Cr and Co diffusion kinetics along alpha-Zr grain boundaries at power reactors normal temperatures. .

C. CORVALÁN; N. DI LALLA

Congreso; CALPHAD XXXVIII. An International Conference on Phase Diagram. Calculations and Computational Thermochemistry.; 2009

Masaryk University, Faculty of Science, Dep Chemistry. Czech Republic.

Atomic transport along grain boundaries, being several orders of magnitude faster than in the crystal, plays a key role in a large number of metallurgical processes as phase transformations, corrosion, surface treatments, sintering, etc. The study of auto and heterodiffusion a matrix give us information that can be used in theoretical calculation and in experimental predictions. At low temperatures, diffusion processes are often not properly considered.  In fact, even when bulk diffusion can be neglected the atomic displacement along grain boundaries is important and the whole diffusion processes becomes relevant. Different models concerning bulk and grain boundary diffusion have been developed and a direct application to the study of policrystals is also possible.             Zirconium, the base material for nuclear devices exhibit singular behaviour in both auto and heterodiffusion of  Fe, Ni, Co and Cr. Autodiffusion shows a non Arrhenius-like behaviour, while the other elements present extremely high difusion coefficients. They are called ultra fast diffusors, several orders of magnitude faster than self-diffusion.             Recent experiments concernig Co and Cr fast diffusion along alpha-Zr grain boundaries as well as the well known bulk ultra fast characteristics of them in Zr based alloys give us the necessary information to study the different kinetics of diffusion for Cr and Co in alpha-Zr. On these basis, the effective diffusion coefficients and mean-square displacement (involving those concerned to GB diffusion in type C kinetics) were calculated. Extrapolations to [373-573] K  temperature range becomes suitable from the basis of measured data: [380-449] K for Cr and [430-487] K for Co. An analysis is made on the basis of the validity of the different regimes in the Harrison´s classification.