On the microstructural evolution and hydriding behavior of dilute Zr-2.5Nb-Y alloys

Zr-2.5Nb alloy is a critical pressure tube material in pressurized heavy water reactors (PHWRs), as its performance directly influences the operational life and safety of the reactor. However, the formation of hydrides in this alloy can detrimentally affect its mechanical properties. In the present study, yttrium was employed as a dilute addition to mitigate hydride embrittlement. Alloys were prepared with varying yttrium contents of 0.5 to 2 wt %. Yttrium addition resulted in the formation of fine yttria precipitates. The hot deformed microstructures of alloys with Y exhibited significant differences in morphology of the phases and prior β phase fraction when compared to reference Zr2.5Nb alloy. Dilatometry studies indicated that yttrium addition led to a reduction in the β-transus temperature of the alloy. The hydride behavior of the alloys was also examined, showing that yttrium significantly reduced hydride size to <20 μm, compared to a range of 10–150 μm in the absence of yttrium (Zr-2.5Nb). This comprehensive study of the microstructure and hydriding behavior, with the addition of yttrium to the Zr-2.5Nb alloy, suggests that yttrium may be considered for improving the alloy's performance in nuclear applications, in view of the mitigation of hydride embrittlement.