Evaluating the Paradox of Strength and Ductility in Ultrafine-Grained Oxygen-free Copper Processed by ECAP at Room Temperature

Oxygen-free copper of >99.95% purity was processed by equal-channel angular pressing at room temperature (RT) for up to 24 passes and then pulled to failure at RT using strain rates from 10-4 to 10-2 s-1. The results show that the microstrain increases with strain at the lower numbers of passes but decreases between 16 and 24 passes. Similar trends were found also for the dislocation density, the Vickers microhardness and the values of the measured yield stresses in tensile testing. X-ray diffraction measurements showed a minor increase in the crystallite size at the high strain imposed by processing through 24 passes. These results demonstrate the occurrence of dynamic recovery at the highest strain. In tensile testing at a strain rate of 10-3 s-1 the results gave a yield stress of ~391 MPa and an elongation to failure of 52% which is consistent with an earlier report using Cu of much higher purity but not consistent with an earlier report using Cu of the same purity

Comparisons of self-annealing behaviour of HPT-processed high purity Cu and a Pb–Sn alloy.

Early published results have demonstrated that high purity Cu and a Pb–62% Sn alloy exhibit very different behaviour during high-pressure torsion (HPT) processing at room temperature and subsequent room temperature storage. High purity Cu showed strain hardening behaviour with a refined grain structure during HPT processing whereas a Pb–62% Sn alloy displayed a strain weakening behaviour because the hardness values after HPT processing were significantly lower than in the initial as-cast condition even though the grain size was reduced. During room temperature storage after HPT processing, high purity Cu with lower numbers of rotations softened with the time of storage due to local recrystallization and abnormal grain growth whereas the Pb–62% Sn alloy hardened with the time of storage accompanied by grain growth. Through comparisons and analysis, it is shown that the low absolute melting point and the high homologous temperature at room temperature in the Pb–62% Sn alloy contribute to the increase in hardness with coarsening grain size during room temperature storage

The stability of oxygen-free copper processed by high-pressure torsion after room temperature storage for 12 months.

Ultrafine-grained copper samples produced by high-pressure torsion were stored at room temperature for 12 months to investigate microstructural stability and the self-annealing phenomena. The results show that samples processed by low numbers of turns exhibit less thermal stability after storage for 12 months by comparison with samples processed by high numbers of turns. A significant decrease in the hardness was recorded near the edges of the discs processed by 1/4, 1/2 and 1 turn due to recrystallization and grain growth whereas a minor drop in hardness values was observed in the samples processed by 3, 5 and 10 turns. This drop in hardness was related to a recovery mechanism

Characteristics of grain refinement in oxygen-free copper processed by equal-channel angular pressing and dynamic testing.

Oxygen-free copper was processed by equal-channel angular pressing (ECAP) at room temperature for 1, 4 and 8 passes and then the ECAP specimens were further deformed by dynamic testing at 298 K using a strain rate of 10 s-1. Experiments were conducted to investigate the influence of the initial microstructures induced by ECAP on the subsequent grain refinement and mechanical properties after dynamic testing. The results show the strength of copper increased with increasing numbers of ECAP passes and a significant additional grain refinement was produced in the ECAP specimens through the dynamic testing. Thus, the initial grain sizes after ECAP for 1, 4 and 8 passes were ~16, ~4.4 and ~2.9 µm, respectively, and these values were reduced to ~400, ~330 and ~300 nm by dynamic testing, The grains were refined by conventional dislocation processes in the 1-pass specimen but there was evidence for dynamic recrystallization in the specimen processed by 8 passes

Effect of high-pressure torsion on microstructure, mechanical properties and corrosion resistance of cast pure Mg

© 2018, The Author(s). High-pressure torsion (HPT) processing was applied to cast pure magnesium, and the effects of the deformation on the microstructure, hardness, tensile properties and corrosion resistance were evaluated. The microstructures of the processed samples were examined by electron backscatter diffraction, and the mechanical properties were determined by Vickers hardness and tensile testing. The corrosion resistance was studied using electrochemical impedance spectroscopy in a 3.5% NaCl solution. The results show that HPT processing effectively refines the grain size of Mg from millimeters in the cast structure to a few micrometers after processing and also creates a basal texture on the surface. It was found that one or five turns of HPT produced no significant difference in the grain size of the processed Mg and the hardness was a maximum after one turn due to recovery in some grains. Measurements showed that the yield strength of the cast Mg increased by about seven times whereas the corrosion resistance was not significantly affected by the HPT processing

Effect of spark plasma sintering and high-pressure torsion on the microstructural and mechanical properties of a Cu–SiC composite

This investigation examines the problem of homogenization in metal matrix composites (MMCs) and the methods of increasing their strength using severe plastic deformation (SPD). In this research MMCs of pure copper and silicon carbide were synthesized by spark plasma sintering (SPS) and then further processed via highpressure torsion (HPT). The microstructures in the sintered and in the deformed materials were investigated using Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM). The mechanical properties were evaluated in microhardness tests and in tensile testing. The thermal conductivity of the composites was measured with the use of a laser pulse technique. Microstructural analysis revealed that HPT processing leads to an improved densification of the SPS-produced composites with significant grain refinement in the copper matrix and with fragmentation of the SiC particles and their homogeneous distribution in the copper matrix. The HPT processing of Cu and the Cu-SiC samples enhanced their mechanical properties at the expense of limiting their plasticity. Processing by HPT also had a major influence on the thermal conductivity of materials. It is demonstrated that the deformed samples exhibit higher thermal conductivity than the initial coarse-grained samples

Mechanical properties and microstructural behavior of a metal matrix composite processed by severe plastic deformation techniques

A severe plastic deformation (SPD) technique was applied to an Al-7075 alloy reinforced with 10 vol.% Al2O3. This processing method of high-pressure torsion (HPT) was performed at room temperature under a pressure of 6.0 GPa through a total number of up to 20 turns. The metal matrix composite (MMC) showed a significant grain refinement from an initial average grain size of ?8 ?m to ?300 nm after processing by HPT through 20 turns which led to an increase in the average values of Vickers microhardness at room temperature

Microstructural saturation, hardness stability and superplasticity in ultrafine-grained metals processed by a combination of severe plastic deformation techniques

A commercial Al-7075 aluminum alloy was used to investigate the significance of the saturation hardness and saturation microstructure after processing by high-pressure torsion (HPT) through up to 20 turns or a combination of equal-channel angular pressing (ECAP) for 4 or 8 passes and HPT. The results show additional grain refinement by initially processing the material by ECAP to produce an ultrafine grain size before conducting the HPT. Thus, the grain size immediately prior to the HPT processing controls the saturation microstructure and hardness. Microstructural images reveal a significant grain refinement from elongated grains in the samples prior to processing with an average grain diameter of ~8 ?m to equiaxed grains with an average grain size of ~200 nm after processing by a combination of ECAP for 8 passes and HPT through 20 turns. This grain refinement leads to an improvement in the mechanical properties of the Al-7075 alloy. Vickers microhardness tests show a significant increase in the microhardness values of the Al-7075 alloy from Hv?102 in the annealed samples to Hv?270 after processing by a combination of ECAP for 8 passes and HPT through 20 turns. Specimens processed by a combination of ECAP and HPT also exhibit excellent superplastic properties with elongations up to >1000% at 673 K

Developing superplasticity in an aluminum matrix composite processed by high-pressure torsion

An Al-7075 alloy reinforced with 10 vol% Al2O3 particulates was processed by high-pressure torsion (HPT) at room temperature under a pressure of 6.0 GPa through up to 20 turns. The metal matrix composite (MMC) showed significant grain refinement from an initial average grain size of ~8 ?m to ~300 nm after processing by HPT through 20 turns. The Vickers microhardness also increased from an initial value of Hv?167 to a saturation value after HPT processing of Hv?260. Tensile testing at 623 K demonstrated the potential for achieving true superplasticity in the HPT-processed MMC with a maximum elongation of ~670% when testing at a strain rate of 1.0×10?2 s?1