Tensile tests of Zircaloy-4 were carried out over the temperature range of 295~950 K and at the strain rates of $6.67×10^{-3}$, $6.67×10^{-4}$, $6.67×10^{-5}$ $s^{-1}$. The anomalous tensile deformation behavior of Zircaloy-4 of the maximum uniform strain, minimum total and necking strain, is observed in the temperature range of 600~700 K(0.32~0.37 $T_m$), and the maximum value of strain hardening exponent is appeared in the same temperature range. These anomalous tensile deformation behaviors are explained in terms of the effect of dynamic strain aging of oxygen atoms.
The static and cyclic creep tests of Zircaloy-4 were conducted at the temperature range of 650~690 K (0.34~70.36 Tm) which is thought to have the strongest dynamic strain aging (DSA) effects within the DSA temperature range of 500$\sim$800 K in which most of Zr alloys are used in operation and stress range of 160~190 MPa ($σ/E=2.12×10^{-3}~2.51×10^{-3}$) with various stress frequencies to study the effect of DSA on the cyclic creep behaviors and the characteristic dislocation structures of Zircaloy-4 during the static and cyclic creep in argon atmosphere. The cyclic creep retardation (CCR) behavior has been observed with increasing stress frequency in Zircaloy-4 under the test condition rather than the cyclic creep acceleration (CCA) behavior which is usually observed in most metals and alloys. The degree of CCR in terms of the ratio of cyclic to static creep rate is observed to be decreased with increasing stress frequency and decreasing temperature under the constant peak stress. This is a very interesting result in that the temperature and cyclic stress frequency dependencies of creep rate ratio of Zircaloy-4 are found to be opposite to the general observation in most metals and alloys at this homologous temperature and stress region in which the CCA is dominant and the creep rate is increased with increasing stress frequency. The creep activation energy for static and cyclic creep were measured at various stress values. The cyclic creep activation energy is observed to be larger than those of static creep and lattice diffusion of Zr. These anomalous creep deformation behaviors are explained in terms of the effect of dynamic strain aging.
The dislocation structure of the Zircaloy-4 crept at the DSA temperature range of 670 K were observed using transmission electron microscopy (TEM) to find the differences in the dislocation configurations developed under the different stress modes and stress frequencies. The feature of the dislocation structure developed under the static stress shows a typical dislocation cell structure, on the other hand, the dislocation structure developed under the cyclic stress with higher stress frequency does not show a cell structure, and dislocations are distributed homogeneously. It is suggested that the recovery process of the static creep which has the highly dense disloaction structure is the climb of the tangled dislocation at cell boundary. However, in the cyclic creep, the deformation controlling process is changed from dislocation climb to dislocation glide as the stress frequency increases. The transient creep strain at the cyclic to static transition period increases as the stress frequency increases in the cyclic creep. This observation is explained using the observed dislocation structures. Under the lower stress frequencies, the dislocation structure shows cellular distribution of dislocation tangles which is sessile, and the transient creep strain at the transition period is small. As the cyclic stress frequency increases, the dislocation structure is changed to plannar arrays which is mobile, and the transient creep strain at the transition period increases.
In this study, the dislocation structure of Zircaloy-4 has been changed by the stressing mode and stress frequency in the DSA temperature range. As a result, the stress frequency plays an important role on the cyclic creep deformation behavior, i.e., the cyclic creep strain and creep rate, the formation of dislocation structures, and controlling process of the creep deformation in the DSA temperature range.
