High temperature mechanical behavior of Zr–2.5% Nb alloy

Zr–2.5% Nb alloy is widely used as structural material for pressure tubes of channel nuclear power reactors cooled by heavy pressured water (CANDU-PHW) and boiler water (RBMK, Fugen). Tensile and creep behavior of Zr–2.5% Nb alloy significantly depend on heat treatment. Zr–2.5% Nb alloy is used after different heat treatments to satisfy design requirements to reactor pressure tubes: α-annealing (reactor RBMK-1000), water or gas quenching and aging (reactor RBMK-1500 and Fugen), cold-work and stress-relief (reactor CANDU). An experimental program has been elaborated to research resistance to plastic flow and rupture of Zr–2.5% Nb alloy at the temperatures 20–1200 °C. Tensile tests were carried out at a constant cross-head speed in the range 10−2 to 10−4 s−1 and creep tests under uniaxial load. Temperature dependence of flow and rupture stress were distinguish in different temperature ranges. Three ranges of temperature were observed where the rate of the strength reduction was significantly different. Extremely large elongation was observed in the 600–800 °C range that is manifestation of superplasticity. The temperature corresponding to maximum elongation was equal ∼700 °C. Increase of strain rate from ∼10−4 to 10−2 s−1 did not changed the temperature ranges of superplasticity observation but resulted in decrease of elongation values.Creep tests were carried out in the temperature range of 500–1000 °C. It was observed that creep curves at the temperatures above 600 °C are not exhibited the hardening and creep rate is increased up to failure. The total elongation is diminished from increase of initial stress value. Results of creep tests (creep rates and time to failure) have been described by power theory of creep the equations.