Double Steady-State Creep Behavior in Solid-Solute-Strengthened Zircaloy-4

Long-term creep tests on the solid-solute-strengthened zirconium alloy, i.e., Zircaloy-4, revealed the presence of double steady-state creep behavior at 623-673 K. The first steady state manifested as a faster creep strain rate at a strain (epsilon) of similar to 0.05, and the second appeared with a slower creep strain rate at epsilon > 0.1. Several electron microscopy revealed a change in the dislocation structure during creep from the motion of individual dislocations to the generation of cell structure. Along with this change, the stress exponent differed in each steady state, increasing from 6.7 in the first steady state to 10 in the second steady state. The temperature dependency also changed because the apparent activation energies were 201 and 298 kJ/mol for the first and second steady states, respectively. These results show that pipe diffusion and self-diffusion are the rate-controlling processes in the first and second steady state, respectively. Because such changes in the deformation mechanism during one creep curve are highly unusual, the double steady state is a characteristic feature of high-temperature deformation in Zircaloy-4. For engineering, observation of the second steady state is necessary in estimating the time-to-rupture by means of the Montanan Grant relationship because creep strain rates at the first steady state deviate from the relationship.