Progress in Severe Plastic Deformation of Metastable Beta Ti Alloys

Metastable β Ti alloys can undergo martensitic transformation under stress. It is shown that such stress‐induced martensitic transformation (SIMT) can significantly facilitate grain refinement by severe plastic deformation (SPD), achieving β grain sizes of <50 nm, compared with >100 nm in β Ti alloys without SIMT. The martensitic α″ formed partitions the β grains and blocks dislocation movement, but undergoes reverse transformation back into β at large strains, leaving a pure β grain structure. The grain refinement can be further enhanced by increasing the strain rate to the order of 10–100 s −1 , leading to β grains of <10 nm. SPD also has significant effect on the morphology of the α precipitates upon aging, transforming it from acicular to equiaxed. Although the α nucleus formed between recrystallized β grains of <≈10 nm follows the Burgers orientation relationship (BOR), the semicoherent interface is lost upon β grain growth during aging, causing its growth into equiaxed shape.