Grain‐Boundary Diffusion of 51 Cr in MgO and Cr‐Doped MgO

The grain‐boundary diffusion product, D'δ , of 51 Cr in MgO and Cr‐doped MgO as a function of grain‐boundary orientation and point‐defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain‐boundary diffusion behavior in MgO. The ratio of D'δ || parallel to D'δ ‖ perpendicular to the growth direction, D' || /D' ‖ , is 10 2 for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D' || /D' ‖ is due to a large increase in D' ‖ with increasing tilt angle. The results indicate that grain‐boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain‐boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for 51 Cr diffusion in undoped MgO.