Comparative Study of Microstructural Characteristics and Hardness of β-Quenched Zr702 and Zr-2.5Nb Alloys.

In this study, two commercial Zr alloys (Zr702 and Zr-2.5Nb) were subjected to the same β-quenching treatment (water cooling after annealing at 1000 °C for 10 min). Their microstructural characteristics and hardness before and after the heat treatment were well characterized and compared by electron channel contrast (ECC) imaging, electron backscatter diffraction (EBSD) techniques, and microhardness measurements. Results show that after the β quenching, prior equiaxed grains in Zr702 are transformed into Widmanstätten plate structures (the average width ~0.8 μm) with many fine precipitates distributed along their boundaries, while the initial dual-phase (α + β) microstructure in Zr-2.5Nb is fully replaced by fine twinned martensitic plates (the average width ~0.31 μm). Differences in alloying elements (especially Nb) between Zr702 and Zr-2.5Nb are demonstrated to play a key role in determining their phase transformation behaviors during the β quenching. Analyses on crystallographic orientations show that the Burgers orientation relationship is well obeyed in both the alloys with misorientation angles between α plates essentially focused on ~60°. After β quenching, the hardnesses of both alloys were increased by ~35%-40%. Quantitative analyses using the Hall-Petch equation suggest that such an increase was mainly attributable to phase transformation-induced grain refinements. Since Nb is able to effectively refine the β-quenched structures, a higher hardness increment is produced in Zr-2.5Nb than in Zr702.