Effect of strain rate on the cyclic hardening of Zircaloy-4 in the dynamic strain aging temperature range

Low cycle fatigue (LCF) tests were conducted in Zircaloy-4 with a total strain range of ±0.5% in the temperature range 573–873 K where dynamic strain aging (DSA) manifestations in uniaxial tensile tests were reported. A time dependent linear cyclic hardening stage is the principal feature observed in this alloy. The cyclic hardening rate exhibits a peak in this temperature range. In order to evaluate the strain rate effects on the cyclic hardening, tests at strain rates 2 × 10−3 s−1 and 2 × 10−4 s−1 were performed. The location of the cyclic hardening rate peak depends on the strain rate suggesting the operation of a thermally activated mechanism. Transmission electron microscopy (TEM) observations show that the linear hardening stage is characterized by a two-dimensional wall structure that evolves from the activation of two slip systems. DSA takes place during strain-controlled fatigue deformation at the same temperature range of that occurring during tensile deformation.