Low cycle fatigue properties and microscopic deformation structure of Zircaloy-4 in recrystallized and stress-relieved conditions

Low cycle fatigue (LCF) tests have been performed on Zr-4 in recrystallized (RZ) and stress-relieved (SR) conditions at RT and 400 degrees C. Results show that Zr-4/RZ displays cyclic hardening at RT and 400 degrees C, whereas Zr-4/SR exhibits cyclic hardening at RT, but cyclic softening at 400 degrees C. Fatigue lifetime curves as a function of plastic strain range imply that Zr-4/RZ has higher lifetime than that of Zr-4/SR at RT and in the low-cycle region at 400 degrees C, and the LCF properties at 400 degrees C are superior compared to those at RT for Zr-4 in both RZ and SR conditions. TEM examination shows that typical dislocation configuration contains some parallel dislocation lines in Zr-4/RZ, and some dislocation lines are visible between the channels of elongated original grain boundaries for Zr-4/SR at RT. Elongated dislocation cells plus dislocation dipole perpendicular to the cell boundary are observed in Zr-4/RZ, whereas rectangle cells in Zr-4/SR at 400 degrees C. Trace analysis indicates that prismatic and pyramidal slips are possible cyclic deformation modes for Zr-4 in different heat treatment conditions and at different temperatures. Finally, the relationships between LCF properties and microscopic deformation mechanisms are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.