Sub-boundaries induced by dislocational creep in uranium dioxide analyzed by advanced diffraction and channeling electron microscopy

Highlights

• Sub-boundaries induced by creep in UO₂ were analyzed by advanced EBSD.

• Very low angle boundaries disoriented down to 0.1° were reliably detected_.

• Dislocations and their arrangements in low angle boundaries were revealed by A-ECCI.

• The prior 20 μm grains were fragmented in average into 5 μm sub-grains.

• Creep increased the average GND density by a factor 10 to reach 7.9 × 10¹² m^− 2.

Abstract

The network of sub-boundaries formed in a sintered UO₂ pellet after dislocational creep was examined. Very low angle boundaries, down to 0.1° were reliably detected by Electron BackScattered Diffraction (EBSD). This angular resolution was achieved by optimizing EBSD data collection and processing. Moreover, Accurate-Electron Channeling Contrast Imaging (Accurate-ECCI) was able to image the dislocations produced by creep, directly on the bulk sample. The dislocations were mostly organized in sub-boundaries with low energy configurations. Only a limited number of isolated dislocations were observed. Finally, the deformation substructure obtained after 8% creep deformation (at 1500 °C under a 50 MPa uniaxial state) was quantified. The original grains with a mean size of 20 μm were in average fragmented in sub-grains of about 5 μm. The geometrically necessary dislocations density (GND) was evaluated from the filtered EBSD data to 7.9 × 10¹² m^− 2. This value was 10 times higher than that measured on the as-sintered sample. This confirms that the GND density calculation is sensitive to the dislocation increase after 8% deformation by creep.

Introduction

Sintered uranium dioxide pellets are commonly used in nuclear pressurized water reactors. In normal operating conditions, their central temperature reaches values of the order of 1000 °C whereas it increases up to 1500 °C, or even more, during power transients. Under these conditions, the pellets undergo significant plastic deformation through creep mechanisms, which influence pellet-cladding interaction processes [1], [2]. These mechanisms are partly reproduced by compression creep tests on non-irradiated pellets [3]. The resulting creep damage in these deformed UO₂ pellets is characterized by the development of inter-granular cavities at low stresses (25 MPa), and crack opening at high stresses (50 MPa) [3]. Inside the grains, dislocations produced during the deformation arrange in sub-boundaries, as previously evidenced by means of Transmission Electron Microscopy (TEM) examinations [4]. However, no scale transition between the SEM observations on bulk UO₂ crept samples and those made by TEM on thin foils, can be found in the literature. In addition, no quantified data about the deformation substructure is available.

Electron BackScattered Diffraction (EBSD) and Electron Channeling Contrast Imaging (ECCI) could contribute to achieve TEM-SEM scale transitions and quantify the microstructure evolution under creep. Indeed, ECCI is a SEM-based technique that allows imaging defects in the near surface of bulk materials [5], [6], [7], [8]. Moreover, in many cases, comprehensive characterization of dislocations is possible using analyses of dislocation contrast with various diffraction conditions [9], [10], [11], [12], [13], [14], [15], [16], [17]. Nevertheless, acquiring high quality ECC images requires the channeling conditions to be precisely satisfied. Electron Channeling Pattern (ECP) limited to single crystals or Selected Area Channeling Patterns (SACP) more suitable for polycrystalline materials are used to control the orientation of the region of interest. Unfortunately, the poor spatial resolution of SACP restricts ECCI applications to large grained materials [9], [11], [18], [19], [20], [21], [22], [23], [24], [25]. A new approach developed recently allows the collection of High Resolution SACPs with a spatial resolution of 500 nm [26]. It opens the way to apply Accurate-ECCI on submicron sized grains [27]. Dislocations in creep deformed UO₂ can now be observed by A-ECCI as demonstrated in [28] while in the past they were only analyzed by TEM [3], [4]. Furthermore, continuous progress in EBSD detectors and diffraction patterns analysis offer now improved angular resolution to reliably quantify the low angle boundaries and the subsequent network of geometrically necessary dislocations (GND) [29], [30], [31], [32], [33].

Thus, this paper performs up to date EBSD analysis and A-ECC Imaging on UO₂ samples to analyze and quantify the deformation substructure induced by dislocational creep. EBSD data were acquired in improved angular resolution mode and post-processed with a routine optimized for porous materials. As a result, very low angle boundaries disorientated down to 0.1° were detected. These sub-boundaries were further observed by A-ECCI to reveal the dislocation network induced by creep deformation. Finally EBSD-based quantities were calculated over large EBSD maps to quantify the deformation substructure at a given creep state. They include the quantification of linear fraction of sub-boundaries and boundaries depending on their disorientation range, the determination of sub-grain average size and the GND density quantification. These quantities are worth to quantify the sub-structured microstructure induced by creep and compare its evolution for different deformation conditions. They may also be useful in physically-based modeling of the creep behavior.