AbstractsEngineering

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 to 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. 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. Interrupte L'évolution de la contrainte d'écoulement et de la recristallisation de la phase beta (beta) de l'alliage de titane IMI834 (Ti-5.8Al-4Sn-4Zr-1Nb-0.5Mo-0.35Si), qui présente une microstructure bimodale, a été étudiée lors d'essais thermomécaniques. Des essais de compression ont été effectués à des températures comprises entre 975°C et 1100°C et à des vitesses de déformation allant de 0,01 à 1s-1. Les échantillons ont ensuite été maintenus à la température de déformation pendant 5 à 420 secondes. Ces essais ont permis de mieux comprendre l'influence de la température et de la vitesse de déformation sur le procédé de forgeage isothermique. Des essais de compression interrompus ont également été effectués pour évaluer l'effet de l'adoucissement sur la cinétique de la recristallisation statique. Un modèle mathématique a ainsi pu être déterminé pour permettre une meilleure compréhension du procédé de forgeage à des températures supérieures et inférieures à la température de transformation allotropique (beta transus). Les courbes de…