Progress in Severe Plastic Deformation of Metastable Beta Ti Alloys

Metastable beta 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 beta grain sizes of <50 nm, compared with >100 nm in beta Ti alloys without SIMT. The martensitic alpha " formed partitions the beta grains and blocks dislocation movement, but undergoes reverse transformation back into beta at large strains, leaving a pure beta grain structure. The grain refinement can be further enhanced by increasing the strain rate to the order of 10-100 s(-1), leading to beta grains of <10 nm. SPD also has significant effect on the morphology of the alpha precipitates upon aging, transforming it from acicular to equiaxed. Although the alpha nucleus formed between recrystallized beta grains of More