Nanocrystalline body-centred cubic beta-titanium alloy processed by high-pressure torsion

AbstractProcessing by high-pressure torsion at room temperature was employed to achieve a uniform nanostructure in a metastable Ti67.4Nb24.6Zr5Sn3 bcc β alloy with grains of ∼20 − 60 nm after five turns. By comparison, there was an ultrafine grain structure after equal-channel angular pressing at 903 K. The majority of refined grains were identified as bcc β-titanium with the presence of a small fraction of orthorhombic α″ martensite. It is proposed that the pronounced grain refinement is facilitated by a stress-/strain-induced β → α″ martensitic transformation. However, the presence of a large fraction of β phase in the high-pressure torsion processed samples is attributed to a reverse α″ → β martensitic transformation stimulated by the applied shear stress and the inhibition effect of the nanoscale grains on martensite formation.