Dynamic recrystallization of pure copper and copper-tin alloys
thesisposted on 17.02.2017 by Cram, Darren Geoffrey
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
A physically-based model for discontinuous dynamic recrystallization (DDRX) in metals is developed using a mean field approach. It couples together polyphase plasticity, grain growth and nucleation models to predict the macroscopic stress and grain size evolution during thermomechanical processing. A new nucleation model is developed based on a recent description for static recrystallization which considers an evolving microstructure. Hot compression experiments are performed for pure Cu and a series of Cu-Sn alloys (Cu-0.2Sn, Cu-2Sn and Cu-5Sn (wt%)) to test the effect of solute in solution on DRX for a range of temperatures (725K - 975K) and strain-rates (0.001s^-1 - 0.2s^-1) to a strain of 1. The macroscopic stress response during straining was recorded by a hydraulic Instron and the resulting microstructures were examined by optical microscopy and electron backscattered diffraction (EBSD) maps. Solute additions are shown to significantly refine the DRX grain size. Sn additions exhibit a non-monotonic effect in composition on DRX which is suggested to be caused by the different antagonistic effects Sn has on the elementary processes of strain-hardening, nucleation and grain growth during DRX. The DRX model applied to pure Cu was able to predict the experimentally observed trends such as the critical strain for DRX initiation, single to multi-peak stress transition, convergence towards a steady-state stress and the DRX grain size as a function of temperature, strain-rate and initial grain size. The model reveals an evolving DRX nucleation rate as a function of strain, and agrees with experimental estimates that it generally increases with increasing strain-rate and decreasing temperature. The model was then systematically extended to include the effects of Sn additions in Cu over different hot-working conditions and Sn concentrations. It was found that Sn must play a significant role to suppress the micro-mechanisms that govern nucleation. With Sn, the model was able to capture the experimental trends such as the stress peak broadening, transition from multi to single-peak stress behaviour and grain refinement. It is envisaged that this model in the future may be used to identify new optimum solutions in alloy design and processing schedules to most effectively refine the grain structure via DRX.