Microstructural Refinement in Single-Phase Copper Solid Solutions by Machining
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Microstructural Refinement in Single-Phase Copper Solid Solutions by Machining S. Swaminathan, S. Chandrasekar, W. D. Compton, K. P. Trumble and A. H. King Schools of Engineering, Purdue University, 315 N. Grant Street, West Lafayette, IN, 47907-2023 ABSTRACT A study has been made of the effect of solute (Mn, Al, Ni) additions on microstructure refinement due to large strain deformation in single phase, copper solid solutions. The solutes were specifically selected for their influence on stacking fault energy (SFE) of copper, and the large strain deformation was imposed by chip formation in machining. The microstructure of Cu0.7at%Ni chip consists of elongated, sub-micrometer sized grains while Cu-7at%Al chip is made up of long, thin microbands with twins. The microstructure of the chip changes as the SFE of the material varies. With all of the solid solutions studied, the hardness of the chips is found to be significantly greater than that of the bulk material. Recrystallization temperature of solid solution chips is found to be higher than those of pure copper chips. INTRODUCTION Recent interest in ultra-fine grained (UFG) materials with enhanced mechanical properties has accentuated interest in severe plastic deformation (SPD) processes by which the grain size of metals and alloys can be made sub-microcrystalline. The SPD processes that have attracted most interest in this regard are: equal channel angular extrusion (ECAE) [1] and high pressure torsion (HPT) [2], to produce bulk and thin samples, respectively. Recently, it has been demonstrated that large plastic strains imposed in a chip, in a single stage of deformation during machining, results in significant microstructure refinement [3-5]. Machining is an attractive method for studying microstructural changes and related phenomena arising from large deformation as the conditions can be readily altered, e.g., to introduce a wide range of strain at different temperatures and strain rates. Additionally, machining is applicable to a variety of materials including high strength steels and alloys. The objective of this study was to understand the effects of solute additions to copper on the properties of resulting chips. Machining has been successfully used to create nanocrystalline copper chips with enhanced hardness [3]. The presence of a solute in copper markedly affects the stacking fault energy (SFE) of the resulting copper solid solution [6-8]. Increasing solute content in an alloy increases the dislocation density [8] and hence, the stored energy corresponding to a given state of strain. The addition of solutes is also likely to have a major influence on timetemperature behavior of an alloy in any post-deformation heat treatment. Solutes retard recovery, recrystallization and grain growth [9]. This should facilitate retention of the ultra-fine grained microstructure, and associated enhancements in mechanical properties, to much higher temperatures. Microstructure refinement and associated mechanical properties of chips were explored in single-phase
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