The Role of High-Index Twinning on Hierarchical α Microstructure in a Metastable β Ti-5Al-5Mo-5V-3Cr Alloy

Hierarchical alpha microstructure in a metastable beta Ti-5Al-5Mo-5V-3Cr (wt%, Ti-5553) alloy has been investigated using scanning electron microscopy and transmission electron microscopy. Novel high-index {10 9 3}(beta) type deformation twins were characterized in the 10% cold-rolled Ti-5553. Without the pre-formed deformation twins, refined alpha microstructure and super-refined alpha microstructure were achieved in Ti-5553 using rapid up-quenching and slow heating treatment, respectively. On the contrary, with the pre-formed {10 9 3} (beta) deformation twins in the beta matrix, hierarchical alpha microstructure was generated in Ti-5553, whether rapid up-quenching or slow heating treatment was adopted. It is observed that a layer of alpha phase precipitates from pre-formed {10 9 3}(beta) twin boundary and grows along the twin boundary. alpha sub-layers and fine-scale alpha precipitates form in the interior of pre-formed {10 9 3}(beta) twin. It is speculated that the large number of dislocations, metastable omega particles and alpha '' phase present as the substructure in the {10 9 3}(beta) twin may act as favorable nucleation sites for alpha sub-layers and fine-scale alpha precipitates. Thus, our work indicates that high-index {10 9 3}(beta) twin may assist the formation of hierarchical alpha microstructure in the metastable beta Ti-5553 alloy.