Thermal stability of nanotwinned and nanocrystalline microstructures produced by cryogenic shear deformation

AbstractNanotwinned microstructures are of significant interest due to their high strength and enhanced thermal stability, attributed to the presence of a dense network of coherent twin interfaces. Propensity for twinning during deformation is known to increase at low temperature and/or high-strain-rate. In this study, we use high-strain-rate (~103 s−1) shear deformation in cutting over a range of strains (γ ~1–5) and temperatures (cryogenic to ambient) to engineer a variety of microstructures in three face-centred cubic (FCC) metals – copper, brass and aluminium. The microstructures include nanocrystalline-equiaxed and densely (nano) twinned types of controllable domain size. The effects of low-temperature deformation and stacking fault energy on the resulting microstructure, hardening, stored energy and associated recrystallization kinetics are established. For copper, the nanotwinned microstructures are found to be thermally more stable and stronger than the equiaxed counterparts comprised of random high-angle grain boundaries. This enhanced effect of nanotwins on microstructure stability is, however, not observed in brass, while aluminium did not show any indications of twinning over the investigated range of deformation conditions.Keywords: nanotwinningnanocrystallinerecrystallizationcryogenicsevere plastic deformation AcknowledgementThe work at The Pennsylvania State University was supported by the National Science Foundation under grant CMMI 1130852. The work at Purdue was supported by the Army Research Office under grant W911NF-12-1-0012. The authors also acknowledge support from the Department of Energy via Third Wave Systems under award DE-EE0005762.