Progress in Severe Plastic Deformation of Metastable Beta Ti Alloys

Abstract

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.

Effect of HPT and accumulative HPT on structure formation and microhardness of the novel Ti18Zr15Nb alloy

“…During the HPT processing of the low-modulus β -Ti alloy Ti-15Mo, a decrease in the size of β -grains/subgrains to 80 nm is observed, and the ω -phase appears at the early stages of HPT. In some β -Ti alloys of the Ti-Mo and Ti-Nb-Zr system grain refinement to 50 nm was revealed during HPT processing, while the alloys remained predominantly in the β -phase [13] , [14] , [15] , [16] .…”