Full length articleMobility of dislocations in zirconium
Graphical abstract
Introduction
Zirconium alloys are used in the nuclear industry as fuel cladding tubes and structural components in light and heavy water reactors [1]. Like in other hexagonal close-packed (hcp) metals, plastic activity strongly varies between the different possible slip systems in zirconium. The principal slip system is controlled by glide of ⟨a⟩ dislocations, i.e. dislocations with Burgers vectors, in the prismatic planes [2], [3]. These ⟨a⟩ dislocations can also cross-slip above room temperatures to glide in the (0001) basal and first-order pyramidal planes [4]. However all these deformation modes only accommodate deformation along ⟨a⟩ directions and other mechanisms, either twinning or glide of dislocations with a ⟨c⟩ component, are needed to allow for a deformation along the axis of the hcp crystal.
Both twinning and glide of dislocations, i.e. dislocations with Burgers vectors, are active in zirconium [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], with slip becoming more active when the temperature increases. Depending of the hcp metals, these dislocations can glide in the first-order or second-order pyramidal planes. Almost all transmission electron microscopy (TEM) observations have shown, by slip traces analysis [7] or stereography [8], [10], [11], [13], [14], [15], [16], that dislocations glide in first-order pyramidal planes in Zr alloys, with possible cross-slip between two pyramidal planes sharing the same direction [14], [15], [16]. Scanning electron microscopy of slip traces on bended micro-cantilever well oriented to activate only slip also concluded that dislocation glide in first-order pyramidal planes, without any trace of slip in second-order pyramidal planes, both for the compressive and tensile parts of the cantilevers [17]. Only Long et al. [16] have recently evidenced that a minority of can also be found in the second order pyramidal plane in a Zr-2.5Nb alloy. Besides, some of these TEM observations indicate that dislocations have a tendency to straighten in the direction corresponding to the intersection of the pyramidal glide plane with the basal plane [8], [11], [15], [16], a feature which is not specific to zirconium and can be found in other hcp metals, like Mg [18], [19], [20], [21], [22], [23] or Ti [24], [25], regardless of the pyramidal glide plane.
The critical resolved shear stress (CRSS) necessary to activate glide of dislocations strongly depends on the temperature but is always much higher than the CRSS of ⟨a⟩ dislocation glide [7], [17], thus explaining the strong plastic anisotropy of zirconium single crystals. An increased activity of slip is observed in irradiated zirconium alloys, as the hardening induced by irradiation defects appears stronger on ⟨a⟩ slip systems than on pyramidal slip [14], [26], thus partly compensating for the friction difference between the two types of dislocations.
Although the mobility of dislocations is a key ingredient of the plastic anisotropy of zirconium, not much is known on this mobility besides the slip plane, the dislocation ability to cross-slip and their tendency to align in a specific direction as mentioned above. In situ TEM straining experiments are a valuable tool to study dislocation mobility. Such experiments in zirconium have already revealed some key features of ⟨a⟩ dislocations [3], [4], [14], [27], [28], [29]. But no such in situ observations have been performed for the glide of dislocations in zirconium. The difficulty arises from the necessity to have grains well oriented to activate slip. The aim of the present article is to present such experiments. In situ TEM straining experiments have been performed in two different zirconium alloys, Zircaloy-4 and pure zirconium, as previous experimental studies on polycrystals [6] have shown a difference in the strain accommodation along the ⟨c⟩ axis between Zircaloy-4 and pure zirconium. The results of these in situ experiments are presented below.
Section snippets
Materials
Two different zirconium materials have been chosen for this study: zirconium sponge, which is essentially pure zirconium, and Zircaloy-4 in the recrystallized metallurgical state, which is often considered as a model material for zirconium alloys. Their chemical compositions are given in Table 1.
Results in recrystallized Zircaloy-4
Besides the in situ TEM straining experiments described below, some post mortem observations on the same recrystallized Zircaloy-4 have been conducted after tensile tests to check that the behavior of dislocations characterized in situ in thin foils concurs with the dislocation microstructure obtained after straining macroscopic samples. These post mortem observations are described in Appendix A.
During the in situ experiments, the observation of dislocation motion was always
Results in pure Zr
The same in situ TEM straining experiments have been performed at room temperature on pure zirconium. Comparing these observations with the previous ones in Zircaloy-4 gives some insights on alloying effects on the mobility of dislocations.
Glide planes and cross-slip
Only glide in first-order pyramidal planes has been observed in our experiments, both in pure zirconium and in Zircaloy-4. This agrees with previous TEM observations [7], [8], [10], [11], [13], [14], [15], [16] which invariably report the same glide plane for dislocations. Only Long et al. [16] have shown that cross-slip in second order pyramidal plane is also possible, with nevertheless first-order pyramidal plane being the main glide plane. As the zirconium alloy studied by Long et al.
Conclusions
In situ TEM straining experiments performed both in pure zirconium and in Zircaloy-4 have shown that dislocations glide exclusively in first-order pyramidal planes at room temperatures, with some cross-slip events between two pyramidal planes sharing the same direction. A characteristic feature of the microstructure is that dislocations align in the ⟨a⟩ direction defined by the intersection of their glide plane with the basal plane, leading to long straight dislocations.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank Framatome for providing the raw Zr sponge and the Zircaloy-4 TREX tube, D. Nunes and S. Urvoy (SRMA/CEA) for the preparation of the pure Zr samples and B. Arnal (SRMA/CEA) for thin foils preparation. This work is funded bythe French Tripartite Institute (CEA-EDF-Framatome) through the GAINE project.
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