Abstract
Two different mesoscale models based on dislocation reactions are developed and applied to predict both the flow stress and the microstructure evolution during the hot deformation of titanium alloys. Three distinct populations of dislocations, named mobile, immobile, and wall dislocations, describe the microstructure, together with the crystal misorientation and the densities of boundaries. A simple model consisting of production and recovery terms for the evolution of dislocations is compared with a comprehensive model that describes the reactions between different type of dislocations. Constitutive equations connect the microstructure evolution with the flow stresses. Both models consider the formation of a high angle grain boundary by continuous dynamic recrystallization due to progressive lattice rotation. The wall dislocation density evolution is calculated as a result of the subgrain size and boundary misorientation distribution evolutions. The developed models are applied to two near-β titanium alloys, Ti-5553 and Ti-17, and validated for use in hot compression experiments. The differences in the predictability between the developed models are discussed for the flow stress, dislocation densities and microstructure evolutions. Only the comprehensive model can predict the different reactions and their contributions to the evolution of mobile and immobile dislocation densities. The comprehensive model also allows for correlating the elastic strain rate with the softening and hardening kinetics. Despite those differences, the selection of the model used has a small influence on the overall prediction of the subgrain size and the fraction of high angle grain boundaries.
Highlights
As a high stacking fault energy (SFE) material, the β-phase in titanium and its alloys undergoes a complex microstructure evolution during hot deformation due to dynamic recovery (DRV) and dynamic recrystallization (DRX) [1,2,3,4,5,6,7,8]
There are some controversies [1,2,3,4], it is generally accepted that the β-phase undergoes DRV [5] followed by continuous dynamic recrystallization [8] or geometric dynamic recrystallization [6,7] at large strains. cDRX occurs by progressive lattice rotation [6,7] and was observed in Ti-64 [5], while gDRX occurs at lower strain rates and lower temperatures [6,7]
Among the most powerful methods to predict the hot deformation of complex parts, the mesoscale models are of great interest [12]
Summary
As a high stacking fault energy (SFE) material, the β-phase in titanium and its alloys undergoes a complex microstructure evolution during hot deformation due to dynamic recovery (DRV) and dynamic recrystallization (DRX) [1,2,3,4,5,6,7,8]. From discrete dislocation dynamics [9] to phenomenological approaches [10,11], the hot deformation behavior of metallic alloys has been modeled and predicted. A similar dislocation density reaction approach was applied to describe the hot deformation of a Ti-5553 alloy [19] and a Ti-17 alloy [20]. Simpler dislocation density-based models, mostly based on Kock–Mecking formalism [21], were developed to predict the deformation behavior of other Ti alloys. The deformation of a Ti-6Al-4V over a wide temperature range was achieved in
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