Microstructural Evolution of the Inner Layer of the High Temperature Oxide Scale on Nickel

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The grains of the inner layer of the scale on pure nickel are equiaxed and uniform in size throughout the layer. These grains also have the same mean intercept size as that of the transverse intercept of the columnar grains of the outer layer. This means that all grains of the scale are growing at the same rate, in correspondence with uniform swelling of the scale throughout its thickness. New grains appearing at the metal‐oxide interface grow at once to the prevailing size. This is because their nucleation is limited to the points of contact of the grain boundary quadruple points with the metal‐oxide interface where the oxygen concentration maximizes. Microcracks appear near the metal‐oxide interface, possibly where subscale particles have surfaced. These are enveloped by the growing scale, become spheroidized, and eventually sinter shut. Accompanying each such microcrack is a nest of fine grains.