Controlling thermal stability in nanostructured materials
Lack of adequate thermal stability has precluded widespread application of nanostructured metals and alloys in structural systems. While approaches to address this problem have been developed, their application to date is mainly for alloy systems containing thermally-stable second-phase particles that retard thermally-driven boundary migration. The goal of the present study is to address the problem of thermal stability in single-component, nanostructured metal systems. A novel approach to stabilize boundary migration using special boundary configurations is proposed. By using high-purity copper as a model system, this approach is demonstrated through the enhanced stability afforded by a dense network of twin boundaries. The specific experiments and characterization include creation of a range of microstructures using machining-based severe plastic deformation (SPD), analysis of the microstructures using electron microscopy, hardness and differential scanning calorimetry, and estimation of the stored energy of cold work. Unique combinations of strengthening and stabilization in the case of heavily twinned microstructures are identified. It is shown that certain types of nano-scale twinned microstructures are thermally more stable and stronger than ultrafine-grained (UFG) microstructures comprised of high-angle boundaries. The process engineering of these special, twinned microstructures with enhanced stability is applicable to the creation of bulk forms. A stability ‘map’ is obtained for ultrafine-grained copper microstructures produced by SPD at strain rates of 1 to 103 s−1, strains of 1 to 7 and temperatures as low as cryogenic. Furthermore, SPD process conditions for creating sub-100 nm (nanocrystalline) copper microstructures comprised primarily of high-angle boundaries, as well as microstructures dominated by nano-scale twinning, are identified. The stability approach and insights developed herein are important for stabilization of single-component, nanostructured metal systems that have hitherto not had a practical approach to stabilization against thermal fields. They also offer insights into potential routes for improving the stability of multi-component nanostructured alloys.