Papers by Keyword: Ultrafine Grained Copper

Abstract: In this paper, ultra-fine grained copper fabricated by equal channel angular pressing method and annealed coarse grained copper were tensioned under both quasi-static and dynamic loading conditions using an electronic universal testing machine and the split Hopkinson tension bar respectively. The rapture surface of specimen was also observed via a Scanning Electron Microscope (SEM). The experimental results show that the ductility of polycrystalline copper decreases remarkably due to the grain refinement. However, with the increase of applied strain rate, ductility of the UFG-Cu is enhanced. The fracture morphologies also give the evidence of enhanced ductility of UFG-Cu at high strain rate. It is believed the enhanced ductility of UFG materials at high strain rate can be attributed to the restrained dislocation dynamic recovery.

Abstract: Technical purity Cu (99.95 wt%) polycrystals have been processed at room temperature by equal channel angular pressing. The results of mechanical tests and the microstructure characterization by various experimental techniques are presented. The yield stress as well as the strength were shown to increase with increasing strain and exceed the respective values of a coarsegrained material. The microstructure development and its fragmentation after ECAP was investigated by the TEM and EBSD. The proportion of high angle grain boundaries was found to increase with increasing strain reaching the value of 90% after 8 ECAP passes. Two kinds of defects were identified in ECAP specimens by positron annihilation spectrometry (PAS): (a) dislocations which represent the dominant kind of defects, and (b) small vacancy clusters (so called microvoids). The main increase of defect density was found to occur during the first ECAP pass. PAS analysis indicated that in the specimens subjected to one ECAP pass the mean dislocation density ρD and the concentration of microvoids cν exceeded the values of 1014 m-2 and 10-4 at.-1, respectively. After 4 passes, the number of defects becomes saturated and practically does not change with increasing strain.

Abstract: Fatigue behaviour of ultrafine-grained copper of purity 99.9 % produced by ECAP technique was studied in a broad region of stress amplitudes. Fatigue strength is by a factor of about 2 higher than that of conventional-grain-size copper in the broad region of fatigue lives from 6x103 to 2x1010 cycles. The grain structure is stable and undergoes only very marginal changes during cycling. Fatigue slip markings on specimen surface follow the trace of the shear plane of the last ECAP pass. Fatigue notch sensitivity is also higher than that of conventional-grain-size copper, but not dramatically. The cyclic stress-strain curve of studied copper is temperature insensitive, while its S-N curve is temperature dependent.

Abstract: Oxygen-free high conductivity copper was subjected to room temperature equal channel angular extrusion of 8 passes using route Bc. The resulting ultra-fine grain copper was then rolled to thickness reductions of up to 96.5% at liquid nitrogen temperatures. Annealed coarse grained copper was rolled to the same strain at room temperature for comparison. Samples from the two routes were isochronally and isothermally annealed, and the microstructure and texture evolution studied by electron back scattered diffraction and x-ray diffraction. Annealing of the ultrafine grained copper led to the development of a strong rotated cube texture from a texture in the rolled material dominated by the Brass component. In contrast the more commonly observed cube texture was found after annealing of the coarse-grained sample. Accompanying the rotated cube texture was the development of a large fraction of boundaries with rotation angle/axis close to 60° <111>.

Abstract: Oxygen-free copper was processed by equal channel angular pressing with different numbers of ECAP process cycles, NP. Tensile strength was increased with an increase in NP, but it tended to saturate after NP = 4. Conversely, elongation was dramatically decreased by first pressing, but it tended to saturate up to NP = 3, followed by slight increasing trend after NP = 4. Fatigue tests of specimens processed with NP = 4 and 8 were performed. The change in surface morphologies during fatigue was monitored successively. In addition to this, the change in surface hardness was measured. Significant decrease in surface hardness due to cyclic stresses was measured. The physical basis of fatigue damage of UFG copper was discussed based on the experimental results.

Abstract: Microstructure evolution and corrosion behaviour of ultrafine-grained copper processed by equal channel angular pressing (route Bc) were studied. The results of TEM investigation of the microstructure evolution are presented along with the measurements of the corrosion potential, the corrosion current density and the anodic current density for two aggressive media, viz. 3% NaCl and 1M H2SO4. An important finding and a good news is that the corrosion behaviour of ECAP copper is not inferior to and does not qualitatively differ from that of the coarse grained material. Moreover, it was shown by SEM investigation that the corrosion damage is more homogeneous in ultrafine grained ECAP processed copper than in its coarse grained counterpart.

Abstract: Ultrafine-grained (UFG) pure Cu processed by equal channel angular pressing (ECAP) was subjected to cyclic deformation and subsequent ageing treatment at room temperature (RT) in order to investigate the stability of defects and grain size. Cyclic deformation for 1000 cycles at RT leads to a large decrease of internal stress. X-ray diffraction (XRD) shows that the stability of defects and grain size at RT in as-cyclic deformed sample is lower than that in as-processed sample and that a reduction of internal stress takes place prior to grain growth. TEM observations show that the microstructural evolution during ageing is characterized by normal grain growth accompanied with recovery within grain interior.