Understanding deformation and fracture mechanism of Ti-55531 alloy under complex loading conditions: a case of pre-tensioned torsion

The deformation and fracture failure of aerospace structural components are primarily affected by complex loading conditions. This study aims to investigate how various pre-tension strains (0%, 4% and 6%) influence the torsional properties, deformation and fracture mechanism of the Ti–5Al–5Mo–5V–3Cr–1Zr (Ti-55531) alloy with the bimodal microstructure. The results indicate that increasing the pre-tension strain gradually decreases the torsional strength of specimens. However, their torsional ductility initially increases (from 0 to 4% pre-tension strain) and then decreases (from 4 to 6% pre-tension strain). This can be attributed to the significant influence of different pre-tension strains on the deformation mechanism of each phase in the alloy. Under pure torsion loading, the primary α (α_p) phase mainly undergoes the {0002} basal slip for deformation. However, at a pre-tension strain of 4%, the torsional deformation mechanism of α_p transforms into crossing reaction between the { \(10\overline{1 }0\) $10\overline{1}0$ } prismatic slips. As the pre-tension strain further increases to 6%, { \(10\overline{1 }1\) $10\overline{1}1$ } pyramidal slips were further activated. Moreover, with an increase in pre-tension strain, there is a significant rise multiple slips probability within the α_p during torsional deformation. On contrary, for the secondary α (α_s) phase, the probability of { \(10\overline{1 }1\) $10\overline{1}1$ }_α twins formation during torsional deformation firstly rises and then reduces. These findings indicate that α phase, particularly α_p, plays a crucial role in accommodating deformation. This discovery offers valuable insights for further adjustments and optimizations of material microstructure and properties. Additionally, modifying external load can alter the stress state of components and enhance their fracture resistance during service, thereby broadening their range of applications and improving material reliability.