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

Mukhtarova, Kamilla and Dirras, Guy and Kawasaki, Megumi and Wiener, Csilla and Windisch, Márk and Heczel, Anita and Dankházi, Zoltán and Radnóczi, György Zoltán and Gubicza, Jenő (2026) Microstructure evolution in additively manufactured Ti-15%Nb-12%Zr alloy during plastic deformation over a wide range of strain up to 20,800%. MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 955. No.-149842. ISSN 0921-5093

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Abstract

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 × 1014 m-2 when the strain rose to 70% during compression. The dislocation density reached extremely large values of 600-700 × 1014 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.

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