Reply to comment on “the ductile-brittle transition of a zirconium alloy due to hydrogen”

Abstract

Bai estimates that the local strain rate at the notch tip in our notch tensile tests is at least equals to 2.4x10 -3 /s. Then, he compares our results with recent results by Bai et a1.[3-5] in which the specimens were tested at 10~/s or 10-3/s. In this work it was found that for the hydrided specimens, those tested at 10-4/s exhibited an usual ductilebrittle transition behavior at room temperature, while those tested at 10-3/S were brittle with elongations less than 2 %, independent of the testing temperatures and the hydrogen contents. Therefore, Bai comments that the different hydrogen contents for ductile-brittle transition may be also attributed to the strain rate effect beside with the notch effect. We can partly agree with this point of view. It is well acknowledged that the local strain rate at a notch tip can be magnified to a large extent and the attendant increase of strain rate can induce brittle behavior of metals. However, from our results on smooth hydrided specimens tensile tested at 5x103/s, as shown in Figure 1, no ductilebrittle transition in reduction of area was found up to a hydrogen content of 340 ppm. Since the local strain rate at the notch tip, as estimated by Bai, is close to the strain rate used in tensile tests for the smooth specimens, it is clear that strain rate per se is not significant for the early occurrence of ductile-brittle transition. In fact, Gill et al.[6] performed notch tensile tests on hydrided zirconium and found an overall lowering of elongation values for notched specimens. This was attributed to the effects of triaxiality in the stress system introduced by notch. The triaxiality can reduce the magnitude of shear stress which are responsible for plastic deformation. Thus, the notched specimens exhibit brittle behavior at a lower hydrogen content. Bai's findings of brittleness of the specimens tested at 103/s were obtained from specimens containing at least 500 ppm of hydrogen. Since we did not perform any tests on specimens with hydrogen content higher than 350 ppm, no direct comparison can be made with Bai's results. The other point mentioned by Bai is the effect of hydriding temperature on the mechanical properties of hydrided specimens and the hydrides themselves. The hydriding temperature in our study was purposely chosen to avoid extraneous changes in alloy microstructure. Since the stress relief temperature of zirconium alloys is 480595 °C [7], hydriding at a temperature above 500°C may produce changes in alloy microstructure and thereby change the mechanical properties of zirconium alloys. Therefore, it is not surprising that the ductile-brittle transition on Zircaloy-4 specimens hydrided at 700°C occurs at a different hydrogen content than those hydrided at 400°C. However, for the specimens hydrided at temperatures lower than the stress relief temperature, such as 400°C or 350°C, it is not expected that any extraneous changes in alloy microstructure from the effect of temperature will occur; as a result, the mechanical properties of the zirconium matrix, regardless hydrided at 350°C or 400°C, will