The effect of hydride on the thermal expansivity of zirconium alloys

Abstract

Dilatometric data were determined for hydrided Zircaloy-4, Zircaloy-2 and zirconium− 2 1 2 wt.% niobium alloys. The thermal expansivity of the hydrided alloys is dependent on the nature and distribution of the hydride phase. At low hydrogen contents up to ~600 p.p.m. by weight, the face-centred tetragonal γ'-hydride phase found at room temperature has a thermal expansivity less than that of the α-Zr phase and when directionally aligned as platelets, it can restrain the thermal dilation of the alloy and lower its thermal expansivity. At higher temperatures, the γ'-hydride phase transforms to the less dense face-centred cubic (δ-hydride phase with a thermal expansivity considerably higher than that of the alpha phase, and there is a marked increase in the thermal expansivity of the alloy to values approaching that of pure (δ-hydride phase. The (α+hydride)/hydride boundary is indicated by a marked reduction in expansivity on heating and a corresponding increase on cooling, when the less dense hydride is dissolved or precipitated from the α-Zr matrix. On cooling below the boundary, the initial formation of δ-hydride is followed by a transformation to γ'-hydride. The transition γ'⇉δ-hydride may be related to the brittle/ductile transition found in hydrided zirconium alloys.