L. Thuinet

Characterization of Zirconium Hydrides and Phase Field Approach to a Mesoscopic-Scale Modeling of Their Precipitation

Zirconium alloys are currently used in nuclear power plants where they are submitted to hydrogen pick-up. Hydrogen in solid solution or hydride precipitation can affect the behavior of zirconium alloys during service but also in long term storage and in accidental conditions. Numerical modeling at mesoscopic scale using a “phase field” approach has been launched to describe hydride precipitation and its consequences on the mechanical properties of zirconium alloys. To obtain realistic results, it should take into account an accurate kinetic, thermodynamic, and structural database in order to properly describe hydride nucleation, growth, and coalescence as well as hydride interaction with external stresses. Therefore, an accurate structural characterization was performed on Zircaloy-4 plates and it allowed us to identify a new zirconium hydride phase called ζ. The ζ phase has a trigonal symmetry and is fully coherent with hcp αZr. The consequences of this new zirconium hydride phase on hydride transformation process and stress-reorientation phenomenon are discussed. A first attempt to numerically model the precipitation of this new zirconium hydride phase has been undertaken using the phase field approach.

New insights on strain energies in hexagonal systems

The preferential habit planes of coherent precipitates, strongly influencing alloy properties, can be investigated by direct-space elasticity methods, providing new insight into delicate issues such as elastic inhomogeneities or anharmonicity. Focusing on the poorly known hexagonal system, this work enlightens important trends overlooked hitherto, such as the critical role of C44, leading to the identification of distinct families of hexagonal alloys for precipitation. Moreover, it demonstrates the complex influence of inhomogeneities for real, finite-thickness morphologies. Finally, it provides the missing material required for atomic-scale studies of precipitation in low-symmetry systems with long-range interactions.

Atomic-Scale Modeling of Fe-Al-Mn-C Alloy Using Pair Models and Monte-Carlo Calculations

The Fe-Al-Mn-C system is widely studied for automotive applications due to its good mechanical properties and relatively low density. Our work is devoted to the atomic-scale modeling of this system and to start with, we focused on the well-documented Fe-Al binary system. More precisely, we tested the capability to reproduce its phase diagram combining ab initio calculations and cluster expansion methods. Several models were built using different input atomic configurations: pure iron, a substitutional aluminium atom diluted in iron, pairs of substitutional aluminium atoms located at different neighbour shells and complementary structures (B2, B32 and D03). Long-range order parameters (occupation of sublattices) were defined to analyse the equilibrium configurations generated by Monte-Carlo runs in the semi-grand canonical ensemble. Phase diagrams were plotted for each model and compare well with experimental ones.

Interfacial properties of hydrides in α-Zr: a theoretical study

In order to better understand hydride formation in zirconium alloys, heterophase interfaces between α-Zr and γ-ZrH are investigated by means of ab initio atomic-scale simulations of multilayers coupled with continuous elasticity. Our approach allows us to separate out the elastic contribution, leading to basal and prismatic [Formula: see text] interface energies around 200 [Formula: see text] and 750 [Formula: see text] respectively, i.e. values noticeably higher than previously found for coherent particles such as ζ-Zr2H. By considering interfacial changes of H contents, the possibility of competing elasticity and chemistry effects for interface stability is analyzed. The effects of the strong anisotropy evident in [Formula: see text] interface energies on the important practical issue of preferential habit planes are discussed, allowing us to propose a plausible explanation for the experimental results.

Zêta Hydride Precipitation in Zirconium Alloys: an Example of Elastically Driven Morphology of Coherent Trigonal Precipitates inside a Сlose-Packed Hexagonal Matrix

The influence of a crystallographic symmetry break on the morphology of precipitates during the coherent precipitation of a trigonal phase in a close packed hexagonal matrix is analyzed. It is pointed out that in spite of the isotropy of the stress free strain of the precipitate in the basal plane, the existence of an extra elastic constant in the precipitate (associated to the loss of symmetry) induces a morphological evolution from a shape having a symmetry of revolution around the threefold axis to a needle-like one oriented along the compact directions in the basal plane. These general considerations are applied to the case of zêta zirconium hydrides the crystallography of which has been recently identified to be coherent with that of the alpha Zr matrix. The influence of symmetry break and elastic heterogeneity on precipitation morphology has been numerically addressed by using different approaches. An analytical approximation to the elastic energy based on Eshelby equivalent inclusion method allows obtaining a qualitative criterion to determine the occurrence of a shape bifurcation of zêta hydrides.