Abstract: The effect of chromium content (0.75, 9.85, 27%) and initial state on the thermal stability of copper-chromium alloys after severe plastic deformation has been studied by microhardness and electrical resistivity measurements. The stability of the structures is established to depend on the initial state of the alloys and on the content of chromium phase. In the low-alloy bronze, quenching before HPT substantially increases the thermal stability of the alloy relative to that observed after annealing. The softening temperature increases with increasing chromium phase content and reaches 450°C for the alloy with 27% Cr.
Abstract: An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to ~150 nm and the Vickers microhardness was significantly increased through the HPT process. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis revealed that fine precipitates with sizes of ~20 nm or smaller were formed in the Cu matrix and some particles consist of Ni and Si with no appreciable amount of Cu.
Abstract: An Fe-50at %Ni alloy was processed by high-pressure torsion (HPT) and annealed at lower homologous temperatures. Vickers microhardness and microstructural evolutions were examined with respect to the annealing time. Disks with 10 mm diameters having 0.85 mm thicknesses were subjected to HPT under a pressure of 6 GPa for 1-10 revolutions at a rotation speed of 1 rpm. The annealing after the HPT processing was conducted at homologous temperatures of ~0.3 for up to 40 days. The hardness increases with straining and saturates to a constant level of ~400 Hv at large equivalent strain. Microstructure analysis using XRD revealed that there was a peak corresponding to an α (Fe, Ni) phase.
Abstract: Nickel of 99.5% purity, with an initial grain size of ~23 μm, was subjected to equal-channel angular pressing (ECAP) up to a strain of ~12 at room temperature via route Bc. Mechanical properties and microstructures are investigated by tensile tests, microhardness tests, TEM, and EBSD observations. Results of mechanical properties show that yield strength and tensile strength increase as strain increase up to a max value( s~1009 MPa, b~1120 MPa) at ~8, and microhardness reaches its maximum of ~370HV after 12 passes. Analysis by TEM showed that grain size of pure nickel was severely refined from ~23 μm to several hundreds of nanometers after ECAP processing. Initial coarse grain are divided with lamellar boundaries and dislocation cell structures at low strain level, there has resulted in a homogenous and fine spacing of lamellar boundaries (~100 nm) after 4 passes of ECAP, low angle characters of those boundaries are revealed from corresponding SAED pattern; equiaxed grains of diameter with ~98 nm come out among lamellar boundaries after 12 passes.
Abstract: The present work deals with the evolution of mechanical properties and structure of low-carbon Fe-1,12Mn-0,08V-0,07Ti-0,1C (wt.%) steel after severe plastic deformation (SPD) and high-temperature annealing. Steel in initial ferritic-pearlitic state was deformed by equal channel angular pressing (ECAP) at T=200°C and high pressure torsion (HPT) at room temperature. The evolution of ultrafine grained structure and its thermal stability were investigated after annealing at 400-700°C for 1 hour. The results shown that SPD leads to formation of structure with an average size of (sub-) grain of 260 nm after ECAP and 90 nm after HPT. Ultrafine grained structures produced by SPD reveal a high thermal stability up to 500°C after ECAP and 400°C after HPT. At higher annealing temperatures a growth of structural elements and a decrease in microhardness were observed.
Abstract: The paper reports on microstructure, strength and fatigue of ultrafine-grained (UFG) samples of the Al-Cu-Mg-Si aluminum alloy processed by high pressure torsion (HPT) at various temperatures. Application of the HPT treatment led to strong grain refinement, as well as to a raise of the mean-root square strains and dynamic precipitation. In case of optimal HPT treatment the UFG samples have demonstrated the enhanced thermal stability, an increase in ultimate tensile strength in 2.5 times and enhancement in fatigue endurance limit by 20 % in comparison with coarse-grained alloy subjected to standard treatment. It is shown that the regime of the HPT treatment governs the volume fraction of precipitates and segregations, thereby affecting a grain size and thermal stability of ultrafine-grained structure.
Abstract: Dynamic plastic deformation of commercially pure titanium in the temperature range of -100-18°C at the strain rates of 3.0×102-2.5×103, as well as at quasi-static compression were carried out by a Split Hopkinson Pressure Bar technique and conventional compression testing machine respectively. The formation of deformation twins plays a key role on the accommodation of a large amount of strain produced by plastic deformation. Grain orientation has a great influence on the formation of twins. Temperature has smaller effects than strain rate on the evolutions of the microstructures and mechanical properties. The area fraction of twins and their intersections increase with the increasing strain rate and the deformation strain, resulting in refined microstructures and higher hardness values. Strain rate also leads to the change of twin shape (type). While more lenticular twins are observed in samples after quasi-static deformation, there are lots of needle-like twins with straight and long boundaries in samples processed via dynamic plastic deformation. This may imply that different twin systems operate at different strain rate. For the needle-like twins in samples after dynamic plastic deformation, the twin area fraction approaches saturation beyond the true strain of about 0.13, which is significant turning point for twinning rate. This saturated trend is not observed in quasi-static deformation.
Abstract: In the paper, the study was conducted on the characteristics of microstructures and orientations of UFG pure copper and 6061 aluminum alloy prepared by AARB, ECAP and CECC severe plastic deformation processes, in which SEM, TEM, EBSD and X-ray diffraction techniques were employed. The result of the study shows that microstructures, mainly composed of subgrains and dislocation kinks, are profuse in the UFG materials by the three SPDs. These kinds of microstructures have the strength of the materials much enhanced and the toughness heavily decreased. In these processes, shear deformation makes gains get finer and finer with the existence of the preferred orientations in the prepared UFG materials. Nevertheless, CECC shows the heaviest effect on the grain refining and the preferred orientation weakening.
Abstract: Microstructure evolution during annealing of a Ni-20%Cr alloy subjected to high-pressure torsion (HPT) at ambient temperature was examined. It was shown that recrystallization processes are dependent on the annealing temperature and strain. Discontinuous static recrystallization (DSRX) took place in the alloy strained to <3. Continuous grain growth (GG) occured under subsequent annealing in the material strained to ≥6. It was attributed to the fact that HPT led to the formation of nanoscale grains with an average size of 50 nm. Increasing fraction of these grains with strain led to transition from DSRX to continuous GG under subsequent annealing. The different annealing behavior was attributed to strong strain dependence for the relationship between driving pressures, which were caused by the stored dislocations and the boundary energy.
Abstract: Equal channel angular pressing (ECAP) is a promising severe plastic deformation technique for production of ultrafine-grained bulk metals with face centered cubic (fcc) structure. However, the process is often complicated in hexagonal close packed (hcp) metals such as magnesium due to its low forming capability. In this contribution, magnesium single crystals were processed by ECAP through a single pass in order to reveal processes taking place in hcp lattice during severe plastic deformation. The microstructure and texture were investigated by SEM-EBSD. The deformed microstructure contains shear bands, recrystallized regions and mechanical twins. Activity of twinning systems and texture formation are discussed regarding different initial orientation of the single crystals.