Abstract
The Zn-22% Al eutectoid alloy and the Pb-62% Sn eutectic alloy were processed by high-pressure torsion (HPT) over a range of experimental conditions. Both alloys exhibit similar characteristics with significant grain refinement after processing by HPT but with a reduction in the hardness values by comparison with the initial unprocessed conditions. After storage at room temperature for a period of time, it is shown that the microhardness of both alloys gradually recovers to close to the initial unprocessed values. Electron backscatter diffraction (EBSD) measurements on the Pb-Sn alloy suggest that the self-recovery behaviour is correlated with the fraction of high-angle grain boundaries (HAGBs) after HPT processing. Thus, high fractions of HAGBs occur immediately after processing and this favours grain boundary migration and sliding which is important in the self-annealing and recovery process. Conversely, the relatively lower fractions of HAGBs occurring after annealing at room temperature are not so conducive to easy migration and sliding.
Highlights
Exceptional grain refinement to the submicrometer or even the nanometer scale is achieved most readily in bulk solids through the application of severe plastic deformation (SPD) [1]
Most emphasis to date has focused on two techniques termed equal-channel angular pressing (ECAP) where a rod or bar is pressed through a die constrained within a channel [2] and high-pressure torsion (HPT) where a disk is subjected to a high applied pressure and concurrent torsional straining [3]
3.1 Self-annealing at room temperature in the Zn-Al alloy after HPT Microstructural observations were undertaken on the Zn-Al disk processed by HPT after 1 turn
Summary
Exceptional grain refinement to the submicrometer or even the nanometer scale is achieved most readily in bulk solids through the application of severe plastic deformation (SPD) [1]. Since processing by HPT may lead to grain refinement within the nanometer range [3], this technique has attracted much attention over the last few years. There are numerous reports available describing the microstructural evolution and the enhancement of mechanical properties in both pure metals and simple alloys. For two-phase alloys, for example, several early reports demonstrated the possibility of achieving significant grain refinement in both the Zn-22% Al eutectioid alloy [4,5] and the Pb-62% Sn eutectic alloy [6]. No information is available at present on the self-annealing behavior of these two alloys after processing by HPT at room temperature. The homogenously dispersed secondary phase particles may hinder any grain growth or grain boundary movements but these two alloys have relatively low melting points
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