Materials Science Forum Vols. 503-504

Abstract: It is known that the severe plastic deformation (SPD) induced by Accumulative Roll Bonding (ARB) results in more important grain refinement as compared to conventional rolling. Since ARB enables production of large amounts of ultra-fine grained (UFG) materials, its adoption into industrial practice is favoured. The paper presents the results of a study of high purity aluminium processed by ARB and cold rolling. Microstructure changes induced by both methods were studied by light and transmission electron microscopy. Dislocation density and arrangement were assessed by positron annihilation spectroscopy. Strength evolution was estimated by hardness measurements. Texture measurements were performed by X-ray diffraction. ARB processing results in over twofold increase in hardness. Hardness increases significantly after two ARB cycles and it raises only a little or decreases during subsequent cycles. The increase in hardness induced by conventional rolling is smaller. Positron lifetime measurements reveal a substantial increase of dislocation density at the first ARB cycle and a moderate increase or even a decrease at further cycles. The high fraction of positrons trapped at grain-boundary dislocations gives evidence for substantial grain refinement confirmed by TEM examinations. Grain size of 1.2 􀁐m in the rolling plane and as small as of 90 nm in the normal direction is obtained. The rolled samples have a typical rolling texture (􀁅-fibre). The 􀁅- fibre of the sample ARB processed to strain of 2.4 is weaker as compared to its rolled counterpart and it presents through thickness variations. The surface layers do not have any 􀁅-fibre orientations but they have ND-rotated cube texture formed by the shear strains induced by lubricant-free rolling.

Abstract: The commercially pure titanium cylindrical samples with a diameter of 11.5mm and a length of 24mm were processed by a new severe plastic deformation process, called the rotary-die equal channel angular pressing (RD-ECAP), under the condition of 773K, 2.4mm/s punch. By the RD-ECAP, ECAP processes of 1-4 passes were possible without sample removal and the temperature of cp-titanium could be simply controlled. After the RD-ECAP process, the cp-titanium samples had no crack. Fine-grained microstructures were observed in the sample on Y plane. Therefore the samples processed by RD-ECAP were expected to have high mechanical strength.

Abstract: Equal channel angular extrusion (ECAE) is a processing method for introducing an ultrafine grain size into a material. In the present study, a two-step severe plastic deformation process was used to produce ultrafine grained copper with significantly enhanced strength. Equal channel angular extrusion was first used to refine the grain size of copper samples. The copper samples were further processed by cold rolling (CR) to a strain of 0.67 (about 50% reduction). This two-step process produced ultrafine grained copper with strengths higher than those of pure copper processed through ECAE only. This paper reports the microstructures and mechanical properties of the copper specimens processed by a combination of room temperature ECAE and CR. The effectiveness of initial processing by ECAE prior to cold rolling is discussed.

Abstract: In the present paper a flow pattern of aluminum alloy and copper was investigated by grid procedure, using 2D-modeling and experimental data for ECAP using backpressure. The most important parameters that influence upon flow pattern and deformed state of billets were revealed. We found the most optimal radii of intersecting channels to press the billets, having 16х16 mm in section, from the aluminum alloy and copper in backpressure conditions.

Abstract: Low temperature dislocation- and point defect-related anelasticity in high pressure torsion (HPT) deformed metallic materials bcc Fe-26Al, fcc Fe-36Ni, hcp Ti is studied by mechanical spectroscopy. Internal friction (IF) peaks, which correspond to these phenomena, have different stability against heating. Up to five IF peaks are recorded, at least some of these peaks can be classify as Hasiguti peaks. Mechanical spectroscopy gives a useful tool to study early stages of severely plastic deformed alloys study.

Abstract: The nanocrystalline (nc) formation was studied in cobalt (a mixture of ε (hexagonal close packed) and γ (face-centered cubic) phases) subjected to surface mechanical attrition treatment. Electron microscopy revealed the operation of { 10 10 }〈 1120 〉 prismatic and {0001}〈 1120 〉 basal slip in the ε phase, leading to the successive subdivision of grains to nanoscale. In particular, the dislocation splitting into the stacking faults was observed to occur in ultrafine and nc grains. By contrast, the planar dislocation arrays, twins and martensites were evidenced in the γ phase. The strain-induced γ→ε martensitic transformation was found to progress continuously in ultrafine and nc grains as the strain increased. The nc formation in the γ phase was interpreted in terms of the martensitic transformation and twinning.

Abstract: This paper investigates microstructures and mechanical properties of the TI-6AL-4V ELI alloy processed by ECAP and extrusion with various morphology of α and β-phase. Preliminary thermal treatment consisted of quenching and further high-temperature ageing. The present work reveals that the decrease of volume fraction of α-phase globular component in the initial billet results in a more homogeneous structure refinement during SPD, lower internal stress, enhancement of microstructure stability and mechanical properties. An ultimate strength of UTS ≥1350 MPa was obtained in the Ti-6Al-4V ELI alloy while maintaining a ductility of δ≥11%.

Abstract: This paper studies the effect of combined SPD treatment on microstructure and mechanical properties of semi-products out of CP Ti. The combined processing, consisting of equal-channel angular pressing and further thermomechanical treatment, produced ultrafine-grained rods out of Grade 2 CP Ti with a diameter of 6.5 mm and a length of up to 1 m. It was established that the formation of homogeneous ultrafine-grained structure in Ti rod with α-grain size of about 100 nm allowed to enhance yield stress by 200% in comparison with initial annealed state.