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MSR: Identification of metallurgical mechanisms and kinetics.

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How do twins form? Thermal twins are relatively well known crystal defects, and yet, the mechanisms through which they appear are surprisingly unknown. Control over the quantity of twins in CFC metal or alloy microstructures after thermomechanical forging is essential in order to obtain desired properties and even more so in a grain boundary engineering context. The ANR Blanc International FORMATING project (2011-2014, NSF-MWN), a collaboration with Pr. A.D. Rollett from Carnegie Mellon University (CMU, Pittsburg USA), obtained significant results which led to a better understanding of these fundamental mechanisms. Two PhD students participated in this project: Yuan Jin, based at the Cemef, and Brian Lin at CMU. In Yuan Jin’s thesis, experiments proved that thermal twining takes place mainly during recrystallization and not during grain growth as much of the literature suggests. Also, the deformation state of the material seems to be the first order parameter while heating rate and annealing temperature seem to have very little influence on quantity of twins found in the final microstructure. This work provides a new topological explanation of thermal twining. One of the articles published on this work [Y. Jin et al., Mat. Sc. Eng. A 597 (2014) p295] was cited 14 times the year after it appeared. Moreover, the numerical methods developed in Yuan Jin’s thesis has allowed for anisotropic interfacial energies to be taken into account in the DIGIMU software.

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The mechanism responsible for abnormal grain growth in nickel based superalloys has been clarified. Andrea Agnoli’s PhD, financed by Snecma and defended in 2013, aimed to understand the reasons why heterogeneous grain growth (leading to unusable components) seems to operate in certain zones of forged Inconel 718 turbine disks, i.e. in certain thermo-mechanical conditions. It turns out that this phenomenon is piloted by energy stored in the microstructure after forging which acts as a strong enough driving force for grain growth to overcome the pinning power of second phase particles located at the grain boundaries [Agnoli et al., Metall. Mat. Trans. A 46 (2015) p4405]. This mechanism only operates in a very narrow critical thermo-mechanical window. The phenomenon was reproduced and explained through finely tuned experimental campaigns and advanced numerical simulations. After this work, the same mechanism was observed in other superalloys (specifically of the γ-γ' variety), therefore extending the conclusions of this first thesis. Also, the numerical tools developed during A. Agnoli’s thesis have enabled the DIGIMU software to take into account pinning phenomena at grain boundaries by second phase particles (Smith-Zener pinning) and opened the discussion regarding the weaknesses of the Smith-Zener maximum grain size model largely employed in the state of the art.

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The discovery of heteroepitaxial recrystallization surrounding primary γ' precipitates in γ-γ' superalloys. The detailed characterization of deformed microstructures, notably through an EDS-EBSD coupling [Charpagne M.-A. et al., J. of Microscopy, 263 (2016) p.106], has led to the discovery of a new γ matrix recrystallization mechanism. This phenomenon leads to a new grain recrystallized by a primary γ' precipitate with the same orientation as the precipitate (in the illustration, on top: γ' in black, recrystallized γ in blue, deformed γ in green, on the bottom: orientation map of both phases). This phenomenon is linked to the inverse precipitation of γ phase which takes place at the edges of γ' precipitates and leads to epitaxial nuclei [Charpagne et al., J. All. Comp. 688 (2016) p685]. This mechanism comes as an addition to the more conventional mechanisms and we also showed that it could be found in other γ-γ' alloys [Charpagne M.-A. et al., Superalloys (2016) p.417]. Marie-Agathe Charpagne obtained the Bodycote-SF2M prize for her thesis work "Evolution de microstructure de l'alliage René65 au cours des opérations de forgeage", which includes the discovery of this mechanism as well as her interactions with other mechanisms and their relative kinetics for a large swath of deformation conditions. The René 65 alloy is also susceptible to abnormal grain growth. As such, the critical condition window in which heterogeneous grain growth is observed were determined and, using a sound knowledge of the physical mechanisms in play, an industrial scale preventive solution was proposed.
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