FIGURE 1: Zr-705 specimens: (a) no deformation, (b) deformed by creep, and (c) deformed by thermal cycling.
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 exist internally, 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 an 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).
Financial Support
This research is funded by the National Science Foundation.
Transformation Superplasticity of Cast, Coarse-Grained Alloys
FIGURE 1: Commercially-pure as-cast titanium sample deformed to an engineering strain of e = 100.1% after 176 cycles at 860 °C, compared with an undeformed sample. The stress was held at 3.0 ± 0.2 MPa during the entire test.
Qizhen is studying TSP on cast Ti and Ti-6Al-4V bars with very coarse grains, with the aim to demonstrate that cast materials can be superplastically deformed without the costly thermo-mechanical processing steps needed to induce fine-grain superplasticity. We are also using cast specimens that have been hot-isostatically pressed (HIP), a common method to close porosity casting, but which also leads to large grains. Finally, we will demonstrate multiaxial TSP deformation on cast plates of these materials.
Financial Support
Industrial (through an SBIR project)
Related References
- D.C. Dunand, C.M. Bedell. "Transformation-Mismatch Superplasticity in Reinforced and Unreinforced Titanium", Acta Materialia, 44, 3, 1063-1076 (1996).
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- D.C. Dunand, S. Myojin. “Biaxial Deformation of Ti-6Al-4V and Ti-6Al-4V/TiC Composites by Transformation-Mismatch Superplasticity”,Materials Science and Engineering A, 230, 25-32 (1997).
PDF - P. Zwigl, D.C. Dunand. "A Non-Linear Model for Internal Stress Superplasticity", Acta Materialia, 45, 12, 5285-5294 (1997).
PDF - M. Frary, C. Schuh, D.C. Dunand. "Strain Ratchetting of Titanium upon Reversible Alloying with Hydrogen", Philosophical Magazine A, 81, 1, 197-212 (2001).
PDF - C. Schuh, D.C. Dunand. "Non-Isothermal Transformation-Mismatch Plasticity: Modeling and Experiments on Ti-6Al-4V ", Acta Materialia, 49, 2, 199-210 (2001).
PDF - D.C. Dunand, P. Zwigl. “Hydrogen-Induced Internal-Stress Plasticity in Titanium”, Metallurgical and Materials Transactions A, 32, 3, 841-843 (2001).
PDF - P. Zwigl, D.C. Dunand. “Internal Stress Plasticity in Titanium by Cyclic Alloying/Dealloying with Hydrogen”, Journal of Materials Processing Technology, 117, 3, 409-417 (2001).
- C. Schuh, D.C. Dunand. "Tensile Fracture during Transformation Superplasticity of Ti-6Al-4V ", Journal of Materials Research, 16, 3, 865-875 (2001).
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