In situ characterization of microstructural instabilities: Recovery, recrystallization and abnormal growth in nanoreinforced steel powder
Introduction
Oxide dispersion-strengthened (ODS) ferritic steels are being intensely investigated for high-temperature nuclear applications due to their excellent creep properties and swelling resistance [1]. These assets are obtained thanks to a fine-grained body-centred cubic microstructure reinforced by a dense and homogeneous precipitation of yttrium nanooxides. The classical processing route of this material is based on powder metallurgy involving high-energy ball milling in order to obtain a homogeneous distribution of yttrium in the ferritic powder. This first step is followed by hot isostatic pressing [2], [3], [4] and hot extrusion [5]. In the most recent processing routes, the material is also regularly consolidated by field-assisted sintering [6], [7]. Plastic deformation during mechanical alloying introduces a high dislocation density and deeply modifies the grain microstructure, which results in nanosized elongated grains [8]. Given the high deformation level and the small grain size, the driving force for microstructural instabilities is particularly high. Yttrium in solid solution precipitates rapidly from 600 °C under annealing. The Zener pinning force is thus also especially elevated due to the dense population of small yttrium precipitates. Having extremely high driving and retarding forces that oppose each other creates an important microstructural instability factor. The presence of an important concentration of particular pinning points, such as triple and quadruple grain boundary junctions, induced by the small grain size, will maximize the Zener force. Indeed, the smaller the grain size, the higher the number of particular points on which pinning is more efficient [9], [10], [11], [12]. Therefore, the probability of triggering unstable phenomena like abnormal grain growth is very high.
When the material is extruded, abnormal grains elongate and are responsible for the large anisotropy of the material microstructure. This anisotropy induces detrimental mechanical properties [5] and particularly poor transverse creep strength [13], which is a key property in fuel cladding applications. Indeed, as internal pressure increases in the tube cladding with the accumulation of gas fission products, the major stress component is applied in the transverse direction. Therefore, the material faces a critical risk of failure and the control of the microstructure is a key issue to ensure that it can safely fulfil the role of first barrier against the release of radioactive elements. Despite its indisputable technological impact, microstructural instability, such as abnormal grain growth, of industrial nanostructured metallic materials remains only partially explored. The scarcity of such studies is primarily due to the difficulties in following the mechanisms leading to microstructural evolution: recovery, recrystallization and grain growth can be concomitant and are unpropitious to quantify in a time-resolved manner. Yet, for powder metallurgy, kinetic studies of the microstructural evolution starting from the as-milled powder and during subsequent annealing are essential to detect the beginning of critical phenomena like abnormal grain growth.
Because of its non-destructive character, X-ray diffraction (XRD) has proved to be an appropriate method for describing dislocation density and crystallite size [14], [15], [16], [17], which are suitable microstructural features for monitoring phenomena such as recovery, recrystallization and grain growth. Nevertheless, the instability and rapid evolution of the ferritic microstructure requires the use of fast and precise in situ characterization methods. Kinetic studies have been successfully carried out by combining synchrotron XRD with fast 2-D detectors to obtain timely, well-resolved diffraction peaks [18], [19]. No time-resolved study of recovery, recrystallization and further grain growth upon heating of a nanostructured ODS steel had been reported to date. In the present work, we focus on the evolution of the crystallite size and the dislocation density upon annealing. First, we report the methodology of the in situ XRD measurements and the adaptation of existing methods to improve data analysis. Since X-ray peak broadening is only sensitive to crystallite sizes below 1 μm, it is expected that this in situ investigation will deliver important information on the recovery and early stages of recrystallization of the ODS steel. These results are discussed in terms of the classical microstructural mechanisms of recovery and recrystallization. Using a special algorithm that captures individual spots of high intensity, a qualitative description of abnormal grain growth is also presented.
Section snippets
Material
A high-chromium ferritic steel powder was produced by ingot gas atomization by Aubert & Duval. The powder particles were then mechanically alloyed with submicronic yttria powder (Y2O3) by Plansee SE using a high-energy attritor. The powder is representative of common industrial nanocrystalline powder widely used to process nanostructured materials. Milling conditions and microscopic evaluation of the as-milled powder are respectively reported in Refs. [8], [20]. Using a focused ion beam to
Low-temperature annealing treatments ( = 600 °C; = 800 °C)
The dislocation density and crystallite size upon heating up to 600 °C are reported in Fig. 3. Before heating, the initial nanostructure contains coherent domains with a diameter of about 23 nm, demonstrating the efficiency of high-energy attrition to produce nanostructured materials [41], [42], [43]. A considerable amount of plastic work is stored during milling. Indeed, the dislocation density is m−2, which is consistent with what is observed ex situ by XRD or electron microscopy [8],
Stage I: mechanisms operating at low temperature
Dislocations structures formed during cold-working of polycrystals are usually very complex. During heating, dislocations with opposite Burgers vectors can annihilate, while others can rearrange by climbing and forming low-angle subgrain boundaries, such as particular tilt boundaries [44]. In this study, the nanocrystalline powder was processed by high-energy mechanical alloying, producing a huge amount of dislocations. Due to the high stalking fault energy of the ferritic steels, these
Conclusion
In situ XRD experiments have been performed to reveal the microstructural evolution process leading to the bimodal grain structure of ODS ferritic steels. Two evaluation methods sensitive to different grain populations were applied to in situ recorded diffraction images in order to capture the recovery and the onset of recrystallization and abnormal grain growth. The growth kinetics of specific abnormal grains was quantified. Regarding the selected grains only, no significant incubation time
Acknowledgements
N.S. was supported by the joint program “CPR ODISSEE” funded by Areva, CEA, CNRS, EDF and Mécachrome under contract n° 070551. X.B. was supported by the European Community within the FP7 Project MATTER and in the frame of a tripartite agreement between the CEA, AREVA NP and EDF. Thanks are due to C. le Bourlot for valuable discussion.
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