A study on the coupled effects of solute and grain size on the work hardening of fine- grained FCC alloys

This thesis develops a physically based model for the work hardening of model (pure or solute strengthened) FCC alloys having grain sizes of between 2 μm and 100 μm. This model builds on a previous study of Sinclair et al. (2006) on fine-grained pure Cu and extends it to Cu16at%Ni, Cu50at%Ni and Cu1.5at%Al alloys. Through careful and systematic mechanical testing coupled with microstructural observations several basic hypotheses were tested. The yielding behaviour of fine-grained materials showed an extension of the elasto-plastic transition (over the generally accepted 0.002 offset strain) with decreasing grain size and increasing solute content. This resulted in Hall-Petch plots which showed that the grain size effect was more pronounced with increasing solute content. In all of the materials tested with a sufficiently fine grain size, the stress-strain plots showed an inflection (i.e. region of a low work hardening or “plateau”). While the work hardening rate dropped significantly in this portion of the test, image correlation was used to show that the drop in work hardening was not sufficient to cause strain localization. The stress-strain plots were differentiated and work hardening behaviour was analyzed using a Kocks-Mecking model. An important observation was that with increasing solute content from pure Cu to Cu50at%Ni, a grain size dependent separation between the work hardening plots appeared for tests performed at room temperature. This observation was initially hypothesized to be due to backstresses as proposed in the original model of Sinclair (Sinclair et al. 2006). This idea was tested using strain-rate sensitivity experiments. Strain-rate sensitivity tests showed that a single mechanism (forest hardening) controls the work hardening behaviour beyond the initial few percentage of straining. To unify all these experimental observations in a self-consistent work hardening model, the original Sinclair model was modified through the addition of a new variable, n*f , which accounts for additional dislocation storage by the forest dislocations blocked at grain boundaries. It was hypothesized that the effects of dislocation/grain boundary interactions on screening/annihilation of dislocations could be used to capture the initial high rate sensitivity at the “plateau” in the stress-strain curve of fine grained alloys.

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