Experimental investigation on kinetic behavior of the deformation-induced α→β transformation during hot working of Ti–6Al–2Zr–1Mo–1V alloy below the β-transus

Abstract

Hot compression test was performed on Ti–6Al–2Zr–1Mo–1V alloy below the β-transus (Tβ) with the temperature range of 940–970 °C. Microstructural evolution, especially on the equiaxed primary α phase (αp), was characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results showed that the αp could transform into β during hot deformation at α+β two-phase region, i.e., deformation-induced transformation of α→β (DIT). DIT, as an additional softening mechanism, caused the unusual stress response on the flow curves, which was reflected by the fact of the steady stress (σst)<initial yield stress (σ0.2). The fraction of transformed αp was influenced by various hot working parameters (such as temperature, strain and strain rate). Even complete dissolution of αp was present in some cases, indicating a premature Tβ during straining. The DIT is considered as a displacive nucleation and diffusion-controlled growth process. Its mechanism can currently be divided into two types: internal penetration of αp and α/β interface migration. The αp particles with higher favorable orientation preferentially undergo phase transformation. A kinetic model based on the transformed αp fraction (ωαp(t)) during DIT was developed by taking into account the thermal activation contribution (corresponding to heating duration) and mechanical driving force contribution (corresponding to deformation). The model was fitted to the ωαp(t) as a sigmoidal function of time (t) based on the classical Johnson–Mehl–Avrami (JMA) equation. Such a quantitative kinetic model for DIT can be used to accurately tailor the required balance between αp and β in Ti–6Al–2Zr–1Mo–1V alloy during hot working.