Igor V. Alexandrov

Bulk nanostructured materials from severe plastic deformation

2. Methods of severe plastic deformation and formation of nanostructures . . . . . . . 105 2.1. SPD techniques and regimes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 2.1.1. Torsion straining under high pressure . . . . . . . . . . . . . . . . . . . . . 106 2.1.2. ECA pressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 2.1.3. Multiple forging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 2.2. Typical nanostructures and their formation . . . . . . . . . . . . . . . . . . . . . . . 115

Paradox of strength and ductility in metals processed by severe plastic deformation

It is well known that plastic deformation induced by conventional forming methodssuch as rolling, drawing or extrusion can significantly increase the strength of metalsHowever, this increase is usually accompanied by a loss of ductility. For example, Fig.1 shows that with increasing plastic deformation, the yield strength of Cu and Almonotonically increases while their elongation to failure (ductility) decreases. Thesame trend is also true for other metals and alloys. Here we report an extraordinarycombination of high strength and high ductility produced in metals subject to severeplastic deformation (SPD). We believe that this unusual mechanical behavior is causedby the unique nanostructures generated by SPD processing. The combination ofultrafine grain size and high-density dislocations appears to enable deformation by newmechanisms. This work demonstrates the possibility of tailoring the microstructures ofmetals and alloys by SPD to obtain both high strength and high ductility. Materialswith such desirable mechanical properties are very attractive for advanced structuralapplications.In this work, we report on how inducing severe plasticdeformation (SPD) by equal channel angular pressing(ECAP) and high pressure torsion (HPT)

Influence of ECAP routes on the microstructure and properties of pure Ti

Abstract Equal channel angular pressing (ECAP) is an innovative technique that can produce bulk ultrafine-grained (UFG) materials in product forms large enough for structural applications. It is well known that ECAP route, defined by the sequence of orientations of the billets relative to the die during the iterative ECAP passes, significantly affects the microstructural development of the work piece. Studies reported in the literature have so far focused on fcc metals such as Al and Cu. In this work, we have studied the influence of ECAP routes on the microstructures and properties of hcp commercially-pure Ti. Three ECAP routes, conventionally defined as BA, BC and C, were used to process the Ti billets. Surface quality, microstructures, microhardness, tensile properties, anisotropy, and thermal stability were studied. The route BC is shown to be the best route for processing hcp Ti.

Grain refinement and properties of pure Ti processed by warm ECAP and cold rolling

Abstract This work explored a two-step severe plastic deformation process to produce ultrafine-grained (UFG) Ti with significantly enhanced strength. Warm equal channel angular pressing (ECAP) was first used to refine the grain size of Ti billets to about 350 nm. The Ti billets were further processed by repetitive cold rolling (CR). This two-step process produced UFG Ti with strengths higher than those of common titanium alloys such as Ti–6Al–4V. This paper reports the microstructures, tensile properties, and thermal stability of these Ti billets processed by a combination of warm ECAP and CR.

Nanostructured materials from severe plastic deformation

Abstract Recent results of the development of the severe plastic deformation methods to fabricate bulk nanostructured materials as well as results of their thorough structural characterization and investigations of their unusual deformation behavior and novel mechanical properties are presented. The structural model of nanomaterials processed by severe plastic deformation methods is developed on the basis of the obtained results.

Microstructural evolution, microhardness and thermal stability of HPT-processed Cu

Coarse-grained copper was subject to high-pressure torsion (HPT) and thermal treatment to study the effects of increasing amounts of deformation and subsequent annealing on the evolution of microstructure and microhardness. Cellular subgrains with low-angle grain boundaries were first formed at low strain. Some of the low-angle subgrain boundaries transformed to high-angle grain boundaries at higher strains, refining the average grain size from 200 μm to 150 nm. X-ray diffraction patterns showed the formation of crystallographic texture. Microhardness increased monotonically with increasing torsional strain. High internal stress and nonequilibrium grain boundaries were observed in unannealed samples. Annealing as-deformed samples at temperatures as low as 50°C decreased the microhardness, indicating a very low thermal stability of the deformation induced microstructures. Differential scanning calorimetry (DSC) revealed an exothermal peak between 180 and 280°C, caused by recrystallization. Annealing twins were also formed during recrystallization.

The formation of PSB-like shear bands in cyclically deformed ultrafine grained copper processed by ECAP

Cyclic deformation was performed on ultrafine grained copper processed by ECAP. Shear bands (SBs) and adjacent microstructures were investigated using electron channeling contrast in scanning electron microscope. The possible formation mechanism of SB was discussed based on the characteristic distribution of defects introduced by ECAP

Consolidation of nanometer sized powders using severe plastic torsional straining

Severe Plastic Deformation (SPD) via torsional straining was used to consolidate nanometer-sized metallic powders and metal-ceramic nanocomposites at room temperature. Materials processed using this technique included copper, Al and nanocomposites based on these metallic nanopowders and SiO2, SiC and Al2O3 nanopowders. The as-processed materials were nearly fully dense. Transmission electron microscopy (TEM) and microhardness measurements were used to characterize the materials. The as-consolidated materials had a grain size smaller than the average particle size of starting powder, indicating grain refinement during severe plastic torsional straining. Due to the oxidation of the initial nanometer sized powders, the as-processed samples were very brittle. Thermal stability was investigated by annealing the as-consolidated samples at various temperatures and then measuring their microhardness.

Densities and character of dislocations and size-distribution of subgrains in deformed metals by X-ray diffraction profile analysis

Abstract The density and the character of dislocations and the size-distribution of grains or subgrains were determined by a new procedure of X-ray diffraction (XRD) profile analysis in copper specimens deformed by equal channel angular pressing (ECA) or cold rolling. The anisotropic strain broadening of diffraction profiles was accounted for by dislocation contrast factors. The screw or edge character of dislocations was determined by analyzing the dislocation contrast factors. Three size parameters and the dislocation density were obtained by the modified Williamson–Hall and Warren–Averbach procedures. Assuming that the grain-size distribution is log-normal, the median, m, and the variance, σ, of the size distribution of grains or subgrains were obtained from these three size parameters.

Structural characterization of nanocrystalline copper by means of x‐ray diffraction

Quantitative x-ray-diffraction measurements were performed on a nanocrystalline Cu sample made by severe plastic deformation. The shape of Bragg reflections was found to be represented primarily by a Lorentzian function. A difference of as much as 6%+/-3% was revealed between the integrated intensities from the nanocrystalline and a reference coarse-grained Cu samples. The broadening of Bragg reflections from the nanocrystalline Cu sample was mainly induced by small crystallite sizes and microstrains inside the grains and/or the deformed layers near the grain boundaries. It was found that the grain sizes of nanocrystalline Cu in different crystallographic orientations are essentially the same, while the microstrains exhibit a significant anisotropy. The Debye-Waller parameter B of the nanocrystalline Cu sample was 0.97+/-0.06 Angstrom(2), which suggests that the atomic displacement from their ideal lattice positions equals on average 0.111+/-0.004 Angstrom or 4.3% of the nearest-neighbor spacing