Thermodynamic modeling of the H-Mg-Na system is performed for the first time in this work in order to understand the phase relationships in this system. A new thermodynamic description of the stable NaMgH 3 hydride is performed and the thermodynamic models for the H-Mg, Mg-Na, and H-Na systems are reassessed using the modified quasichemical model for the liquid phase. The thermodynamic properties of the ternary system are estimated from the models of the binary systems and the ternary compound using CALPHAD technique. The constructed database is successfully used to reproduce the pressure-composition isotherms for MgH 2 + 10 wt.% NaH mixtures. Also, the pressure-temperature equilibrium diagram and reaction paths for the same composition are predicted at different temperatures and pressures. Even though it is proved that H-Mg-Na does not meet the DOE hydrogen storage requirements for onboard applications, the best working temperatures and pressures to benefit from its full catalytic role are given. Also, the present database can be used for thermodynamic assessments of higher order systems.
A scanning electron microscopy (SEM)-based electron backscatter diffraction (EBSD) analysis showed that the discontinuous precipitation (DP) reaction rate was dependent on the geometry of the grain boundary in Mg-9Al-1Zn (wt.%) alloys. DP converted a supersaturated solid solution, δ 0 (magnesium (Mg)rich solid solution), into a two-phase δ + γ aggregate, with δ being a precipitated Mg 17 Al 12 (intermetallic phase) behind a migrating reaction front. The near-special grain boundary was rather inactive, whereas most of the random high-angle boundaries promoted the reaction. Prior deformation (hot rolling to achieve up to 80% thickness reduction) had no effect on the frequency of special-grain boundaries.
An AZ31 twin roll cast (TRC) magnesium alloy was subjected to rolling at 450 °C to different final thicknesses. This processing resulted in a gradual grain refinement and microstructure homogenization. Annealing treatments at 400 and 450 °C led to normal grain growth in all the samples. A stable grain size is attained and the grain growth kinetics can be described by D n = D n R + kt, where n (n = 5 to 8) is a constant essentially dependent on the annealing temperature. The annealing texture is a retained basal deformation texture and is stable upon ageing.résumé -On a soumis un alliage de magnésium de coulée en bande à double rouleau (TRC), AZ31, au laminage à 450 °C avec différentes épaisseurs finales. Ce traitement a résulté en un raffinement graduel du grain et en une homogénéisation de la microstructure. Des traitements de recuit à 400 et à 450 °C ont conduit à une croissance normale du grain dans tous les échantillons. On a atteint une taille de grain stable et l'on peut décrire la cinétique de la croissance du grain par D n = D n R + kt, où n (n = 5 à 8) est une constante dépendant essentiellement de la température de recuit. La texture de recuit est une texture de déformation basale retenue qui est stable lors du vieillissement.
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