions of the sample. This processing method gives grain refinement in polycrystalline materials with as-pressed grain sizes typically in the submicrometer range. This paper describes the factors influencing the development of a homogeneous microstructure through ECA pressing including the influence of the pressing speed and the angle contained within the special die, the effect of rotating the sample between consecutive pressings and the significance of pressing to a high total strain. An experimental example is presented to illustrate the potential for using ECA pressing to achieve a superplastic forming capability at very rapid strain rates.

Grain refinement of metallic materials may be achieved by imposing severe plastic deformation through procedures such as equal-channel angular pressing or high pressure torsion straining. The grain sizes produced by these techniques are generally below 1 μm and it is therefore necessary to make use of advanced analytical procedures in order to characterize the fine-grained microstructures. This report describes the application of high-resolution electron microscopy (HREM) to atomic-scale observations of the ultrafine-grained structures produced in Cu, Ni and alloys of Ni-Al-Cr and Al-Mg by torsion straining. It is demonstrated that the grain boundaries in these materials are in a high-energy nonequilibrium condition, with irregular arrangements of facets and steps at the interfaces. The thermal stability of these ultrafine-grained microstructures is examined by annealing the samples at elevated temperatures.

In the last decade investigations of plastic deformation have focussed on the large strain ranges, i.e. stage IV and V of deformation [1–5]. Although several models have been developed to explain the work hardening behavior in these stages [2, 6–8], the existing experimental findings are not sufficient to identify the real microstructural processes governing stage IV and V hardening. This situation arises mainly from two facts: (i) traditional methods were not convenient to measure dislocation densities, local internal stresses and misorientation of the substructure, and (ii) most of microstructural investigations were done without relation to the specific mechanical properties. Two new methods, X-ray Bragg Profile Analysis (XPA) [9–12] using a rotating anode and/or synchrotron radiation, and Electron Back Scatter Patterning (EBSP) [13, 14], are effectively for studying microstructures induced by large strains. The XPA-method implemented with a rotating anode generator allows investigation of microstructural evolution within one or more grains by using a focal spot size of several tenth of a mm [15]. Using synchrotron radiation with intensities up to 1012 photons/mm/s allows a reduction in the footprint of the beam on the sample to an order of a few tens of microns. This allows the investigation of the microstructural evolution within a single grain [16, 17]. The EBSP method evaluates the Kikuchi-line pattern from back scattered electrons in a Scanning Electron Microscope (SEM) with a spatial resolution down to 0. 5 μm. This is done by computer support in a very efficient way so that a very large number of different lattice sites can be studied in short time.

Ultrafine-grained (UFG) high purity nickel samples produced by equichannel angular pressing were submitted to thermal treatment and cyclic plastic deformation at different temperatures in order to investigate the stability of the defect structure. Recrystallization was observed already after annealing above 425K. Cyclic plastic deformation at 300K and 425K leads to a coarsening of grains and to a dynamic recrystallization, respectively. Investigations performed by means of synchrotron radiation diffraction revealed that the mean volume expansion, long-range and short-range internal strains are diminished in consequence of the cyclic plastic deformation.

A promising way of generating novel physical properties of solids is possible by the modification of their microstructures. In nanostructured solids (crystalline size ca. l0 nm) the physical properties are strongly modified by the disordered structure of grain boundaries (GB). Nanocrystals are produced under severe (high pressures and heavy deformations) or nonequilibrium (growth from an amorphous state) conditions. Much attention is presently given to studies of nanocrystalline solid as nonequilibrium systems whose mechanical and physical properties are related to collective phenomena in the system of GB defects. The GB defects are thermodynamically nonequilibrium, but can be mechanically stable at ambient temperatures. The nonequilibrium nature of GBs may originate from the intrinsic properties of the dislocation structures that form these mesoscopic defects, as well as from the interactions in the system of GB defects. Since the effect of GB defects on the mechanical properties of polycrystals should be considered as a cooperative phenomenon (i.e., as the effect of the whole ensemble of GB defects), a statistical approach seems to be most relevant in accounting for both the nonequilibrium nature of a separate mesoscopic defect (a grain boundary) and the interactions of GB defects in the ensemble. In this work, we attempt to construct a statistical theory of the polycrystalline state that explicitly allows for the presence of GB defects and defect structures that develop in the ensemble of such GB defects.

A reduction of the mean grain size in metallic alloys is expected to increase their yield stress at room temperature and to promote superplastic properties at higher strain rates and/or lower temperatures than those conventionally used in superplastic forming. By conventional thermomechanical treatments, grain sizes of about 10 µm are developed for aluminium-magnesium alloys and superplastic deformation is thus typically obtained at a temperature close to 500°C and a strain rate in the range 10−4–10−3 s−1 [1,2]. Very fine microstructures can be produced by severe plastic deformation. Equal channel angular extrusion (ECAE) is one of the most popular techniques and has been applied to aluminium alloys [3], particularly in the case of Al-Mg alloys due to their large industrial use, especially in terms of superplastic forming [1,2].

Physical modelling of the macroscopic mechanical properties of metallic materials requires a coupling between these properties and the underlying micro- and substructural features. The substructure development under plastic deformation up to large strains at low and intermediate temperatures is characterized by the coexistence of two substructures on different size and misorientation scales, namely of a cell structure and a fragment (cell block) structure [1,2]. While the cell structure saturates with respect to size and misorientation, the mean fragment size decreases and the mean misorientation between fragments increases monotonously. Long-range stresses are present in the cell as well as in the fragment interiors. The present model describes the cell and the fragment structure development, that means the substructure development on the microscopic and the mesoscopic length scales, through separate, but coupled evolution equations for dislocations carrying deformation and work-hardening on the microscopic scale and for disclinations carrying them on the mesoscopic scale. The authors propose non-compensated nodes of fragment (cell block) boundaries, that means triple junctions with an orientational mismatch around them, to have a disclination character, and, thereby, to be sources of long-range stresses.

In the last two decades, a new scientific trend, the physical mesomecanics of materials [1–6], which links continuum mechanics, physics of plasticity and strength (dislocation theory), and physical materials science has been developed in Tomsk.

The microstructure and micro-strain level during severe plastic deformation (SPD) of 30 heat-resistant Ti-based billets has been characterized using of OM, TEM, EDS and XRD methods. Samples of Ti-based alloys was produced using SPD with simultaneous high speed heating (up to 250 C/sec), and titanium aluminide with rapid solidification. Bragg reflections analysis for different order of diffraction shows metastable state of phases after SPD. The micro-strain make major contribution to broadening of the peaks. The subfine-grained microstructure and texture change have been found in a zone of maximum deformation. The synergetic approach has been used for explanation of the peculiarities of the system of micro- and macro- strains and microstructure evolution during SPD with simultaneous supply of energy.

When severe plastic deformation (SPD) is applied to metals and alloys, the processed materials can possess ultrafme-grained nanostructures having highly non-equilibrium grain boundaries and a distorted crystal lattice. These new states can lead to novel properties of SPD materials. The present paper considers the effects of nanostructures on the unusual mechanical properties in several metals and alloys subjected to severe plastic deformation.

Tensile superplasticity has been observed in a number of severe plastic deformation (SePD) processed alloys with nanocrystalline microstructure. The observations of superplasticity in nanocrystalline materials are briefly reviewed with emphasis on the aspects that are different from superplasticity in microcrystalline materials. The temperature for onset of superplastic elongation coincides with microstructural instability. The important features include, high strain rate superplasticity in an aluminum alloy, low temperature superplasticity, extensive strain hardening and high flow stresses. A comparison of the experimental results with existing models shows the difference in superplastic deformation kinetics. The deformation mechanisms for microcrystalline materials are not simply scaleable to nanocrystalline range. It is difficult to establish the parameters for deformation mechanism because of grain growth. The observations of low temperature and high strain rate superplasticity in nanocrystalline materials with some unique features opens up new possibilities for scientific and technological advancements.

With the development of new materials processing techniques such as severe plastic deformation (SPD) with the equal-channel angular (ECA) extrusion technique [1], bulk structural materials of very fine grain size (UFG: ultrafme grain size) and extraordinary strength are becoming a reality. Among the mechanical properties of interest, the fatigue strength is considered to be of special importance. It is therefore timely to consider critically to what extent a drastically reduced grain size, compared to conventional grain size, will be expected to affect the fatigue behavior. Because of the lack of detailed studies so far, such considerations must of necessity be based not only on the limited data on fatigue of SPD-material but also largely on some basic considerations and “extrapolations” from work on materials of conventional grain size.

Mechanical properties of bulk nanostructured Cu and Ni with 200 nm grains (manufactured by the equal channel angular pressing) are measured under uniaxial compression at 300, 77 and 4.2 K. Measured mechanical characteristics exceed several times those of coarse grained polycrystalline Cu and Ni.

Nanostructured (NS) materials (grain size d ≤ 100 nm) are currently being intensively investigated. This special attention is due to the distinct physical, mechanical and other properties compared to when they are coarse-grained (CG) [1]. In particular, it has been found that diffusion coefficients (D) in NS materials exceed by several orders of magnitude the respective values in CG materials [2–4]. However, the physical reasons for the anomalously high values of D in these materials are debatable. The authors of [4] suggest that the self-diffusion in nanostructured Ni prepared by the inert gas condensation method occurs mainly along the boundaries of clusters and on the surface of pores located there. Clusters (having size 1–10 µm) consist of the complexes of nanoparticles having grain size d ≤ 100 nm. Diffusion along nanoparticles boundaries inside a cluster does not differ from grain boundary diffusion in CG materials, while diffusion along cluster boundaries closely resembles surface diffusion.

Severe plastic torsion straining was used in the present work for processing of the metal matrix Cu + 0.5%Al₂O₃ nanocomposite. The mean grain size, particle size distribution and elastic strain were studied by TEM and XRD. It is shown that high ultimate strength (680 MPa) and microhardness (2300 MPa) as well as high thermal stability and electrical conductivity are the features of the nanocomposite samples. The decrease of the creep rate by an order magnitude and the increase of the time to failure by a factor of 4–5 is revealed in nanocomposite as compared the extruded sample.

Pure copper and copper alloys containing different volume fractions of nano size A1₂0₃ dispersoids were processed by ECAE at 25–250°C, annealed at 25–500°C, and microstructural refinement and mechanical property improvement were studied. In samples processed by ECAE to strains of 5, hardness values increased by 100% in copper specimens and 40–50% in Cu-Al₂0₃ specimens. A drastic increase in hardness was observed after a strain of 1, and hardness values leveled off beyond a strain of 2. Samples were annealed at 25–500°C, to determine the extent of recovery of hardness values. In ECAE processed copper samples, significant reduction in hardness occurred between 100–200°C, and higher temperature annealing resulted in hardness values close to the values before ECAE processing. However, in Cu-Al₂0₃ specimens, the increased hardness was maintained in samples annealed at as high as 500°C. Strength and ductility values determined from 3 point bend tests showed strength values of 150–200 MPa for copper and 650–800 MPa for Cu-Al₂0₃ samples with excellent ductility in copper as well as Cu-Al₂0₃ samples. The observed results were analyzed in terms of the magnitudes of strengthening contributions from dislocation substructure, fine grains, and incoherent dispersoids.

Due to their elevated temperature properties, such as high strength, melting temperature and oxidation resistance, the chromium-based alloys are potentially very attractive for variety of structural applications, including their use as low activation materials for fusion reactor components [1]. To date, however, the potential of this class of alloys has not been translated into practice, because, though they are strong and hard, they are inherently brittle. In fact, they exhibit little or no ductility at low temperatures together with an unacceptably high ductile-to-brittle transition (DTBT) temperature. The sixties and early seventies, marks the period when mechanical properties of these alloys were actively studied and their inherently brittle behavior was amply demonstrated. However, the mechanical results from some of the studies, published during this period, appear to suggest that the low temperature ductility and fracture toughness properties of these alloys could be markedly improved by grain refinement. Among the results reported during this period, are those of Wilcox [2], Gilbert [3], and Wain [4] and their respective co-workers, all of which point to the critical role of grain size on the resulting mechanical properties. For example, Wilcox and co-workers showed that decreasing the grain size of chromium from 37 to 2.4 µm by the addition of a few percent of thoria particles resulted in a significant decrease in DTBT temperature (from 140 to 15°C) together with significant improvements both in the ductility and strength [2].

Superplastic deformation usually requires testing temperature above 0.5T_m when an average grain size is less than 10 µm (T_m is the melting point) [1]. This is because the superplastic deformation is connected closely with thermally-activated processes such as dislocation climb and grain-boundary diffusion. It has long been recognized that the grain size affects the temperature at which the superplastic deformation occurs. To achieve low temperature superplasticity, several Al-alloys have been fine-grained by the equal channel angular pressing (ECAP) technique [2]. The properties of the superplastic materials have primarily been studied in monotonie straining while the limited results have been reported on their cyclic behavior [3]. Hence, it seems reasonable to utilize the superplastic alloys fine-grained by ECAP to perform a precise fatigue experiment at room temperature. In the present work, we carried out cyclic tests at room temperature on the ECAP fabricated Pb-62%Sn and Zn-22%A1 eutectic alloys. The attention is paid particularly to the strain-rate dependence of the stress amplitude.

Grain growth in ultrafine-grained (UFG) copper during isothermal annealing and heating at a constant rate was studied by TEM, DSC and microhardness measurements. Three stages of thermal structure evolution in UFG copper were identified. These stages are connected with partial relaxation of the defects, migration of non-equilibrium grain boundaries and grain growth. The values of the activation energy in Stage II and Stage III were estimated.

Lower-Bound Ductility (LBD) is the critical strain at the moment of fracture as a function of stress state and temperature. Measurements of LBD are normally performed by testing in a hyperbaric chamber. There is an alternative approach for determination of the LBD based on the cumulative nature of damage. The basic idea of this new approach is to apply two successive loadings with different stress ratios that lead to fracture, and then to calculate the LBD function.The processes of equal channel angular extrusion and drawing are used to introduce some damage in the specimen for a specified stress — temperature history. The amount of plastic deformation is determined by the angle between the two intersecting channels, while the stress ratio is varied by applying different back-pressures. The rest of the damage required to cause fracture is introduced by a subsequent tensile test. The LBD diagram for continuously cast Al 6061 has been deduced using this method.

The equal channel angular pressing method was used to strengthening the Fe-36%Ni invar alloy and of some industrial maraging steels by severe plastic deformation. The features of the dislocation structure formed on this process, as well as mechanical and thermophysical properties of deformed alloys have been studied.

Data on ambient temperature tensile properties of commercial aluminium alloys processed from cast ingots are reviewed. The properties of alloys in the severely deformed condition and after annealing are compared with standard values for conventionally treated commercial wrought products. It is shown that with non-heat treatable alloys, processed by severe deformation, a significant increase in tensile strength is obtainable. The combination of strength and ductility achievable is comparable to, or even better than, that found in many high-strength conventionally treated precipitation-hardened alloys. For most of heat treatable alloys, the use of severe deformation for improving mechanical properties is ineffective.

A wide variety of natural and technical materials exhibit superplastic or superplastic-like flow under external or internal stress in certain temperature regimes. Phenomenological analysis of mass-transfer mechanics producing such flow gives the basis for dividing these materials into two principal groups: (1) polycrystalline superplastic materials which exhibit so called fine-structure superplasticity i.e. superplastic flow where mass-transfer units are crystalline grains of size in micro-, submicro- or nanometer range; and (2) materials exhibiting superplastic-like flow where the basic mass-transfer units are either single atoms (molecules) or groups of them. The latter group includes inorganic non-metallic and metallic glass-forming systems and polymers. Amorphous alloys (metallic glasses) with small volume fraction of nanocrystalline dispersed phase are known to have very high mechanical strength and low ductility at low and high temperatures compared with those for crystalline alloys. On the other hand, some recently developed bulk amorphous alloys with large glass-forming ability have shown striking superplastic-like behavior and very high formability in supercooled liquid state which seems to be promising for future development as a new type of superplastic metallic materials [1–3]. One of the most powerful tools for producing bulk amorphous alloys is severe plastic deformation (SPD) in the form of mechanical alloying. Other forms of SPD can be used to yield traditional fine-structure superplastic alloys.

The severe plastic deformation (SPD) of metals is known to modify their crystal structure. Particularly, the average size of the crystalline grains considerably decreases while the extent of crystal lattice distortion and amorphous phase both increase. Such a state is characterized as non-equilibrium one. When the metal is in this state, it undergoes a polymorphous transformation at temperatures below the melting point (most intensive at T ~ 0.4 T_meit).

The practical application of nanostructured materials obtained by severe plastic deformation involves two main problems at least. Firstly, there is the instability of non-equilibrium structure connected with high energy accumulated by severe plastic deformation [1]. The instability is the reason for intense softening at relatively low temperature. This temperature is close to room temperature for several nanostructured materials, for example, copper [2]. Secondly, there is the increased diffusivity of nanostructured material state. This leads to high sensitivity to the influence of surrounding. One of the most important type of this action is grain boundary fluxes from an external source, for example, coating [3, 4].

The status and prospectus for severely deformed materials are overviewed from the perspective of interrelated trends in materials characterization, modeling, and processing. Technological drivers for accelerating the development of these materials are summarized. Technological barriers to understanding and applying severely deformed materials are highlighted. Recommendations are made to direct future work, including facilitation of more rapid commercialization.

Ultrahigh-carbon steel wire can achieve very high strength after severe plastic deformation, because of the fine, stable substructures produced. Tensile strengths approaching 6000 MPa are predicted for UHCS containing 1.8%C. This paper discusses the microstructural evolution during drawing of UHCS wire, the resulting strength produced and the strengthening mechanisms active in these materials. Drawing produces considerable alignment of the pearlite plates. Dislocation cells develop within the ferrite plates and, with increasing strain, the size normal to the axis (d) decreases. These dislocation cells resist dynamic recovery during wire drawing and thus extremely fine substructures can be developed (d < 10 nm). Increasing the carbon content reduces the mean free ferrite path in the as-patented wire and the cell size developed during drawing. For UHCS, the strength varies as d-n, where n is 0.5 to 1. The influence of processing and composition on achieving high strength in these wires during severe plastic deformation is discussed.

Bulk samples of commercially pure (CP) titanium were processed by severe plastic deformation (SPD), namely, equal channel angular (ECA) pressing in combination with thermal mechanical treatment, to produce ultrafine-grained (UFG) microstructures. It is shown that by altering SPD processing parameters one can form distinct types of UFG microstructures, which differ in grain shape, size and intragrain defect density and defect distribution. These UFG microstructures in CP titanium result in strength and fatigue properties similar to those of Ti-alloys. We are thus able produce CP titanium suitable for replacing Ti-6A1-4V in biomedical and orthopaedic applications such as dental implants, medical instruments, and trauma fixation devices (nails, plates, screws, and wires).

Severely drawn steel wire is nanostructured. This leads to very high strengths thus opening a number of interesting engineering applications. Several of these applications require a change in strain path subsequent to the drawing strain. This has important influences on material flow, on the occurrence of shear bands and on fatigue behavior, hence on residual ductility during subsequent forming and on product performance. The underlying reasons are the very strong crystallographic and structural anisotropy as well as the polarity of the dislocation arrangements introduced by the drawing process. The present contribution focuses on the first mentioned aspect, namely the influence of structure induced by a previous strain mode on material flow in a subsequent strain mode.