I am not sure whether this is really a standard terminology, but here goes.
I believe that inward and outward are references to the diffusion of Ni in intermetallic layers grown by aluminizing Ni-base superalloys (used in gas turbine engines). This is a function of the aluminum activity, temperature and time when pack aluminizing the Ni-based part in a mixture of Al(pure or alloyed), alumina and a halide at a temperature in the range 750-1100
oC.
The intermetallics NiAl
3, Ni
2Al
3, NiAl and Ni
3Al are present in the Al-Ni phase diagram (
The morphology of the aluminized Ni-base alloy can be controlled by the Al activity of the pack. “A high activity pack causes Ni
2Al
3 to be the predominant phase with the diffusion of Al
inward [my emphasis] being greater. Most processes aim at the beta NiAl phase which is more suitable and resistant with favourable mechanical and oxidation properties.
…
“Aluminum can move at a relatively high rate through Ni
2Al
3 in the inward diffusion type of coating while in NiAl only Ni can diffuse significantly and does so at a relatively low rate.”
Thus, a low Al activity pack allows a greater proportion of the more desirable NiAl phase (which grows by
outward diffusion of Ni) in the coating formed. Alloying additives such as Cr form Cr-rich interlayers which act as barriers against Al diffusion. These alloying elements also affect the morphologies of the phases; columnar morphology is bad.
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Metallic and ceramic Coatings, M. G. Hocking, V. Vasantasree and P. S. Sidky, chapter 5 (1989).
An online description of the above:
Coatings for High-Temperature Structural Materials: Trends and Opportunities (1996), pp. 78-82, ‘Manufacturing Technologies of Coating Processes.’