An intermediate cooling rate from в phase, in a Zr-2.5wt.%Nb alloy, has resulted in a decrease in creep rate by 100 times over cold worked mate­rial and rendered higher anisotropy at 450°C. This increase in creep rate is attributed to segregation of Nb in grains that are favorably oriented for easy slip.133

The recent work on the thermal creep of Zr-2.5%Nb alloy by Kishore et at.134 indicates that a microstructure containing a stable phase creeps faster than one with a meta-stable phase and a phase redistribution is established (Fig. 3.31). During creep deformation the stable в phase (with 80 wt.%Nb) dissolves and re-precipitates as в phase (with ~35wt.%Nb), this resulting strain due to phase change adds to the creep strain. Similar phase transfor­mation is reported by Griffiths wherein the Zr-2.5wt.%Nb alloy after 2-14 years of in-reactor service shows that the в phase has a distribution of com­position, the Nb concentration varying from 37% to 75%.135 However, the effect of this phase change on the creep deformation is not well studied.