An Overview of the Thermomechanical Processing of α/β Titanium Alloys: Current Status and Future Research Opportunities

Current understanding of the principles underlying the thermomechanical processing (TMP) of α/β titanium alloys is reviewed. Attention is focused on the formulation of constitutive descriptions for plastic flow under hot-working conditions, the evolution of microstructure, the occurrence of defects, and novel/emerging TMP techniques. With regard to constitutive behavior, descriptions of the plastic flow of the individual phases and two-phase alloys per se are summarized. The important influence of phase morphology, size, and volume fraction on plastic flow is emphasized. Mechanisms which underlie microstructure evolution include beta recrystallization (in the high-temperature β field), the development of dislocation substructure and its effect on dynamic and static spheroidization of colony microstructures (in the two-phase field), static and dynamic coarsening of primary α, and the development of deformation and transformation textures. In the area of defects, the effect of TMP variables and starting microstructure on the formation of cavities, the persistence of microtexture, and the development of undesirably-coarse β grain structures are described. The current status of relatively new processing techniques for α/β titanium alloys such as low-temperature superplastic forming and solid-state joining (via linear friction or friction-stir methods) are also briefly reviewed. Last, R&D which could help to resolve deficiencies in the current knowledge base for TMP of α/β titanium alloys are summarized for each of the areas.