The processing regime relevant to superplasticity in the Ti-6Al-4V alloy is identifed. The
effect is found to be potent in the range 850 to 900 deg ?C at strain rates between 0.001/s
and 0.0001/s. Within this regime, mechanical behaviour is characterised by steady-state grain
size and negligible cavity formation; electron backscatter diffraction studies confirm a random
texture, leaving grain boundary sliding as the overarching deformation mechanism. Outside of the
superplastic regime, grain size refinement involving recrystallisation and the
formation of voids and cavities cause macroscopic softening; low ductility results.
Stress hardening is correlated to grain growth and accumulation of dislocations.
The findings are used to construct a processing map, on which the
dominant deformation mechanisms are identified. Physically-based constitutive
equations are presented which are faithful to the observed deformation mechanisms.
Internal state variables are used to represent the evolution of grain size,
dislocation density and void fraction. Material constants are determined using
genetic-algorithm optimisation techniques. Finally, the deformation behaviour
of this material in an industrially relevant problem is simulated: the deformation of
diffusion-bonded material for the manufacture of hollow, lightweight structures, e.g.
those used for fan blades of aeroengines.
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