Dynamic recrystallization of Ti-5553 alloy during sub-transus thermomechanical processing: Mechanisms and its role in formation of a bi-modal structure

Formation of a bi-modal structure, which is strongly affected by the dynamic recrystalliaztion induced by the sub-transus thermomechanical processing, is an effective way to improve ultimate tensile strength, plasticity and high cycle fatigue strength of Ti-5Al-5Mo-5V-3Cr (Ti-5553) titanium alloy. By using isothermal compression machine, transmission electron microscopy and electron backscattered diffraction, we have investigated the dynamic recrystallization behavior of lamellar Ti-5553 alloy during the sub-transus thermomechanical processing at 780 °C and the effect of dynamic recrystallization on bi-modal microstructure transformation for Ti-5553 alloy. The critical strain for the dynamic recrystallization of the alloy ranges from 0.2 to 0.3, at which low angle grain boundary has formed in Beta grains. The dynamic recrystallization of Beta phase goes through the following steps, including formation of dislocation tangles (ε = 0.1), formation of cell blocks (ε = 0.15), formation of density dislocation walls (ε = 0.2) and intersection of dense dislocation walls (ε = 0.3). If the compression strain reaches 0.9, low angle grain boundary could transform to high angle grain boundary. The critical strain for the dynamic recrystalliaztion of Alpha phase is approximate 0.2, which is lower than that of Beta phase. Here we have established a link between sub-transus thermomechanical processing and bi-modal microstructure transformation of Ti-5553 alloy: dynamic recrystallization of Beta phase should be an essential condition on bi-modal microstructure transformation. The main results of this work might be beneficial to the homogeneity optimization of large scale bi-modal Ti-5553 alloy bulks.