An investigation of bulk nanocrystalline copper fabricated via severe plastic deformation and nanoparticle consolidation

Ultrafine grained (UFG) and nanocrystalline materials have attracted considerable interest because of their unique mechanical properties as compared with coarse grained conventional materials. The fabrication of relatively large amounts of these materials still remains a challenge, and a thorough understanding of the relationship between microstructure and mechanical properties is lacking. The objective of this study was to investigate the mechanical properties of UFG and nanocrystalline copper obtained respectively by a top down approach of severe plastic deformation of wrought copper and a bottom up approach of consolidation of copper nanoparticles using equal channel angular extrusion (ECAE). A critical assessment and correlation of the mechanical behavior of ECAE processed materials to the microstructure was established through the determination of the effect of strain level and strain path on the evolution of strength, ductility and yield anisotropy in UFG oxygen free high conductivity copper in correlation with grain size, grain morphology and texture. ECAE was shown to be a viable method to fabricate relatively large nanocrystalline consolidates with excellent mechanical properties. Tensile strengths as high as 790 MPa and fracture strain of 7 % were achieved for consolidated 130nm copper powder. The effects of extrusion route, number of passes and extrusion rate on consolidation performance were evaluated. The relatively large strain observed was attributed to the bimodal grain size distribution and accommodation by large grains. The formation of bimodal grain size distribution also explains the simultaneous increase in strength and ductility of ECAE processed wrought Cu with number of passes. Texture alone cannot explain the mechanical anisotropy in UFG wrought copper but we showed that grain morphology has a strong impact and competes with texture and grain refinement in controlling the resulting yield strength. Tension-compression asymmetry was observed in UFG wrought copper. This asymmetry is not always in favor of compression as reported in literature, and is also influenced by grain morphology through the interaction of dislocations with grain boundaries. Different prestrains in tension and compression should be experimented to have a better understanding of the encountered anisotropy in Bauschinger parameter in relation with the observed tension-compression asymmetry

Characterizations of severely deformed and annealed copper

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 120-123).Issued also on microfiche from Lange Micrographics.The objectives of this study were to characterize severely plastically deformed and recrystallized oxygen free high conductivity copper, to determine texture transformation potential of Equal Channel Angular Extrusion (ECAE) and to investigate the possibility of converging different initial textures. The initial material was processed to have an average grain diameter of 50[u]m. The effects of different combinations of heat treatment, routes of extrusion (A, B and C) and plastic strains of 2.32 and 4.64 were examined. Hardness measurements were used to determine the recrystallization temperature of each ECAE path. Optical microscopy and X-ray diffraction were used for grain morphology and texture analyses. Optical microscopy revealed uniform microstructures for different processing routes. After recrystallization heat treatment, the nucleation of new grains from heavily deformed material was found to occur along sites with heavy distortions. Nucleation sites for recrystallized grains start in shear bands with subsequent growth in the direction of slip lines. Processing that causes intersection of shear planes creates more sites for nucleation and leads to a shifting of the recrystallization curve to lower temperatures. The one dimensional nature of shear bands lessens the opportunity for nucleation in route A decreasing consequently the beginning of nucleation. All recrystallization curves exhibit a slight increase in hardness prior to the sharp drop that accompanies recrystallization. Relatively weak textures are developed during processing of copper 101 via ECAE. The texture after multipass processing via route A is found to be near the {110}. An intermediate rotation of the billet of ±90° during processing is found to produce a partial fiber texture: this texture is not as pronounced as a typical drawing texture and can be best described by partial fiber textures and along the extrusion direction. Route C leads to the formation of a sheet texture which is not removed when a reversal strain is applied during the even pass but rather is intensified by decreasing the component parallel to the rolling direction. ECAE is found to be a powerful tool to converge different initial textures in copper. Recrystallization leads to similar textures for copper processed by different multipass routes