Distinct Microstructure and Hardness of Zr-2.5Nb Alloy Annealed in Lower and Upper Dual-Phase Regions: the Role Played by Nb

A 50%-rolled Zr-2.5Nb alloy sheet was annealed in lower (700 °C) and upper (800 °C) α-Zr + β-Zr dual-phase regions, respectively, followed by water quenching. Microstructural features were meticulously characterized and analyzed using electron channeling contrast imaging and electron backscattering diffraction techniques. After annealing at 700 °C, the alloy obtains a mixed structure of recrystallized α grains and granular β phase, and the preservation of β phase can be associated with the enrichment of Nb. For the 800 °C sample, its microstructure comprises recrystallized α grains and ultrafine plates produced by martensitic transformation. There exist numerous nanotwins inside these martensitic plates, which are related to the Nb-induced reduction of martensite start temperature (M_S). Hardness tests reveal that compared to the as-rolled sample (249.8 ± 11.0 HV), the hardness of the 700 °C sample slightly drops (240.9 ± 6.7 HV) due to increased α-grain sizes and the reduced deformation defects. In contrast, there appears a notable increase in hardness for the 800 °C sample (290.0 ± 5.5 HV), which is attributed to synergistic effects of multiple mechanisms including grain refinement strengthening of martensitic laths, nanotwin boundary strengthening, and solid-solution strengthening.