Abstract
The presence of hydrostatic pressure is a general crucial characteristic of severe plastic deformation methods for reaching high strains and for introducing large quantities of lattice defects, which are necessary to establish new grain boundaries. Insights into the processes occurring during deformation and the influence of hydrostatic pressure are necessary to help better understand the SPD methods. A special experimental procedure was designed to simulate the hydrostatic pressure release: High pressure torsion (HPT)-deformed microstructure changes related to the release of hydrostatic pressure after the HPT deformation of copper and nickel were studied by freezing the sample before releasing the pressure. High-resolution in-situ X-ray diffraction of the heating process was performed using synchrotron radiation in order to apply X-ray line profile analysis to analyze the pressure release. The results on copper and nickel generally indicated the influence of hydrostatic pressure on the mobility and interaction of deformation-induced defects as well as the resulting microstructure.
Highlights
Severe plastic deformation (SPD) imposes strains up to several orders of magnitude, which can be provided by special deformation techniques and tools
In the presentation of results and their discussion hereafter, the following expressions will be used to describe the sample/material treatment and condition: Loaded state: The material was deformed by high-pressure torsion to a certain strain, the torsion was stopped but the hydrostatic pressure was maintained—experimentally, this “deformed state under pressure” was conserved by cooling down the sample with liquid nitrogen, releasing the pressure after cooling, and keeping it cooled there until the diffraction experiment
Unloaded state: This refers to after the hydrostatic pressure was released—the material was slowly warmed from liquid nitrogen to room temperature—the in-situ diffraction experiments investigated this phase
Summary
Severe plastic deformation (SPD) imposes strains up to several orders of magnitude, which can be provided by special deformation techniques and tools. In addition to the principles of conventional plastic deformation, the presence of hydrostatic pressure is a key characteristic of most SPD methods. It is usually achieved with special tool designs that prevent the material from free flow. Iterative folding and deformation sequences feature a hydrostatic pressure component to some extent. All together these allow very high strains to be achieved, and subsequently enable the generation of extremely high amounts of lattice defects, which are responsible for grain fragmentation down to submicron- or even nanometer-scale. The hydrostatic pressure affects intrinsic material properties such as the Young’s modulus and the flow stress, and in [1] was estimated to increase the flow stress by 15% per 1 GPa pressure
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