Abstract Pure ZA27 alloys and ZA27 alloys with Cu and Mg additions were cast and deformed by compression. Samples of both alloys were heat treated at 460°C after deformation and the evolution of the microstructure as a function of time measured. The results showed that the average grain size increases with the increase of time for both alloys, with a coarsening rate which is 55% lower for the alloyed ZA27 alloy than the rate of the pure ZA27 alloy. X-ray diffraction analysis indicated that the ZA27 alloys containing Cu and Mg additions present the ε phase which is not present in the non-alloyed ZA27 alloy; and that the α and η phases transform to the β phase during heat treatment. Microhardness measurements showed that the ZA27 alloy with Cu and Mg additions has higher microhardness than the pure ZA27 alloy.

ABSTRACT The use of nuclear reactor as a source of electric energy has been extended and consolidated during the second half of last century and the beginnings of XXI century. The continuity of this resource relies on the reliability and security of nuclear plants, requiring the improvement of all features related to their operation. Among them, the structural elements integrity that depends strongly on the mechanical stability of protective oxides must be taken into account. Zircaloy-4 (zirconium based alloy with Sn, Fe and Cr additions) is used for the coolant channels at CNA I and CNA II plants. However, this alloy suffers corrosion problems which have caused, for example, the replacement of coolant channels at CNA I in 1988. In this work we analyze from functional density theory (DFT) the properties of low index hexagonal Zr surface and of the experimentally observed (001) tetragonal ZrO2 surface. Our results predict Zr (1120) and O terminated ZrO2(001) with Zr subsurface, as the surfaces with the lowest superficial energy. The Zr(1010) surface exhibits the most inhomogeneous charge density. This feature could favor oxygen adsorption and then the oxide growth, in agreement with the experimentally observed Zr(1010)/ZrO2(001) interface.

RESUMEN El uso de los reactores nucleares como fuente de energía se ha extendido y consolidado en la segunda mitad del siglo XX y en los inicios de este siglo XXI. La continuidad de este recurso depende en gran medida de que las plantas nucleares sean cada vez más seguras y confiables, lo cual implica un mejoramiento de todos los aspectos relacionados con su operación. Entre ellos se cuenta la integridad de los elementos estructurales, que depende fuertemente de la estabilidad mecánica de los óxidos protectores. En las centrales CNA I y CNA II el Zircaloy-4 (aleación base Zr con adiciones de Sn, Fe and Cr) se utiliza en los canales refrigerantes. Sin embargo, esta aleación presenta problemas de corrosión que, por ejemplo, motivaron el reemplazo de los canales refrigerantes de CNA I en 1988. En este trabajo analizamos desde la teoría de la funcional densidad (DFT) las propiedades de superficies de bajo índice de Zr hexagonal y de la superficie (001) observada experimentalmente en circonia tetragonal. Nuestros resultados predicen a la superficie libre de Zr (1120) y a la de ZrO2 (001) terminada en O con subsuperficie de Zr como las más estables. Además encontramos que las superficies Zr(1010) tienen la mayor inhomogeneidad de la distribución de carga, lo que favorecería el crecimiento del óxido. Esta observación coincide con la evidencia experimental, que indica que la interfaz observada del óxido protector es Zr(1010)/ZrO2(001).