Environmental Engineering Reference
In-Depth Information
Figure 6.50
Schematic illustration of formation of NiAl coating by diffusion anneal-
ing from Ni
2
Al
3
layer on a nickel-based superalloy in a high-activity process [70].
which is used more often, Ni
2
Al
3
phase is initially formed with preferential diffu-
sion of aluminum during aluminizing treatment in the temperature range 973-
1123 K. Subsequently, during its diffusion annealing in argon atmosphere at tem-
peratures of 1273-1373 K in the absence of aluminum source, Ni
2
Al
3
transforms
to NiAl by reacting with the substrate. The development of an NiAl coating
during diffusion annealing, by interdiffusion between a layer of Ni
2
Al
3
and the
nickel-based superalloy substrate, is illustrated schematically in Fig. 6.50a-c.
The final layer of NiAl on the alloy substrate, as depicted in Fig. 6.50c, consists
of three distinct regions. The external zone is almost as thick as the initial Ni
2
Al
3
layer, containing various precipitates that existed in the initial layer. The central
region is devoid of precipitates. The internal zone consists of precipitates similar
to those observed in the internal zone of coatings obtained by a ''low-activity''
aluminizing treatment (Fig. 6.49). In fact, the last two zones may be considered
to be a low-activity coating, with the initially formed Ni
2
Al
3
layer virtually play-
ing the role of cement. So it is the external zone containing precipitates formed
in the initial Ni
2
Al
3
layer that distinguishes the high-activity NiAl coatings from
the low-activity ones. The typical microstructures of the two types of aluminide
coatings formed on U-700 alloy by high- and low-activity processes are illus-
trated in Fig. 6.51a and b, respectively. The structural features of the coatings
can be explained in terms of the diffusion mechanism wherein outward diffusion
of Ni through
-NiAl phase occurs during the entire coating process in a low-
activity mode, whereas inward Al diffusion followed by outward Ni diffusion
during the subsequent heat treatment takes place in a high-activity mode. How-
ever, it is to be noted that the composition and microstructure of the coatings
are dependent on the composition of the substrate alloy, making it necessary to
optimize the process parameters for a particular alloy. This implies that the coat-
β
