**Published:** Jan 1987

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**Source: **STP939-EB

The object of the present work is to study the load and strain cyclic behavior of Zircaloy-4 in the temperature range 573 to 873 K. Load cyclic tests are performed at 673 and 873 K for different values of *R* = *σ*^{min}/*σ*^{max}, keeping *σ*^{max} and the load rate constant at each temperature. At 873 K the lifetime and the cyclic creep rate show a time-dependent behavior, with the mean strain being activated by the mean stress. For zero values of the mean stress (*R* = −1) no appreciable net strain accumulation occurs and failure by fatigue precedes failure by cyclic creep. At 673 K the mean strain is activated by the mean stress only in the range 1 > *R* > 0. In the 0 > *R* > −1 range the cyclic creep becomes faster on decreasing the mean stress up to some *R* value between −0.9 and −1. This effect is consistent with a dislocation model where the substructure formed in tension is unstable when the loading direction is reversed.

Strain-controlled cyclic tests with a total strain rate of 0.01 and a strain rate of 2 × 10^{−3}s ^{−1} are also carried out in the range 573 to 873 K. The peak tensile stress response during strain cycling shows a cyclic hardening behavior in the range 623 to 823 K. In the region 673 to 743 K this effect is more prominent and the hysteresis loops are nearly independent of temperature and strain rate. This phenomenon and the well-defined cell structure observed by transmission electron microscopy are attributed to strain aging. Below and above this temperature region (at 573 and 873 K) the total stress range remains approximately the same after a few cycles up to near fracture. The dislocation structures observed at both temperatures are consistent with the lack of cyclic hardening or softening.

**Keywords:**

Zircaloy, cyclic creep, low-cycle fatigue, dislocations (materials), high temperature

**Author Information:**

Armas, AF *Instituto de Física Rosario, Rosario,*

Alvarez-Armas, I *Instituto de Física Rosario, Rosario,*

**Paper ID:** STP28147S

**Committee/Subcommittee:** B10.02

**DOI:** 10.1520/STP28147S