The effect of Co additions on the microstructure and mechanical properties of solute lean ⍺ + β Ti alloys

Ti-6Al-4V is an α + β Ti alloy of great commercial significance to the aerospace sector, due to a good balance of mechanical properties. However, alloys that can be processed at lower temperatures whilst retaining these favourable mechanical properties could offer attractive financial and environmental benefits. As such, there has been recent interest in developing solute lean α + β Ti alloys which have been reported to improve on the room temperature ductility of Ti-6Al-4V. Despite this, there is currently a limited understanding of the role that specific alloying additions, namely Co, have on achieving these favourable properties. As such, this study considered the microstructure and mechanical properties of a systematic series of Ti-Al-V-Co alloys. Co was seen to influence the size of the different α populations in a bimodal microstructure and result in a shallower β approach curve. Furthermore, greater Co contents were seen to result in higher work hardening rates in the material. However, the precipitation of intermetallic phases identifies a limit to the fractions of Co that can be introduced before the ductility of such alloys becomes compromised.