Tensile Deformation Behavior of Bulk Ultrafine Grained Copper

Tensile deformation behavior including strain hardening and failure mechanism of bulk ultrafine grained (grain size 100 - 300 nm) Cu has been systematically analyzed and compared with that of nanocrystalline and coarse grained Cu. Bulk ultrafine grained Cu has been prepared by conventional rolling of coarse grained Cu at cryogenic temperature followed by controlled recrystallization. Microstructural characterization has been performed by using x-ray diffraction analysis and transmission electron microscopy and the observations are supplemented by electrical conductivity and hardness measurements. Microstructural investigations suggest that controlled annealing of heavily cold worked Cu results into ultrafine grained structure with bimodal grain size distribution, where small volume of micrometer sized grains are embedded inside a matrix of ultrafine grains. Evaluation of tensile properties indicates that ultrafine grains significantly improve the strength of Cu in comparison to its coarse grained counterpart, and the measured strength values follow the conventional Hall-Petch relationship within the investigated grain size range. The inhomogeneous microstructure induces strain hardening and increases the strain hardening exponent values, specifically at early stage of deformation, and thereby stabilized deformation leading to higher tensile ductility than that commonly reported for the nanocrystalline/ultrafine grained materials. Present study indicates that conventional rolling at subzero temperature followed by controlled recrystallization may be utilized as an effective method for development of bulk Cu with enhanced toughness.