A TEM metallographic study of hydrides in a Zr-2.5wt%Nb alloy

A thin-foil electron metallographic and electron diffraction study was made of hydrides in Zr-2.5wt%Nb alloy nuclear reactor pressure tubing containing 7–300 ppm by weight hydrogen. At the higher hydrogen contents and for slow cooling rates from above the solvus temperature, there was an increasing tendency to form the equilibrium δ-hydride (ZrH1.66) rather than the metastable γ-hydride (ZrH). The γ- and δ-hydrides differed in morphology; the γ-hydrides were either acicular or of flake form, whereas the δ-hydrides were larger and more blocky in appearance. The large δ-hydrides often formed in a radial through-wall orientation which could lead to through-wall cracks in the pressure tubes.