Occurrence of Dynamic Alpha-Phase Nucleation in Ti-5553 during Hot Deformation

Abstract

This paper discusses the microstructural changes during hot deformation, mainly dynamic nucleation and growth of a precipitates, occurring in the b metastable titanium alloy Ti-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe). The effect of process variables on flow response and microstructure evolution during hot working of the alloy with initial bimodal microstructure was established using isothermal hot-compression tests. Testing was conducted at 4 strain rates between 0.001 and 1 s−1 and 5 temperatures between 720 °C and 850 °C, on material with prior b grain size of 450 μm, a nodular size of 3 µm and a known primary a-phase fraction. All flow curves exhibited a peak stress followed by moderate flow softening in the two-phase domain. Flow softening was interpreted in terms of deformation heating and substructure or texture evolutions. The dependence on strain rate and temperature of the kinetics of dynamic a-phase nucleation during straining is complex and appears to be of second-order importance compared to the effects of strain. This suggests that the nucleation and growth of a phase in the temperature range between 720 °C and 990 °C results from a mixed-mode displacive-diffusional transformation, similar to the austenite / ferrite transformation above the Ae3 temperature reported by some authors.