| Transformation-mismatch plasticity is a deformation mechanism that occurs in polymorphic materials when a small external stress is imposed during their phase transformation. Internal mismatch, due to the density difference between polymorphs, is accommodated plastically in the weaker phase, and the accommodation strains are biased to occur in the direction of the external stress. This leads to average Newtonian flow, which can be accumulated after multiple transformations to achieve superplastic strains (i.e., transformation superplasticity).; In this thesis, transformation-mismatch plasticity and transformation superplasticity are investigated through analytical modeling and detailed experiments on titanium-based materials, in which the transformation mismatch is accommodated by time-dependent creep flow. First, the prevailing model of transformation-mismatch plasticity is tested rigorously against new experimental data on a titanium-aluminide intermetallic alloy. A new analytical model is then developed for transformation-mismatch plasticity during non-isothermal phase transformations, and validated against new experimental data on the engineering alloy Ti-6Al4V. Next, the evolution of structure during superplastic elongation of Ti-6Al-4V is investigated, including the macroscopic development of plastic instabilities (necks) and the realignment of a reinforcing whisker phase. Finally, the phenomenon of chemical cycling is considered, first theoretically and then experimentally for Ti and Ti-6A1-4V, in which internal mismatch stresses are developed through a compositional excursion rather than through a thermal excursion. |
