temperature oxidizing conditions as seen in fig2 a protective alumina scale has formed on the gamma and beta phases the thickness of the layers varies according to the underlying phase no such scale has formed from the carbide phases and the carbide phases 3 have been partially oxidized forming flaws in the coating down which oxygen can pass also shown in fig2 is some preferential oxidation along the grain boundaries between the gamma and beta phases caused by the difference in thermal expansion between the gamma and beta phases which causes cracking of the protective alumina layer the detriment caused by the carbide phases and the property differences between the gamma and beta phases is largely due to the relatively large size of the phases and would not be a problem for example if the carbide phases did not extend deeply into the coatings fig3 shows the coating of fig1 after surface melting the effect of the surface melting is to produce a visually homogeneous surface layer 20 in which the various phases cannot be optically resolved the secondary phases cannot be optically resolved at a magnification of 1000 times fig4 shows a high magnification view of a portion of fig3 illustrating the presence of fine carbide gamma and beta phases visible only at magnifications greater than 1000 times fig5 shows the coating of fig4 after high temperature exposure a uniform alumina scale 40 has formed over a depletion zone 41 the depletion zone 41 is comprised of the gamma phase formed by outward diffusion of aluminum to form the alumina scale 40 the carbide 42 and beta 43 phases have grown in size but the protective alumina scale remains intact in order to accomplish this result the coating must be surface melted.