Abstract
Metals with a fine-grained microstructure have exceptional mechanical properties. Severe plastic deformation (SPD) is one of the most successful ways to fabricate ultrafine-grained (UFG) and nanostructured (NC) materials. Most of the SPD techniques employ very low processing speeds. However, the lowest steady-state grain size which can be obtained by SPD is considered to be inversely proportional with the strain rate at which the severe deformation is imposed. In order to overcome this limitation, methods operating at higher rates have been envisaged and used to study the fragmentation process and the properties of the obtained materials. However, almost none of these methods, employ hydrostatic pressures which are needed to prevent the material from failing at high deformation strains. As such, their applicability is limited to materials with a high intrinsic ductility. Additionally, in some methods the microstructural changes are limited to the surface layers of the material. To circumvent these restrictions, a novel facility has been designed and developed which deforms the material at high strain rate under high hydrostatic pressures. Using the facility, commercially pure aluminum was processed and analysis of the deformed material was performed. The microstructure evolution in this material was compared with that observed in static high pressure torsion (HPT) processed material.
Highlights
Fine-grained metals have been investigated extensively over the last decade which has culminated into development of many Severe plastic deformation (SPD) techniques
high pressure torsion (HPT) has been one of the most successful methods for SPD and has been employed to process a variety of materials, such as Al, Cu, Al-Cu system, Fe, Ti-Fe alloy system, Nd-Fe-B alloys system, etc. This is because HPT is capable of introducing very large shear strains in the material in a single cycle of operation as opposed to other SPD methods, Equal channel angular pressing (ECAP) or Accumulative roll bonding (ARB), where multiple passes are required to accumulate the SPD strain
A novel dynamic SPD facility, named as dynamic high pressure torsion (DHPT), is introduced wherein HPT like deformation conditions generate the SPD deformation while split Hopkinson torsion bar (SHTB) like mechanism delivers the deformation at high strain rate, onto the material held under hydrostatic pressure conditions
Summary
Fine-grained metals have been investigated extensively over the last decade which has culminated into development of many SPD techniques. During SPD, after reaching a stable region, further fragmentation of microstructure and as a result refinement of grain size cease, even if straining is continued to very large strain values. This is due to the occurrence of a steady-state regime which is considered to be largely a function of the specific deformation technique at a given temperature and strain rate [3]. Metals deforming via slip exhibit similar deformation structures when processed at higher strain rates or at lower temperatures or when the material stacking fault energy (SFE) is low. Under high strain rates suppression of the thermally activated dislocation processes can lead to high flow stresses, enough
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