The effects of forging and rolling on microstructure in O+BCC TiAlNb alloys

The effects of hot upset forging and hot pack rolling on microstructure of orthorhombic (O)+body-centered cubic (BCC) TiAlNb alloys was investigated. The starting materials were melted ingots of nominal compositions: Ti25Al25Nb(at.%), Ti23Al27Nb(at.%), and Ti12Al38Nb(at.%). Smaller cigar-shaped Ti25Al25Nb ingots were examined to understand the effect of rolling preheat treatment on microstructure. It was found that super-transus preheat treatment results in large prior BCC grains and surface edge cracking. For larger castings, forging and rolling procedures were carried out after heating the materials between 932–1000°C. These temperatures were below the BCC-transus temperature for Ti23Al27Nb and Ti25Al25Nb and above the transus for Ti12Al38Nb. This resulted in a significantly larger grain size for the as-processed Ti12Al38Nb compared with the other two alloys. The Ti25Al25Nb alloy required the greatest forging and rolling loads, while the fully-BCC Ti12Al38Nb alloy exhibited the best workability and required the lowest forging and rolling loads. This was related to the alloys’ aluminum contents and O-phase volume fractions. Sub-transus processing of the near Ti2AlNb alloys proved to be a viable technique for obtaining homogeneous microstructures containing fine O and BCC phases and lacking large prior BCC grains, which can be detrimental to the mechanical performance.