Microstructure evolution in additively manufactured Ti-15%Nb-12%Zr alloy during plastic deformation over a wide range of strain up to 20,800 %

The evolution of the microstructure during plastic deformation was studied in an additively manufactured Ti-15%Nb-12%Zr (at.%) alloy. The applied equivalent strain ranged extremely widely from 8% to 20,800%. Strains below 100% were achieved by uniaxial compression, while the high strains were obtained by the high-pressure torsion (HPT) technique. In addition to the as-built material, the same investigation was conducted on the alloy after heat treatment at 550 °C for 1 hour after additive manufacturing. The as-built alloy had a fully body-centered cubic structure, while the annealing step resulted in the development of a hexagonal close-packed secondary α-Ti phase with a fraction of about 10%. Compression up to a strain of 70% resulted in an increase in the secondary phase fraction to 12% and 22% for the as-built and heat-treated alloys, respectively. HPT processing at high strains yielded a lower secondary phase fraction due to reverse martensitic transformation. The dislocation density increased to 70-100 × 10 14 m -2 when the strain rose to 70% during compression. The dislocation density reached extremely large values of 600-700 × 10 14 m -2 at a strain of about 20,800% achieved by HPT. Although the dislocation density increased by two orders of magnitude and nanocrystallization also occurred when the strain increased to 20,800%, the hardness was enhanced only by 10-30%. The unexpectedly low hardness increase after HPT was attributed to unique deformation mechanisms such as the kink pair related dislocation motion and grain boundary sliding. • Additively manufactured Ti-Nb-Zr alloy was deformed up to the strain of 208 • Effect of heat treatment on the deformation behavior was investigated • Texture ⟨100⟩ changed to ⟨110⟩ at high strains • Many-fold increase of dislocation density and reduction of grain size occured • Only a very low hardness increase was observed between the strains of 0.7 and 208