Solute clustering and its role in a titanium alloy made by laser powder bed fusion

In recent years, atomic clusters and composition undulation have been observed in a number of concentrated solid-solution alloys and were found to be beneficial for strength-ductility synergy. Some reports are also available on the observation of atomic clusters in body-centred cubic metastable β titanium alloys but the role of these atomic clusters in mechanical property development is unclear. In this study, we report the formation of solute atomic clusters in an additively manufactured and solution-treated metastable titanium alloy and investigate their role in deformation and strengthening mechanisms. It was found that the atomic clusters together with ultrafine ω precipitates present in the material effectively pin dislocation motion and contribute to an ultrahigh yield strength. The interaction between dislocations and solute atomic clusters leads to an increased shear stress necessary for dislocation motion. The atomic clusters and the resulting chemical undulation generate pronounced atomic strain fluctuations and distortions with alternating tensile and compressive strain fields, which lead to large local internal stresses and thus resistance to dislocation glide. Cross slip has also been frequently observed in the additively manufactured and solution-treated metastable titanium alloys, which is beneficial for strain hardening and ductility. We also reveal that thermal cycling in the alloy can lead to a significant influence on the microstructural evolution. During solidification and cooling upon laser powder bed fusion, only β and athermal ω precipitates form. With repetitive thermal cycling, the microstructure transforms into α + β grains together with isothermal ω particles. The present findings suggest that solute atom clustering can be introduced through proper selection of solute elements, providing a new strengthening mechanism for titanium alloys.