Formation of an ultrafine-grained structure during equal-channel angular pressing of a ß-titanium alloy with low phase stability

A uniform, well-defined ultrafine-grained structure was attained by equal-channel angular pressing in a metastable Ti67.4Nb24.6Zr5Sn3 β-titanium alloy with low phase stability. In contrast, coarser β grains of several micrometers were yielded upon equivalent plastic strain in a relatively stable Ti73.5Nb19.6Ta4.5Cr2.4 alloy. The pronounced deformation-induced grain refinement is proposed to mainly result both from the stress-assisted β → α″ martensitic and α″ → β backward transformations during severe plastic deformation, and from the interaction between α″ and dislocations in the β phase.