Search

A magnesium/aluminium composite was produced by room temperature consolidation through high-pressure torsion (HPT) processing. Half-discs of the pure metals were placed side-by-side and subjected to different numbers of turns. The initially reduced interface between the phases gradually increased with increasing rotation. The composite displayed a significant ductility even after 10 turns. The distribution of hardness in the HPT-processed discs was bi-modal in the early stages of processing. As the number of turns increased and the thickness of the phases decreased there was a noticeable increase in hardness. The hardness values of the composite further increased after thermal treatment due to the formation of intermetallics within the interface between the magnesium and aluminium-rich phases.

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.

An Al-7075 aluminum alloy and a ZK60 magnesium alloy were processed by a combination of equal-channel angular pressing (ECAP) for 4 passes and high-pressure torsion (HPT) through total numbers of up to 20 turns. Processing by ECAP and HPT were performed at 473 K and room temperature, respectively. Mechanical testing showed an increase in the hardness value of the Al-7075 alloy after a combination of ECAP and HPT whereas in the ZK60 alloy the hardness was reduced. Microstructural images of the Al-7075 alloy revealed very significant grain refinement after a combination of ECAP + HPT compared to the individual processing techniques. By contrast, and consistent with the hardness measurements, the average grain size of the ZK60 alloy was larger after processing by the two-step SPD technique. These results are examined and an explanation is presented based on the available microstructural evidence.

Oxygen-free copper of 99.95Â wt.% purity was severely deformed at room temperature by two modes of severe plastic deformation, equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). ECAP was performed using 4, 16 and 24 passes, and HPT was performed using 1/2, 1 and 10 turns. The results show that while recovery occurs during both ECAP and HPT processing, copper shows a faster recovery rate with HPT processing than ECAP. The occurrence of recovery was observed at an equivalent strain exceeding â¼12 that led to an enhancement in the uniform plastic deformation. The influence of recovery behaviour on the mechanical properties was investigated using X-ray diffraction, microhardness and tensile testing. Keywords: Ductility, Equal-channel angular pressing, High-pressure torsion, Recovery, Strain rate sensitivity, Work hardening

High pressure torsion provides an opportunity to process materials with low formability such as magnesium at room temperature. The present work shows the microstructure evolution in commercially pure magnesium processed using a pressure of 6.0 GPa up to 10 turns of rotation. The microstructure evolution is evaluated using electron microscopy and the hardness is determined using dynamic hardness testing. The results show that the grain refinement mechanism in this material differs from materials with b.c.c. and f.c.c. structures. The mechanism of grain refinement observed at high temperatures also applies at room temperature. The hardness distribution is heterogeneous along the longitudinal section of the discs and is not affected by the amount of deformation imposed to the material.

Ultrafine-grained materials have a strong tendency to transform into coarse-grained materials due to the high density of grain boundaries at elevated temperature. In this study, pure titanium was processed by high-pressure torsion for 10 turns to give an ultrafine-grained (UFG) structure with an average grain size of ~96 nm. The recrystallization behavior of the UFG Ti was investigated by in-situ transmission electron microscopy (TEM). It is found that the gradient microstructures with average grain size ranging from ~129 nm to ~655 nm are formed under in-situ TEM heating up to 800 °C for 30 min. The Kossel-Mollenstedt (K-M) fringes in a convergent-beam electron diffraction (CBED) pattern were used to provide an accurate measure of the sample thicknesses. The results demonstrate that grain growth is significantly suppressed in the UFG pure Ti thin film compared to bulk material. Mechanism analysis shows the combined effects of driving force and drag force on grain boundary migration is the primary cause of the grain growth inhibition in the UFG pure Ti thin film.

An investigation was conducted to examine the mechanical behavior and microstructure evolution during deformation of ultrafine-grained pure magnesium at low temperatures within the temperature range of 296–373 K. Discs were processed by high-pressure torsion until saturation in grain refinement. Dynamic hardness testing revealed a gradual increase in strain rate sensitivity up to m ? 0.2. High ductility was observed in the ultrafine-grained magnesium including an exceptional elongation of ?360% in tension at room temperature and stable deformation in micropillar compression. Grain coarsening and an increase in frequency of grain boundaries with misorientations in the range 15°–45° occurred during deformation in tension. The experimental evidence, when combined with an analysis of the deformation behavior, suggests that grain boundary sliding plays a key role in low strain rate deformation of pure magnesium when the grain sizes are at and below ?5 ?m.

An Al-7075 alloy reinforced with 10 vol% Al 2 O 3 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 μm 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 .

High purity copper was processed by high-pressure torsion (HPT) at room temperature and then stored at room temperature for periods of up to 6 weeks to investigate the effect of self-annealing. Hardness measurements were recorded both at 48 h after HPT processing and after various storage times. The results show the occurrence of recovery near the edges of the discs after processing through 1/2 and 1 turn and this leads to a significant drop in the measured hardness values which is accompanied by microstructural evidence for abnormal grain growth. Conversely, there was no recovery, and therefore no hardness drops, after processing through 5 and 10 turns. X-ray line profile analysis was used to determine the crystallite sizes and dislocation densities at 1 h after HPT and after storage for different times. The results show a good thermal stability in high purity Cu after processing through more than 1 turn of HPT but care must be exercised in recording hardness measurements when processing through only fractional or very small numbers of turns.

A grade 2 pure titanium with an initial grain size of ≈50 μm is processed by high-pressure torsion (HPT) at room temperature under an imposed pressure of 6.0 GPa. The microhardness variations are examined and the results show that the disks are reasonably homogeneous after 10 turns of torsional straining. The microstructural evolution is systematically characterized by optical microscopy, X-ray diffraction, and transmission electron microscopy to provide information on the effect of shear strain on grain size and microstructure. The results demonstrate that the initial coarse structure is gradually refined from the edge to the center of the disk under the shear stress during HPT processing and an ultrafine-grained pure Ti is achieved with an average grain size of ≈96 nm after 10 turns. A model is developed by considering the formation of subgrain boundaries, twins, and high-angle grain boundaries for the grain refinement of pure Ti processed by HPT.

We examine the effect of grain size on the dynamic failure of tantalum during laser-shock compression and release and identify a significant effect of grain size on spall strength, which is opposite to the prediction of the Hall-Petch relationship because spall is primarily intergranular in both poly and nanocrystalline samples; thus, monocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultra-fine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra - and intergranular in the poly and ultra-fine-grained crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is a strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. The length of geometrically-necessary dislocations required to form a grain-boundary (intergranular) void is lower than that of grain-interior (intragranular) void with the same maximum diameter; thus, the energy required is lower. Consistent with prior literature and theory we also identify an increase with spall strength with strain rate from 6 × 106 to 5 × 107 s−1. Molecular dynamics calculations agree with the experimental results and also predict grain-boundary separation in the spalling of polycrystals as well as an increase in spall strength with strain rate. An analytical model based on the kinetics of nucleation and growth of intra- and intergranular voids and extending the Curran-Seaman-Shockey theory is applied which shows the competition between the two processes for polycrystals.

A ZK60 magnesium alloy with an initial grain size of ~10 μm was processed by high-pressure torsion (HPT) through 5 revolutions under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. An average grain size of ~700 nm was achieved after HPT with a high fraction of high-angle grain boundaries. Tensile experiments at room temperature showedpoor ductility. However, a combination of reasonable ductility and good strength was achieved with post-HPT annealing by subjecting samples to high temperatures in the range of 473 to 548 K for 10 or 20 min. The grain size and texture changes were also examined by electron back scattereddiffraction (EBSD) and the results compared to long-term annealing for 2500 min at 450 K. The results of this study suggest that a post-HPT annealing for a short period of time may be effective in achieving a reasonable combination of strength and ductility.

The influence of High-Pressure Torsion (HPT) on texture and superplasticity in an Al-7075 was studied using X-ray diffraction and tensile testing. The alloy was processed by HPT at room temperature under a pressure of 6.0 GPA up to a maximum of 20 turns. The pole figures were measured at mid-radius of the disks after 1, 5, 10 and 20 turns. The results show the presence of a typical torsion texture during HPT, in particular, the C{001} component was found to develop preferentially. With increasing deformation, the A {111} and the C components are reinforced after 5 turns and the texture tends to be random with the presence of a fibre texture near the center. Moreover, the fraction of C components tends to gradually decrease and a fairly isotropic microtexture is apparent after 20 turns. Tensile testing showed the development of excellent superplastic properties in this alloy with elongations up to ~700% when testing at a temperature of 623 K.

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.

Early published results have demonstrated that high purity Cu and a Pbâ62% 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 Pbâ62% 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 Pbâ62% 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 Pbâ62% Sn alloy contribute to the increase in hardness with coarsening grain size during room temperature storage. Keywords: High-pressure torsion, High purity Cu, Pbâ62%Sn alloy, Self-annealing

An extruded ZK60 magnesium alloy was processed by high-pressure torsion (HPT) at room temperature for up to 5 turns under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. This processing produced an average grain size of ∼700 nm. The grain size distributions and textures were examined by electron backscatter diffraction (EBSD) and this revealed some multi-modality in the microstructure at different stages of straining with fractions of both coarse grains and ultrafine grains. EBSD analysis at the mid-radius positions of unprocessed and HPT-processed materials revealed a gradual evolution from a prismatic { 10 1 ¯ 0 } fiber to an ultimate basal { 0001 } fiber texture with the c-axis parallel to the normal direction. The majority of grain boundaries had misorientations larger than 15° throughout the processing. The strain hardening tended towards a reasonable hardness homogeneity with a hardenability exponent, η, of 0.07 up to strains of ∼20 and with a subsequent hardness saturation at Hv ≈ 125.

Experiments were conducted on a commercial Al-7075 alloy by processing by ECAP for 4 passes and then processing by HPT for up to a maximum of 20 turns. Measurements show that the grains were refined to ~680 nm after ECAP and to ~310 nm in the center of the disk after ECAP+HPT. Tensile testing at a temperature of 623 K revealed lower flow stresses and higher elongations to failure after processing by ECAP+HPT. Thus, the alloy was not superplastic after processing by ECAP but superplasticity was achieved with elongations up to ~800% after processing by ECAP+HPT. By plotting the Vickers microhardness against equivalent strain, it is shown that the hardness saturates at Hv≈250 after ECAP+HPT. This saturation hardness is higher than the value of Hv≈230 recorded after processing by HPT without a preceding step of ECAP. The results demonstrate that processing by ECAP+HPT produces higher hardness and greater grain refinement than processing only by HPT.

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, 〈a〉-type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for 〈a〉-type dislocations, leading to a slight strain softening. The 〈c〉-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and 〈c〉-component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.

Processing by high-pressure torsion (HPT) was performed on disks of an Al-7075 alloy at room temperature. The alloy was initially annealed at 753 K and then processed by HPT under a pressure of 6.0 GPa up to a maximum of ten turns. Measurements of the Vickers microhardness showed lower values at the centers of the disks after small numbers of turns but higher numbers of turns led to a reasonable hardness homogeneity across each disk. After five turns, the grain size at the edge of the disk was ~250 nm. It is demonstrated that results from mechanical testing are consistent with the hardness and microstructural data.

High-pressure torsion provides the opportunity to introduce significant plastic strain at room temperature in magnesium and its alloys. It is now established that this processing operation produces ultrafine-grained structures and changes the properties of these materials. The present paper shows that the mechanism of grain refinement differs from f.c.c. and b.c.c. materials. It is shown that fine grains are formed at the grain boundaries of coarse grains and gradually consume the whole structure. Also, the processed material exhibits unusual mechanical properties due to the activation of grain boundary sliding at room temperature.