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Monday, June 23, 2008 - 1:30 PM

Low-Temperature Superplastic Flow of Ultrafine Ti-6Al-4V

G. A. Sargent, University of Dayton, Dayton, OH; D. Li, RTI International Metals, Inc., Niles, OH; S. L. Semiatin, Air Force Research Laboratory, Wright-Patterson AFB, OH

The superplastic-flow behavior at low temperatures (650 – 800°C) and a range of strain rates (10-4 to 10-2 s-1) of Ti-6Al-4V with an ultrafine two-phase (alpha/beta) microstructure has been established. Program materials comprised billet product manufactured by warm isothermal (‘abc’) forging, tested in compression, and sheet fabricated by warm rolling, tested in tension. Extensive metallography on undeformed and deformed samples water quenched from the various test temperatures was conducted using backscattered-electron imaging in an SEM to characterize microstructure stability. Despite the low deformation temperatures, both lots of material showed similar (measurable) dynamic coarsening, whose kinetics mirrored the flow-hardening observed during compression and tension tests. The plastic-flow phenomenology was interpreted in the context of the classical Bird-Mukherjee-Dorn relation. Over the entire test-temperature range, the stress and grain-size exponents of the strain rate were ~1.4-2 and ~2, respectively, for strain rates of 10-4 and 10-3 s-1. The constitutive analysis suggested that multiple (dislocation glide-climb and diffusional) mechanisms control deformation, thus complicating the interpretation of the apparent activation energy derived from the plastic-flow data.

+ Currently Consultant with UES at the Air Force Research Laboratory, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433.


Summary: The superplastic-flow behavior at low temperatures (650-800C) and a range of strain rates (10-4 to 10-2 s-1) of Ti-6Al-4V with an ultrafine two-phase (alpha/beta) microstructure has been established. Program materials comprised billet product manufactured by warm isothermal (abc) forging, tested in compression, and sheet fabricated by warm rolling, tested in tension. Extensive metallography on undeformed and deformed samples water quenched from the various test temperatures was conducted using backscattered-electron imaging in an SEM to characterize microstructure stability. Despite the low deformation temperatures, both lots of material showed similar (measurable) dynamic coarsening, whose kinetics mirrored the flow-hardening observed during compression and tension tests. The plastic-flow phenomenology was interpreted in the context of the classical Bird-Mukherjee-Dorn relation. Over the entire test-temperature range, the stress and grain-size exponents of the strain rate were ~1.4-2 and ~2, respectively, for strain rates of 10-4 and 10-3 s-1. The constitutive analysis suggested that multiple (dislocation glide-climb and diffusional) mechanisms control deformation, thus complicating the interpretation of the apparent activation energy derived from the plastic-flow data.