Effects of boron additions and solutionizing treatments on microstructures and ductility of forged Ti-6Al-4V alloys

The effects of boron additions on the microstructure and mechanical properties of forged Ti-6Al-4V alloys in different heat-treatment conditions have been characterized by both experimental studies and thermodynamic calculations. The results indicate a combination of proper post-forging treatments and B additions are helpful for control of the prior-β grain size and the volume fraction of α phase, thereby tuning the ductility of the forged Ti-6Al-4V alloys. However, the B-containing alloys exhibit a significant drop in ductility if the solutionizing temperature is too high, and this embrittlement is mainly due to the coarsening of brittle TiB borides. The mechanism in this case is due to the cleavage fracture of TiB rather than its debonding with the matrix, as indicated by the observation of the aligned TiB borides on the matching areas of both halves of the fracture surfaces. Thus, the TiB size and orientation, the prior-β grain size, and the volume fraction of the α phase all play important roles in controlling the mechanical properties of the forged Ti-6Al-4V alloys. The current findings shed light on the composition-microstructure-ductility relationship in the forged Ti-6Al-4V alloys.