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

Processing by high-pressure torsion at room temperature was employed to achieve a uniform nanostructure in a metastable Ti67.4Nb24.6Zr5Sn3 bcc beta alloy with grains of similar to 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 beta-titanium with the presence of a small fraction of orthorhombic alpha '' martensite. It is proposed that the pronounced grain refinement is facilitated by a stress-/strain-induced beta -> alpha '' martensitic transformation. However, the presence of a large fraction of beta phase in the high-pressure torsion processed samples is attributed to a reverse alpha -> beta martensitic transformation stimulated by the applied shear stress and the inhibition effect of the nanoscale grains on martensite formation.