1,721,022 research outputs found
Concurrent microstructural evolution of ferrite and austenite in a duplex stainless steel processed by high-pressure torsion
No full textA duplex stainless steel with approximately equal volume fractions of ferrite and austenite was processed by high-pressure torsion. Nano-indentation, electron backscatter diffraction and transmission electron microscopy were used to investigate the hardness and microstructure evolutions of the steel. Despite the different strain-hardening rates of individual ferrite and austenite, the microstructures of the two phases evolved concurrently in such a way that the neighbouring two phases always maintained similar hardness. While the plastic deformation and grain refinement of ferrite occurred mainly via dislocation activities, the plastic deformation and grain refinement process of austenite were more complicated and included deformation twinning and de-twinning in coarse grains, grain refinement by twinning and dislocation–twin interactions, de-twinning in ultrafine grains and twin boundary subdivision
A visualization of shear strain in processing by high-pressure torsion
No full textOptical 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 vortice
The role of stacking faults and twin boundaries in grain refinement of a Cu–Zn alloy processed by high-pressure torsion
No full textA recent model developed to predict the smallest grain sizes obtainable by severe plastic deformation has worked well for materials with medium to high stacking fault energies (SFEs) but not for those with low SFEs. To probe this issue, experiments were conducted using a Cu–30 wt.% Zn alloy with a very low SFE of 7 mJ/m2 as the model material. High-pressure torsion was used as the grain refinement technique. The results indicate that stacking faults and twin boundaries play a key role in the grain refinement process such that the smallest achievable grain size is determined by the highest stacking fault and twin density that the system is able to produce. An amorphization of grain boundaries was also observed in the final structure. These observations are very different from those reported for materials having medium to high SFEs and they confirm the operation of a different grain refinement mechanism.<br/
Grain boundary formation by remnant dislocations from the de-twinning of thin nano-twins
No full textWe report a grain boundary formation mechanism in face-centred cubic metals with low stacking fault energies. Severe plastic deformation produces primary nano-twins with a twin boundary spacing of several nanometres, followed by secondary twinning through the activation of Shockley partial dislocations. The partial dislocations interact with primary twin boundaries, resulting in de-twinning of the primary twins and producing very high densities of sessile dislocations. The accumulation of these dislocations produces new grain boundaries with neighbouring grains having similar orientations
Influence of equal-channel angular pressing on pPrecipitation kinetics in an Al-Zn-Mg-Cu alloy
No full textProcessing by equal-channel angular pressing (ECAP) affects the morphology of ? precipitates in an Al–Zn–Mg–Cu (Al-7136) alloy. It is shown by transmission electron microscopy that ECAP changes the orientation of precipitates and this influences the atomic configuration and the interfacial energy at the ?/?-Al interfaces. Consequently, ? precipitates adopt an isotropic growth mode and evolve into equiaxed particles. A three-dimensional atom probe analysis demonstrates that large ? precipitates formed in different numbers of ECAP passes are of similar composition. The coalescence of smaller precipitates, rather than the fragmentation of larger precipitates, dominates the precipitate evolution
The effect of dislocation density on the interactions between dislocations and twin boundaries in nanocrystalline materials
No full textThe interactions between dislocations and twinboundaries (TBs) are significantly affected by both intrinsic material properties and extrinsic factors, including stacking fault energy, the energy barriers for dislocation reactions at TBs, twin thickness and applied stress. In this study, dislocation–TB interactions in grains with different dislocationdensities were investigated and we conclude that the dislocationdensity also affects the dislocation–TB interactions. In a twinned grain with a low dislocationdensity, a dislocation may react with a TB to fully or partially penetrate the TB or to be absorbed by the TB via different dislocation reactions. Alternatively, in a twinned grain with a high dislocationdensity, dislocations tangle with each other and are pinned at the TBs, thereby making it unfavourable for further dislocation reactions to mediate dislocation penetration across the TB. This leads to an accumulation of dislocations at the TBs, raising the local strain energy, which, in turn, is released by the activation of secondary twins by partial dislocation emissions from the other side of the T
De-twinning via secondary twinning in face-centered cubic alloys
No full textWe report a de-twinning process via secondary twinning in face-centred cubic structures with low stacking fault energies. A duplex stainless steel was deformed using high-pressure torsion. Primary twins with an average twin boundary (TB) spacing of ?7 nm formed in the early stages of the deformation and this was followed by secondary twinning. The partial dislocations from the secondary twinning subsequently interacted with the primary TBs, leading to the de-twinning of the primary twins. As a result of the de-twinning process, the secondary twins with an average TB spacing of ?1.7 nm reached a maximum length of ?200 n
Microstructural evolution of Fe-rich particles in an Al-Zn-Mg-Cu Alloy during equal-channel angular pressing
No full textThe microstructures of a severely deformed Al–Zn–Mg–Cu (AA7136) alloy have been characterized carefully using transmission electron microscopy and three-dimensional atom probe analysis. The Fe-rich intermetallic particles are predominantly Al13Fe4 type in the as-extruded alloy. Significantly, equal-channel angular pressing (ECAP) at 200 °C refines Fe-rich particles from 1 to 2 ?m to as small as 50 nm after 4 passes processing, and effectively narrow down their size distribution with the increase of number of ECAP passes. In addition, small Fe-rich particles evolve into spherical morphology and are in a more uniform distribution. The formations of Fe-rich phases in AA7136, the kinetic and thermodynamic effects in relation to the refinement of Fe-rich particles and their morphology evolution during ECAP processing are discussed.<br/
Twinning via the motion of incoherent twin boundaries nucleated at grain boundaries in a nanocrystalline Cu alloy
No full textUnusual macroscopic shearing patterns observed in metals processed by high-pressure torsion
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