Creep of zircaloy-2 at low stresses

Abstract

Low stress creep behaviour of Zircaloy-2 of grain size 8, 12 and 17 μm has been investigated in the temperature range 783 to 993 K at stresses below 10 MN/m². Grain boundary diffusion controlled Coble creep governs flow in the material with 8 and 12 μm grain size. However, a switch to lattice diffusion controlled Nabarro-Herring creep occurs when Zircaloy-2 is treated to develop the coarser 17 μm grain size. It is suggested that an explanation for this change in creep behaviour is contained in the transmission electron microscopy observations with regard to grain boundary precipitation. In Zircaloy of 8 and 12 μm grain size, Zr₂ (Ni,Fe) type precipitates occur at grain boundaries, while grain boundaries appear to be the preferred sites for the more refractory Zr(Cr₂Fe)₂ type precipitates in the sample heat-treated to produce the 17 μm grain size.