Flow and microstructure development of a near-alpha titanium alloy during thermomechanical processing

The flow and beta recrystallization behaviour during thermomechanical processing of near-alpha titanium alloy IMI834 (Ti-5.8Al-4Sn-4Zr-1Nb-0.5Mo-0.35Si), with an initial bimodal alpha+beta microstructure, has been investigated. The effects of temperature and strain rate were characterized and modelled at beta and alpha+beta hot working temperatures near the beta→alpha+beta transition temperature (beta transus) to study the quantitative differences in one- and two-phase isothermal forging. The experimental work for characterization and modelling was based on compression testing of lab-scale specimens at temperatures of 975-1100°C, strain rates of 0.01-1s-1, and post-deformation annealing times of 5-420s. Supplementary interrupted compression testing was also performed at 975-1000°C to evaluate the applicability of fractional softening in the determination of static recrystallization kinetics.;A finite element model of the experimental setup, coupling heat transfer, flow behaviour and microstructure evolution, has been developed using constitutive equations adapted from available literature. The agreement between finite element model predictions and raw experimental data served as a validation of the data analysis. Stress was modelled through a self-consistent method capable of predicting flow partitioning between phases. The flow model, which had been originally developed for alpha+beta alloys, was extended to stress prediction in IMI834 and other near-alpha alloys. The microstructure was subsequently modelled through beta static recrystallization kinetics using an Avrami-type relationship.;The stress-strain analysis, which employed corrections for friction and deformation heating, showed increasing stress at increasing strain rates and decreasing temperatures. The stress-temperature dependence increased below the beta transus due to the increasing alpha phase fraction with decreasing temperature. Microstructural observation through optical microscopy indicated dynamic recrystallization occurred, although complete grain refinement and homogeneity was only achieved following static recrystallization. Quantitative measurement via image analysis revealed static recrystallization kinetics increased with temperature for single phase beta pre-deformation microstructures (1060-1100°C). However, bimodal alpha+beta microstructures (1000-1025°C) displayed greater recrystallization rates with decreasing temperature. This behaviour was attributed to the associated increase in alpha phase fraction, which yielded a refinement in initial beta grain size and an increase in favourable nucleation sites. Interrupted compression testing of initial alpha+beta microstructures with lamellar alpha in the beta matrix (975°C) indicated that static recrystallization kinetics may have been comparable to that obtained at 1000°C, although confirmation through optical microscopy and image analysis was not feasible. At all temperatures, increasing strain rate accelerated recrystallization kinetics due to less time for dynamic recovery.