Roberto B. Figueiredo

Significance of stacking fault energy on microstructural evolution in Cu and Cu-Al alloys processed by high-pressure torsion

Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting minimum grain size (d min) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of d min on the SFE decreases when the severity of the external loading conditions is increased.

Twenty-five years of severe plastic deformation: recent developments in evaluating the degree of homogeneity through the thickness of disks processed by high-pressure torsion

The processing of disks by high-pressure torsion leads to an inhomogeneous distribution in strain with a high strain around the perimeter of the disk and a zero strain in the center. Despite this apparent inhomogeneity, there are now many experiments showing that the hardness values on the surfaces of disks gradually evolve with increasing strain to give a reasonably high level of homogeneity. Experiments were conducted to determine whether this high level of homogeneity extends also through the thickness of the disks or whether inhomogeneities occur in the axial direction. Results are presented for high-purity aluminum and a magnesium AZ31 alloy as two representative materials showing different hardness characteristics.

Analysis of plastic flow during high-pressure torsion

Finite-element modeling (FEM) was used to simulate processing by high-pressure torsion under quasi-constrained conditions using three different material conditions: strain-hardening, perfect-plastic, and flow-softening. The results show there is a tendency for flow localization during processing and this becomes more obvious during the processing of perfect-plastic, and flow-softening materials or when processing samples having high thickness to diameter ratios. The analysis demonstrates the effect of the material condition, the disk aspect ratio and the effect of friction between the disks and the anvil walls. It is demonstrated that the predictions from FEM correlate well with published experimental results.

Effect of temperature on the processing of a magnesium alloy by high-pressure torsion

It is now well known that processing by SPD can significantly increase the strength of metallic materials by refining the grain structure and increasing the density of defects. The rapid increase in strength observed in the early stages of deformation is expected to slow down and saturate at large strains because of an increasing recovery of the material. Therefore, a saturation strength is anticipated that will depend on the processing temperature. This investigation analyses this parameter by determining the evolution of hardness of a magnesium alloy processed by high-pressure torsion at different temperatures.

The nature of grain refinement in equal-channel angular pressing: a comparison of representative fcc and hcp metals

Abstract 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.

Fabricating Ultrafine-Grained Materials through the Application of Severe Plastic Deformation: a Review of Developments in Brazil

Considerable attention is now being devoted to the fabrication and properties of ultrafine-grained materials processed through the application of severe plastic deformation. The two main processing techniques for SPD are equal-channel angular pressing and high-pressure torsion. Ten years ago, in 2002, the first Brazilian research paper was published describing the results obtained from a material processed using an SPD technique. Since that time, Brazilian materials scientists have made, and are continuing to make, major contributions to this important field. This tenth anniversary, and the introduction of a new Brazilian research journal, provides an excellent opportunity to summarize the main principles of SPD processing and then to review the major contributions from Brazil in the field of SPD.

Microstructure and texture evolution in a magnesium alloy during processing by high-pressure torsion

Magnesium alloys often exhibit cracking and segmentation after equal-channel angular pressing (ECAP) at room temperature. With torsion shear deformation and a hydrostatic stress, high-pressure torsion (HPT) has an advantage over ECAP in the processing of hard-to-deform materials like magnesium alloys at room temperature. In this report, HPT was used on extruded AZ31 Mg alloy at temperatures of 296, 373 and 473 K for 1 and 5 turns. After HPT processing, the hcp crystal c-axis rotated from the disc (r,θ) plane towards the torsion axis. The angle between the c-axis and the torsion axis (Φ) has a relationship with the HPT processing temperature. It was found that the c-axis was 10o from the torsion axis at 296 and 373 K but 5o from the torsion axis at 473 K. The activity of the basal slip and the twinning exert significant contributions to the deformation. Microstructural features such as the grain size and grain size distributions were examined and correlated with the mechanical properties through the microhardness values.

Three-dimensional analysis of plastic flow during high-pressure torsion

The use of imposed plastic deformation as a single parameter to compare results of samples processed by severe plastic deformation is not always accurate. Therefore, this report describes the theoretical plastic flow occurring during high-pressure torsion and presents finite element modeling of this technique to complement the theory. The results demonstrate the influence on plastic flow of the material behavior, the sample aspect ratio, the processing pressure, and the contact friction between the sample and the anvil. It is shown that heterogeneous flow is primarily observed near the edges of the samples. The present results are in general agreement with published experimental observations.

Grain Refinement of Commercial Purity Magnesium Processed by ECAP (Equal Channel Angular Pressing)

Grain refinement in magnesium is evaluated in the present paper. Equal Channel Angular Pressing is used to process commercially pure magnesium. Processing was carried out at 523 K which is lower than the temperature used in other papers on the literature. The grain structure was evaluated throughout the deformation zone. The low processing temperature prevents significant grain growth. The evolution of the grain structure is compared to a recent model for mechanism of grain refinement in magnesium. The present results confirm the validity of the model.

The characteristics of superplastic flow in a magnesium alloy processed by ECAP

Abstract It is well known that Equal-Channel Angular Pressing (ECAP) introduces an ultrafine grain size and leads to exceptional superplastic properties in a ZK60 (Mg-5.5 % Zn-0.5 % Zr) alloy. This paper presents an analysis of the flow behavior during superplastic deformation in the ZK60 alloy. It is shown that an increase in the ECAP processing temperature enhances the thermal stability of the grain structure. In addition, the optimum superplastic properties occur at higher strain rates when testing at higher temperatures thereby introducing the potential for achieving high strain-rate superplasticity.