Oxide grain boundaries

The HREM observations revealed that well-structured grain boundaries in which the lattice structure is being maintained right up to the boundary are the 

An HREM image of the = 5, (310) symmetric tilt grain boundary in NiO 

---

H. Li, F. Czerwinski, J.A. Szpunar. The role of oxide grain boundary character distribution in nickel oxidation kinetics // Defect Diffusion Forum.- 2001.- V. 194-199.-P.1683-1688. 

The experimental results concerning the segregation of Y and Zr at oxide grain boundaries are consistent with the observations of Pint et al. [13] from NiAl - 0.2 Zr. 

After oxidation at 1200 C a-A1203 was the only A1203 phase present in the oxide scale. Analytical studies revealed the segregation of reactive elements (Zr, Y) at the oxide grain boundaries as well as at the metal/oxide interface. Analytical studies on NiAl-0.1 Y further more indicate that Y can getter the S in the alloy. 

Express the mobility of an oxide grain boundary in which the migration is governed by the diffusion of segregated aliovalent dopant atoms. 

Thus, in the system coolant — oxide layer the Co ions are subjected to an adsorption—desorption process leading to a cobalt concentration in the oxide layer on the surface of the materials which is controlled by the cobalt input into the coolant. In the case of very low or zero cobalt input, the cobalt concentration in the oxide (and, simultaneously, the Co activity concentration) decreases steadily according to a power-law decline (Lister, 1992). As yet it is not known whether the release of chemically bound cobalt from the chromites would be fast enough to explain the observed processes or whether the above-mentioned oxide grain boundary diffusion of Co is the rate-controlling process. 

According to the reactive element effect, the reactive element ion, such as beryllium, diffuses in to the native oxide grain boundaries and prevents the outward diffusion of substrate metal cations (Czerwinski and Smeltzer, 1993 Czerwinski and Szpunar, 1998 Czerwinski, 2000, 2004). The inhibitory effect of boron on aluminum alloy oxidation is clearly a surface phenomenon given the effectiveness of very low levels of boron. This could occur through a combination of boron migration into the MgO lattice and/or boron bonding to the defect-rich MgO surface (Choudhary and Pandit, 1991). 

STEM bright field image (a) and corresponding elemental maps for Hf (b) and Y (c). Oxide grain boundary on alloy +Hf after 3100 h air oxidation at 1200°C. 

Formation of new oxide within growing oxide scale by counter-diffusion of cations and anions along oxide grain boundaries normal to the scale/ substrate interface. 

---