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X20CrMoV12-1 steel is one of the most common martensitic steel grades that is known for its excellent heat-resistant and superior performance against oxidation and creep. Consequently, it has been used as a replacement for the low-alloy ferritic 2.25Cr-1Mo steel reheater pipes in a desalination power plant. Fracture occurred unexpectedly in some X20 steel reheater pipes of the boiler. Visual inspection revealed clear signs of wall thinning and corrosion at the fireside of the failed tube. Assessment of the tube cross-section included metallurgical examination, indicating that the microstructure is tempered martensite with no clear indications of thermal degradation or voids formation by creep. This was consistent with the microhardness measurements showing a typical average Vickers microhardness of 240 Hv. In addition, evaluation of oxide layer was made using Energy-dispersive X-ray spectroscopy (EDS) showing formation of corrosive deposits on top of the protective oxide layer at the fireside. The deposits contain excessive amounts of sulfur and vanadium that were initially present in the firing fuel. Remedial actions should be necessarily taken to minimize high-temperature corrosion by including fuel additives such as magnesium oxide or alumina to increase the melting point of vanadium and sulfur compounds. Also, improving the combustion efficiency, e.g. use of improved atomizer, with less air should be considered to minimize corrosion attacks.

Tests are conducted to evaluate the effect of long-term storage on the microstructure and microhardness of an oxygen-free high conductivity (OFHC) copper after processing by high-pressure torsion (HPT) for various numbers of revolutions at ambient temperature. Results are presented for samples subjected to storage at room temperature through periods of either 1.25 or 7 years. The results show that an increase in storage time leads to a coarsening of the ultrafine-grained structure produced by HPT processing and a corresponding decrease in the microhardess where this is associated with the occurrence of recrystallization and grain growth. Plots of hardness against equivalent strain reveal a three-stage behavior with much lower hardness values over a range of equivalent strains of ~2-8. This behavior is similar after both storage periods but the hardness values are lower and the grain sizes are larger after storage for the longer time. The results demonstrate that long-term storage has a significantly detrimental effect on the microstructure and hardness of ultrafine-grained OFHC Cu

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.

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

Disks of an Al-6061 metal matrix composite, reinforced with 10 vol.% Al 2 O 3 particles, were processed by high-pressure torsion (HPT) at room temperature for 1/4, 1/2, 1, 5 and 10 turns under an applied pressure of 6.0 GPa. The evolution of microstructure was investigated using optical microscopy and scanning electron microscopy. During HPT processing the average grain size within the aluminum matrix decreased from ∼ 35 μm in the unprocessed condition to ∼ 170 nm after processing through 10 turns but there was no significant effect on the size and distribution of the alumina particulate clusters. The values of the Vickers microhardness were recorded across the surface of each disk and then plotted as two-dimensional and three-dimensional color-coded contour maps. The results show the hardness increases from ∼ 56 Hv in the initial condition to ∼ 165 Hv after HPT for 10 turns. The results demonstrate that, as in many unreinforced metallic alloys, the evolution of hardness with strain exhibits strain hardening without any significant recovery.

A commercial Al-7475 alloy with an initial grain size of ~40 μm was processed by high-pressure torsion (HPT) for up to 2 turns at room temperature under a pressure of 6.0 GPa. The mechanical properties of the processed materials were evaluated by the ball-indentation technique to give information on the yield strength and the ultimate tensile strength. Following HPT, microhardness measurements revealed a steady increase in the hardness values from the centers of the samples towards the edges. After 2 turns, the ultimate tensile strength was ~1050 MPa at the edge of the disk and the measured grain size was ~70 nm. The results demonstrate the potential for using HPT to achieve excellent grain refinement in the Al-7475 alloy.

Aluminum of commercial purity was processed by equal-channel angular pressing (ECAP) through two, four and eight passes at room temperature. A series of [1 1 4] convergent-beam electron diffraction (CBED) zone axis patterns were obtained using an electron probe with a diameter of 20 nm. Observations were recorded both immediately adjacent to the grain boundaries and in the grain interiors. Symmetry breaking of the higher-order Laue zone (HOLZ) lines was observed adjacent to the boundaries after two and four passes but not in the grain interiors. Pattern simulation of the CBED patterns taken from the two- and four-pass samples adjacent to the boundaries revealed a homogeneous strain with compressive and shear components. The presence of these homogeneous strains demonstrates that the internal stresses associated with the deformation of aluminum at room temperature are localized in the close vicinity, to within ∼20 nm, of the grain boundaries.

Processing by high-pressure torsion was conducted on four different materials: high-purity (99.99%) aluminum, commercial purity (99.5%) aluminum, an Al-1% Mg solid solution alloy and a commercial aluminum Al-6061 alloy. Disks of each material were processed through 1/4, 1 and 5 turns and detailed microhardness measurements were recorded to permit the construction of color-coded hardness maps and three-dimensional representations of the hardness distributions. There are significant differences between these four materials. Whereas the hardness is initially high in the centers of the HPT disks of high-purity aluminum, the hardness is initially low in the centers of the disks for the other three materials. The hardness achieves saturation values after 5 turns in high-purity Al and the commercial purity Al but more torsional strain is required to achieve homogeneity in the other two alloys. There is evidence that it is difficult to achieve a well-defined saturation hardness in the Al-6061 alloy.

Experiments were conducted on commercial purity titanium to evaluate the microstructure and mechanical properties after processing by equal-channel angular pressing (ECAP) at room temperature. Samples were successfully processed through 1 and 2 passes at two different pressing speeds using an ECAP die with a channel angle of 135?. Following ECAP, a microstructural evaluation showed evidence for the occurrence of basal, prismatic and pyramidal slip together with {1 0¯1 2}? ¯10 1 1?twinning. Hardness measurements confirmed the occurrence of homogeneous strengthening throughout the samples. Tensile testing revealed a high ultimate tensile strength of ?750 MPa after 2 passes of ECAP but with a reduction in the measured elongation to failure.

An electrochemically deposited nanocrystalline supersaturated face-centred-cubic Ni–21 at.% Fe alloy with an initial average grain size of ∼21 nm was processed using high-pressure torsion (HPT) that resulted in grain growth via grain rotation and coalescence to an average grain size of ∼53 nm. Atom probe tomography investigations revealed that the supersaturated Ni–Fe solid solution was stable under HPT and that C and S atoms, which are the major impurities in the material and segregated to the grain boundaries (GBs) of the as-deposited material, migrated from disappearing GBs to the remaining GBs during HPT. We propose that the elemental redistribution was facilitated by GB diffusion and the motion of a large volume of HPT-induced defects at the GB regions during the grain growth process. This elemental redistribution process is different from other HPT-induced elemental redistribution processes reported in the literature.

The strain response of an electrochemically deposited nanocrystalline Ni–20 wt.% Fe alloy processed by high-pressure torsion (HPT) was investigated by monitoring changes in hardness. Strain hardening was observed in the very early stage of HPT, followed by strain softening before the onset of a second strain hardening stage. Structural investigations revealed that the two hardening stages were associated with an increase in dislocation density, whereas the strain softening stage was accompanied by a reduction in the dislocation and twin densities, thereby demonstrating the main dependence of hardness on the dislocation density in this material. Grain growth occurred during HPT and its role in the hardness evolution is also discussed.

Billets of a commercial-purity aluminum Al-1050 alloy were processed by equal-channel angular pressing (ECAP) at room temperature for up to six passes and microhardness measurements were recorded on the longitudinal and cross-sectional planes of each billet. Large numbers of datum points were recorded in order to minimize the errors in the results. The measurements show the hardness increases significantly after the first pass and then increases by very small amounts in subsequent passes. There are regions of lower hardness running in bands near the top and bottom surfaces of each billet. With increasing numbers of passes, the lower hardness region near the top surface disappears and the region near the lower surface remains in place but becomes less extensive. Neglecting the very small region of lower hardness near the bottom surface, the results show there is a potential for achieving excellent three-dimensional homogeneity after six passes of ECAP.

The structural evolution of a nanocrystalline Ni–Fe alloy induced by high-pressure torsion (HPT) was investigated. HPT-induced grain growth occurred via grain rotation and coalescence, forming three-dimensional small-angle sub-grain boundaries. Further deformation eliminates the sub-grain boundaries from which dislocations glide away on different {1 1 1} planes. A significant number of these dislocations come together to form Lomer–Cottrell locks that effectively increase the dislocation storage capacity of the nanocrystalline material. These observations may help with developing strong and ductile nanocrystalline materials.

High-pressure torsion (HPT) was used to process Al–2% Si and Al–2% Si–0.25% Sc alloys for up to five turns and the mechanical properties of the processed materials were evaluated using the ball indentation technique (BIT). The results show that the presence of Al3Sc precipitates is effective in producing higher strength levels and greater grain refinement in the Al–2% Si–0.25% Sc alloy. The introduction of scandium reduces the grain size of the Al–2% Si alloy from ∼0.38 to ∼0.15 μm after 5 turns of HPT and the corresponding maximum tensile strength is increased from ∼325 to ∼375 MPa. The grain and substructure formation in the Al–2% Si alloy is similar to aluminum with dislocation cell formation and a reasonably recovered microstructure whereas in the Al–2% Si–0.25% Sc alloy it is non-homogeneous with arrays of non-equilibrium boundaries and dislocation tangles within the grains.

The effect of grain size on the deformation twinning and de-twinning in a nanocrystalline Ni-Fe alloy was investigated using transmission electron microscopy. Specimens with different grain sizes were obtained by severely deforming an electrochemically deposited nanocrystalline Ni-20wt.% Fe alloy using high-pressure torsion, which resulted in continuous grain growth from an average grain size of ~ 21 nm in the as-deposited material to ~ 72 nm for the highest strain applied in this study. Results show that deformation de-twinning occurs at very small grain sizes while deformation twinning takes place when the grain size is larger than ~ 45 nm. The mechanism of the observed grain size effect on twinning and de-twinning is briefly discussed.

Disks of as-extruded Mg-4Nd alloy were processed by high-pressure torsion (HPT) through ¼ to 5 turns at room temperature. The first 1/4 turn of HPT induces large numbers of twins and some dislocation tangles in the center region of the disk. With increase of torsional straining, the twinning is inhibited gradually and the dislocation density increases relating to the formation of dislocation substructures and ultimately transforming to high fractions of equiaxed gains which have an average grain size of ~200 nm and high-angle boundaries. HPT significantly improves the values of microhardness of this alloy. The hardness values in both the central and edge regions show a sharp rise after HPT for 1/4 turn and exhibit nearly saturation after 1/2 turn although there is a trend of a slight increase with increasing numbers of turns. The experimental results suggest more homogeneous microstructures may be produced by larger numbers of turns in the HPT process.

Disks of a commercial purity aluminium Al-1050 alloy and Al-1%Mg alloy were processed by high-pressure torsion (HPT) at room temperature for up to a maximum of 5 turns under a pressure of 6 GPa. Following processing, hardness measurements were recorded across the surfaces of the disks. These measurements showed low values of hardness at the center and high values near the edges of the disks and the hardness increased in both alloys with increasing numbers of turns. The evolution of homogeneity in hardness was rapid in Al-1050 compared to the Al-1%Mg alloy. After 5 turns of HPT under a pressure of 6 GPa, the hardness was fully homogeneous across the total surface of the Al-1050 disk whereas there was a region of lower hardness around the center of the Al-1%Mg disk. The results reveal the significant difference between both alloys where the higher rate of recovery in the Al-1050 alloy leads to a rapid evolution of the hardness homogeneity.

Room temperature equal-channel angular pressing (ECAP) was employed on commercial purity titanium in the present work. Mechanical twining was observed in ~90% grains while the grains were not significantly refined (~10 μm) after ECAP. TEM observations showed that the twins observed under OM are usually composed of a serial of parallel twin bands with a width of ~1 μm. Microhardness and tensile tests showed that such a coarse-grained microstructure combined with a high fraction of mechanical twins has a microhardness of ~ 240 Hv, yield strength of ~ 730 MPa, tensile strength of ~ 740 MPa and elongation to failure of ~ 16%. This yield strength is much higher than 620-640 MPa, the yield strength of ultrafine-grained titanium by 8~12 passes of ECAP at 450 oC with a grain size of 200-300 nm, and is close to ~ 790 MPa, the yield strength of commercial Ti-6Al-4V alloys.

Al-2 wt. % Si alloys with and without 0.25 wt. % scandium additions were processed by high-pressure torsion up to five turns at room temperature under a pressure of 6.0 GPa. Microstructural examination of the as-cast Al-2Si-0.25Sc alloy revealed the presence of Al3Sc precipitates which refined the Al grain structure, whereas no major changes were observed in the morphology of the Si particles. Processing by HPT of both experimental alloys revealed submicrometer grains with uniformly distributed Si particles. The mechanical properties were obtained using hardness measurements and the ball-indentation technique. The results show the hardness increased in the first turn of HPT and further increased with increasing numbers of turns. In addition, the hardness values were lower at the centers and continuously increased towards the edges of the disks. The difference in hardness values between the centre and the edge decreased with increasing turns, thereby suggesting an increasing homogeneity with increasing processing. The scandium addition and HPT processing of the Al-2Si alloy strongly influences the grain refinement and mechanical properties. The grain size reduction in the Al-2Si alloy was similar to Al whereas the presence of Sc in Al-2Si during HPT processing was responsible for large precipitation networks and a submicrometer grain formation.

A duplex stainless steel was processed by high-pressure torsion (HPT) and then examined by optical microscopy. The results reveal unusual flow patterns including double-swirl strains, shear vortices, and the presence of significant local turbulence. Similar flow behavior was also visible in disks of high-purity aluminum and a Zn–22%Al eutectoid alloy. These complex flow patterns and the presence of double-swirls are consistent with the presence of a Kelvin–Helmholtz instability during HPT processing where this may arise if there are local shear velocity gradients between adjacent positions within the HPT disks.

Optical microscopy was used to examine the shear strain imposed in duplex stainless steel disks during processing by high-pressure torsion (HPT). The results show a double-swirl pattern emerges in the early stages of HPT and the two centres of the swirl move towards the centre of the disk with increasing revolutions. Local shear vortices also develop with increasing numbers of revolutions. At 20 revolutions, there is a uniform shear strain pattern throughout the disk and no local shear vortices.

An ultra-high purity oxygen free high conductivity (OFHC) Cu was investigated to determine the evolution of microstructure and microhardness during processing by high-pressure torsion (HPT). Disks were processed at ambient temperature, the microstructures were observed at the center, mid-radius and near-edge positions and the Vickers microhardness was recorded along radial directions. At low strains, Σ3 twin boundaries are formed due to dynamic recrystallization before microstructural refinement and ultimately a stabilized ultrafine grain structure is formed in the near-edge position with an average grain size of ~280 nm after 10 turns. Measurements show the microhardness initially increases to ~150 Hv at an equivalent strain of ~2, then falls to about ~80 Hv during dynamic recrystallization up to a strain of ~8 and thereafter increases again to a saturated value of ~150 Hv at strains above ~22. The delay in microstructure and microhardness homogeneity by dynamic recrystallization is attributed to the high purity of Cu that enhances dislocation mobility and causes dynamic softening during the early stages of straining.

Pure aluminum deformed in pure shear at elevated temperature reaches a broad “peak” stress and then undergoes about a 17 pct decrease in flow stress with deformation with, roughly, 1 to 2 equivalent uniaxial strain. Beyond this strain, the flow stress is approximately constant. The sources for this softening are unclear. The suggested basis includes texture softening, microstructural softening, enhanced dynamic recovery, and discontinuous dynamic recrystallization. Experiments were performed in which specimens were deformed in torsion to various strains within the softening regime followed by compression tests at ambient and elevated temperature. Analysis of the compressive yield strengths indicate that the softening is most likely substantially explained by a decrease in the average Taylor factor.

The effect of the initial annealing temperature on the evolution of microstructure and microhardness in high purity OFHC Cu was investigated after processing by HPT. Disks of Cu were annealed for one hour at two different annealing temperatures, 400 and 800°C, and then processed by HPT at room temperature under a pressure of 6.0 GPa for 1/4, 1/2, 1, 5 and 10 turns. Samples were stored for 6 months after HPT processing to examine the self-annealing effects. Electron backscattered diffraction (EBSD) measurements were recorded for each disk at three positions: center, mid-radius and near edge. Microhardness measurements were also recorded along the diameters of each disk. Both alloys showed rapid hardening and then strain softening in the very early stages of straining due to self-annealing with a clear delay in the onset of softening in the alloy initially annealed at 800°C. This delay was due to the relatively larger initial grain size compared to the alloy initially annealed at 400°C. The final microstructures consisted of homogeneous fine grain sizes having average sizes of ~0.28 and ~0.34 μm for the alloys initially annealed at 400 and 800°C, respectively. A new model is proposed to describe the behavior of the hardness evolution by HPT in high purity OFHC Cu.

An aluminum-copper alloy (Al-2024) was successfully subjected to high-pressure torsion (HPT) up to five turns at room temperature under an applied pressure of 6.0 GPa. The Al-2024 alloy is used as a fuselage structural material in the aerospace sector. Mechanical properties of the HPT-processed Al-2024 alloy were evaluated using the automated ball indentation technique. This test is based on multiple cycles of loading and unloading where a spherical indenter is used. After two and five turns of HPT, the Al-2024 alloy exhibited a UTS value of ~1014 MPa and ~1160 MPa respectively, at the edge of the samples. The microhardness was measured from edges to centers for all HPT samples. These results clearly demonstrate that processing by HPT gives a very significant increase in tensile properties and the microhardness values increase symmetrically from the centers to the edges. Following HPT, TEM examination of the five-turn HPT sample revealed the formation of high-angle grain boundaries and a large dislocation density with a reduced average grain size of ~80 nm. These results also demonstrate that high-pressure torsion is a processing tool for developing nanostructures in the Al-2024 alloy with enhanced mechanical properties.