Investigation on microstructural evolution and strengthening mechanism in Zr-2.5Nb alloy with multi-scale lamellar structure

Zr-2.5Nb alloy was subjected to a solid solution treatment at 900 °C for 30 min, followed by aging between 500 and 700 °C for 60 min. It was found that there were two distinct martensitic microstructures after the solid solution treatment: nano-twins and dislocation lamellar structures. As the aging temperature increased, the α’ martensite lath gradually decomposed, which was mainly manifested in the precipitation of the β-Zr phase, annihilation of nano-twins, and reduction of dislocation density. EBSD experiments revealed that the precipitated phase formed during aging treatment shared its (101) plane with the (011¯1) plane of nano-twins. The precipitation of the β-Zr phase along the twinning plane resulted in twin shrinkage and gradual decomposition. The yield strength of the Zr-2.5Nb alloy initially increased and then decreased with an increase in aging temperature. It is worth noting that the sample aged at 600 °C exhibited excellent strong plasticity matching, which can be attributed to the combined effect of dislocation strengthening, grain boundary/twin boundary strengthening, solid solution strengthening, and precipitation strengthening.