Electron microscopy of oxide scales modified to resist high‐temperature oxidation

SUMMARY The high temperature oxidation of metals and alloys is slowed to a useful extent by the presence of certain elements or their oxides in the metal or on its surface. Particles of the reactive element oxide are found in part of the oxide scale and, by an unknown mechanism, retard the diffusion of metal through oxide grain boundaries. Changes in texture are also observed, which might affect geometry of individual grain boundaries. In this study the rotational geometry of grain boundaries in nickel oxide scales has been characterized both in the presence of particles of cerium dioxide and in their absence. Proximity of the relationship of neighbouring grains to a coincidence orientation could have an important influence on diffusion rate. Ways of testing this are compared and a new method for analysing orientation data from electron diffraction is presented, which allows statistically sound conclusions to be drawn. The frequency of occurrence of coincidence orientations in the presence of CeO2 is little greater than random, so one possible mechanism for the ‘reactive element effect’ may be rejected.