Kinetics of Phase Layer Growth During Aluminide Coating of Nickel - Part II.
CARNEGIE-MELLON UNIV PITTSBURGH PA METALS RESEARCH LAB
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The diffusion coating of nickel with aluminum was studied by a two-step aluminizing pack process involving initially an influx of aluminum at the surface step 1 and later a partial homogenization of the aluminum-rich region under conditions of zero surface flux step 2. The process was studied in the temperature range from 870 to 1000C. Step 1 was characterized mainly as the rapid, parabolic growth after an initial transient period of the Ni2Al3 phase gamma as a surface layer with concurrent growth of a thinner NiAl gamma layer. Step 2 was characterized mainly as the rapid loss of the aluminum gradient in the gamma layer followed by parabolic growth of the delta layer primarily by the solution of the gamma phase. Mathematical models were developed, using numerical methods and computer techniques as well as closed-form solutions, which yielded growth rate predictions in agreement with the experimental data.
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