Development of ultra-fine grain microstructures in a metastable-β titanium alloy via phase transformation assisted recrystallisation

Generally, in metallic alloys, attaining ultra-fine sub-micron grain sizes by recrystallisation requires severe plastic deformation (SPD), however SPD techniques are difficult to apply to large quantities of material. In this work, we explore a strategy to reach an ultra-fine grained microstructure in a metastable-beta titanium Ti-20Nb-6Zr (at.%) by controlled recrystallisation after conventional rolling and annealing. The thermally stable sub-micronic dual phase microstructures are associated with the formation of strain-induced martensite during rolling and its subsequent reversion during annealing. Some specimens were cold-rolled to change most of the beta microstructure to stress-induced martensite (SIM); others were "warm rolled" just above the critical temperature for formation of SIM (453 K). The role of SIM in grain refinement was isolated by comparing the microstructural evolution of the two sets of specimens during annealing treatments. The cold rolled specimens produced an effective ultrafine grain recrystallisation, which was identified to start from 723 K, and a final equiaxed alpha-beta microstructure of ~250 nm. In warm rolled specimens without SIM, no new beta grains were observed, only an array of intragranular acicular alpha precipitates of approximately 50 nm thickness.