Two-Stage Steady State Creep Behavior of Zircaloy-4 in High Temperature Creep Condition

Abstract

To identify the creep behavior of Zircaloy-4 during operating time of nuclear power plants, creep tests were performed at 673 K. The alloy showed an abnormal creep behavior, i.e., two-stage steady state creep, at low stresses of less than 66 MPa. The first steady state was observed at strain of 0.05, whereas the second one appeared at that of 0.15 at 55 MPa. Moreover, the first steady state creep rate was faster than the second one. It resulted in the changing of the stress exponent from 4.9 to 14. Microscopy after the creep tests revealed that dislocation structures at each stage were totally different. In the first stage, EBSD results showed that strain distributed uniformly in grain interiors, where dislocations moved individually. In the second stage, the interaction among dislocations became stronger with increasing strain because cell structure was generated. Therefore, the creep was suppressed and the creep rate decreased in the second stage. It concluded that the changing of dislocation structure during creep led to two-stage steady state creep behavior in Zircaloy-4.