Annealing-induced evolution of microstructural, mechanical properties and the spheroidization mechanism of nanoscale FCC-Zr precipitates in cold Pilger-rolled Zr–Nb alloys

The microstructural evolution and mechanical properties of a cold Pilger-rolled Zr–Nb alloy during annealing in the temperature range of 500–580 °C were systematically investigated, along with the effect of cooling conditions. The results indicate that incomplete static recrystallization (SRX) occurred after annealing at 500 °C for up to 5 h, regardless of whether the sample was air-cooled or furnace-cooled. Increasing the annealing temperature and time promoted more complete SRX, with nearly full recrystallization achieved at 540 °C. The average size of statically recrystallized grains remained stable between 4.3 and 5.7 μm throughout the annealing process. The rolling texture with < 0001 > //ND was slightly weakened after annealing under most conditions, except for 500 °C/5 h and 540 °C/5 h and followed by furnace cooling samples. With increasing annealing temperature and holding time, the tensile strength decreased while the elongation improved slightly. Importantly, while strain-induced nanoscale lamellar FCC-Zr precipitates are known, this study for the first time reveals their annealing-induced spheroidization, accompanied by a transition in the orientation relationship with the matrix from conventional B/P-type to a new [01 1 ̅ 1] HCP //[001] FCC variant. The spheroidization mechanism of these precipitates and the associated crystallographic reorientation were analyzed, which is a result of the synergistic reduction of interfacial and strain energies. This work advances the understanding of microstructure and phase evolution in the annealed Zr alloys and offers valuable guidelines for tailoring their mechanical properties through thermal processing.