Dynamic Recrystallization During High-Strain-Rate Tension of Copper
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he term recrystallization refers to a process where a deformed microstructure is replaced by new defect-free grains in order to decrease the stored energy from plastic deformation. When recrystallization occurs during on-going deformation, it is referred to as dynamic recrystallization (DRX), which is an important phenomenon in many metal working processes, such as hot rolling, extrusion, and forging, as well as during creep deformation.[1] DRX can occur in a continuous or discontinuous fashion, depending on the material and deformation mode. The continuous process involves rotation of subgrains, which occurs simultaneously in the entire microstructure, whereas discontinuous DRX involves an inhomogeneous nucleation step followed by growth of the recrystallized grains by high-angle grain boundary (HAGB) migration.[1] Moreover, continuous DRX is favored by a low stacking fault energy (SFE) due to the easier cross-slip and recovery, whereas a high SFE typically results in a discontinuous process.
NOOSHIN MORTAZAVI, Doctoral Student, and MAGNUS HO¨RNQVIST COLLIANDER, Associate Professor, are with the Department of Physics, Chalmers University of Technology, Gothenburg, Sweden. Contact e-mail: [email protected] NICOLA BONORA, Professor, and ANDREW RUGGIERO, Associate Professor, are with the Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Cassino, Italy. Manuscript submitted December 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
DRX can also occur under high-strain-rate conditions, typically for combinations of shear strains exceeding around 3, shear strain rates above 104 s1, and homologous temperatures above approximately 0.4 to 0.5 Tm.[2] For practical reasons, dynamic testing is usually performed in shear[3–6] or shear–compression.[7] Under such loading conditions, DRX occurs locally through a mechanism similar to the continuous process described above, involving the formation of elongated subgrains or bands, which break up into an equiaxed structure by local rotation of subgrain walls.[3] This takes place also in high SFE materials such as copper, due to the localization of the plastic deformation into shear bands resulting from the adiabatic heating and/or geometrical constraints.[3] Through the introduction of a new test method, called dynamic tensile extrusion (DTE), very high strains and strain rates can be now achieved in uniaxial tension.[8] Recently, it has been shown that DRX in oxygen-free high-conductivity (OFHC) copper could be obtained in the uniaxially extruded section of a DTE tested specimen,[9] where tensile strains in the order of 5 were reached in combination with strain rates of around 106 s1 and quasi-adiabatic heating to temperatures exceeding 0.75 Tm. Although grain growth primarily occurred during post-test cooling of the specimen in the die, the nucleation was suggested to occur during deformation since the kinetics of static recrystallization is too slow. The occurrence of DRX could potentially suppress necking and thus be expect
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