Heterogeneous structure controlled by shear bands in partially recrystallized nano-laminated copper

The recrystallization behavior of ultra-fine grained (UFG) copper and nano-laminated (NL) copper, prepared by equal channel angular pressing (ECAP) and ECAP with subsequent cryogenic rolling (CYR), was investigated. According to the course of isothermal annealing, the significant effect of inhomogeneous deformation on the subsequent recrystallization was confirmed. In uniform UFG copper, the recrystallized grains are equiaxed and distributed randomly. In contrast, in non-uniform NL copper with dense shear bands (SBs), as introduced by CYR, recrystallized grains are severely clustered and exhibit abnormal growth related to the preferential recrystallization behavior of the SBs. The overall recrystallization kinetics curve shows that NL copper has a much lower Avrami exponent than UFG copper, which is explained using the modified two-stage kinetics Johnson-Mehl- Avrami-Kolmogorov model incorporating the non-uniform microstructure. The recrystallized microstructures originating from the SBs are found to be not limited to within the SBs regions because the following abnormal growth was at the consume of the deformed NL matrix. Furthermore, the abnormal growth rate of the recrystallized SBs was controlled by the stored energy in the NL matrix. A Vickers microhardness test indicated that the strength-ductility tradeoff may be overcome by exploiting the two-stage recrystallization behavior to control heterogeneous microstructures in partially recrystallized NL copper.