ON THE MICROSTRUCTURE AND THERMOMECHANICAL PROCESSING OF TITANIUM-ALLOY IMI685

The microstructure of near-alpha titanium alloys is very sensitive to the thermomechanical processing conditions, which in turn have a significant influence on the mechanical properties of the material. It is recognized that a smaller prior beta-grain size is the key microstructural feature necessary to achieve an improved combination of properties, but refining the grain size is a difficult task. The present work investigates first the grain growth behaviour of a near-alpha titanium alloy, IMI685, and then the effect of various thermomechanical routes on its microstructure, in an attempt to refine the grain structure. Both optical and transmission electron microscopy were used to examine the microstructural variations during each step of the thermomechanical treatment. The results show that grain refinement is very difficult in the case of beta or upper alpha + beta processing, owing to insufficient dynamic recrystallization during hot deformation and to the rapid grain growth during the beta-solution treatment. Grain refinement has been effectively achieved using a lower alpha + beta processing route, and the desired lamellar morphology can be regained by means of a beta-solution treatment.