Huge local elastic strains in bulk nanostructured pure zirconia materials

From the liquid state to room temperature, two successive solid-state phase transitions occur in pure zirconia. It is well-known that the last one (tetragonal to monoclinic) is martensitic and induces large volume variations and shear strains. Elastic and inelastic behaviors of zirconia-based materials are strongly influenced by this transition and the associated strain fields that it induces. Knowledge of strain and stress at the crystal scale is thus a crucial point. Using fully dense pure zirconia polycrystals obtained by a fuse casting process, we have determined at a sub-micrometric scale, by X-ray Laue microdiffraction, the strains map at room temperature in as-cast specimens and after a post elaboration high temperature thermal treatment. We observed that the fluctuation of deviatoric elastic strain is huge, the standard deviation of normal component being in the range of 1–2%. The heat treatment tends to even further increase this range of fluctuation, despite the development of a multiscale crack network formed during the cooling. Correspondingly, the associated stress level is also huge. It lies in the 5 GPa range with stress gradient amounting 1 GPa μm−1.This work was done in the frame of the ASZTECH research program funded by the ANR (ANR-12-RMNP-0007). We acknowledge the ESRF and the French Collaborating Research Group (F-CRG) for provision of synchrotron radiation facilities beamtimes and beamline staff for their assistance. The authors are thankful to I. Cabodi and O. Bories (Saint- Gobain CREE) for the supply of the bulk zirconia-based materials

Synthesis of Fe-4.6 wt% B alloy via electro-deoxidation of mixed oxides

Fe-4.6 wt% B alloy was synthesized via electro-deoxidation of the mixed oxide precursor. The oxides, Fe2O3 and B2O3, mixed in suitable proportions were sintered at 900 A degrees C yielding pellets with a two-phase structure; Fe2O3 and Fe3BO6. The sintered pellets, connected as cathode, were then electro-deoxidized in molten CaCl2 or in CaCl2-NaCl eutectic, against a graphite anode at 3.1 V. The electrolysis at 850 A degrees C has successfully yielded a powder mixture of Fe and Fe2B. Sequence of changes during the electrolysis was followed by interrupted experiments conducted at 850 A degrees C. This has shown that iron is extracted quite early during the electrolysis through the depletion of oxygen from the starting oxide; Fe2O3, forming the other iron oxides in the process. Boron follows a more complicated route. Fe3BO6, the initial boron-bearing phase, was depleted in the early stages due to its reaction with molten salt. This gave rise to the formation of calcium borate. Boron was extracted from calcium borate in later stages of electrolysis, which appeared to have reacted in situ with the iron forming compound Fe2B

Coupling between elastic strains and phase transition in dense pure zirconia polycrystals

A deep understanding of the solid-state phase transition processes of zirconia is a mandatory requirement for the development of new zirconia-based materials useful for many industrial applications. For five decades, the monoclinic ⇔ tetragonal phase transition is described as a martensitic one and it is well known that it is associated with a large unit cell volume variation that promotes the appearance of elastic strains and also microcracking. In the present paper, we study, through in situ high temperature x-ray diffraction experiments, the coupling between strain relaxation and the martensitic phase transition into a pure zirconia bulk polycrystal. Quantitative analysis of the diffraction signal allows us to disentangle, with respect to the temperature variation, the phase transition and the microcracking processes, and we demonstrate that a high temperature postelaboration thermal treatment induces an increase in the stored elastic energy. Finally, we show that in such polycrystals exhibiting a crystal size distribution and in which the crystals are under internal stresses, the tetragonal to monoclinic phase transition process evolves from a first order one to a second order one when the temperature decrease

LaueNN: neural-network-based hkl recognition of Laue spots and its application to polycrystalline materials

A feed-forward neural-network-based model is presented to index, in real time, the diffraction spots recorded during synchrotron X-ray Laue microdiffraction experiments. Data dimensionality reduction is applied to extract physical 1D features from the 2D X-ray diffraction Laue images, thereby making it possible to train a neural network on the fly for any crystal system. The capabilities of the LaueNN model are illustrated through three examples: a two-phase nanostructure, a textured high-symmetry specimen deformed in situ and a polycrystalline low-symmetry material. This work provides a novel way to efficiently index Laue spots in simple and complex recorded images in <1 s, thereby opening up avenues for the realization of real-time analysis of synchrotron Laue diffraction data