Transformation Superplasticity of Metals, Composites and Intermetallics

Internal-stress plasticity is a mechanism used to increase the deformation rate of metals and alloys deforming by creep or low-temperature plasticity. When mismatch stresses or strains externally, they can be biased in the direction of an external stress, resulting in a strain increment in the same direction as the biasing stress, and with a magnitude proportional to the biasing stress. If the internal mismatch is constantly regenerated (usually through thermal cycling), this leads to an average strain-rate proportional to the applied stress, with a average strain rate sensitivity of unity which results in tensile strains well in excess of 100%, a phenomenon called internal-stress superplasticity.

One common method to produce repeatable internal stresses is to cycle the temperature around a phase transformation temperature, where the two coexisting allotropic phases have different densities. Transformation mismatch plasticity or transformation superplasticity (TSP) by thermal cycling has been observed in many allotropic metals and alloys and is particularly well-studied in Ti subjected to thermal α-β cycling. Recently, Zwigl and Dunand showed that chemical cycling at constant temperature could also produce transformation superplasticity: due to the very high diffusivity of hydrogen, α-Ti can be rapidly alloyed with hydrogen by exposure to a hydrogen-bearing atmosphere, which leads to the formation of the β-Ti phase; upon exposure to vacuum or a hydrogen-free atmosphere, the hydrogen diffuses out of the titanium, which results in a transformation back to the α-Ti phase.

In current work, we investigate hydrogen-induced TSP in titanium with the goal of experimentally separating phase transformation mismatch from lattice swelling mismatch. We explore the underlying deformation mechanisms and their dependencies on the processing parameters (hydrogen partial pressure, half cycle time and applied stress). Additionally, we investigate transformation superplasticity in other materials (Zr, Nb) and different geometries (wire vs. bulk).