Aluminizing of Nickel Alloys by CVD. The Effect of HCl Flow

Abstract

The solid-state diffusion in binary and ternary systems is overall treated and modeled using the bi-velocity method. The mathematical formulation is given, and the boundary conditions discussed. The model allows the calculation of concentration profiles, spatial distribution of the entropy production rate, and diffusion path. The last two can be used to identify the phases present in the diffusion zone of the ternary system, and determine the thicknesses of the respective layers. The model is applied for the first time to simulate diffusion during aluminization of nickel and its super-alloys, MAR-M200 þ Hf and CMSX-4, and the predictions are compared with the results of the experiments performed by the authors. The results give a new quantitative explanation of the high-active and low-active regimes of aluminization. It is, in particular, confirmed that the high-active and low-active regimes of nickel aluminization can be carried out by a variation of the HCl/H 2 flow ratio. The most important, and a brand new, result concerns the entropy production rate. It is shown that its maxima coincidence with the two-phase zone boundaries in real space, which gives new insight into the modeling of the diffusion in ternary systems.

The interdiffusion in copper-nickel alloys

“…The interdiffusion coefficients in bNiAl phase exhibits strong dependence on the concentration. In this work the Campbell approximation [21,22] was used. The integrated diffusion coefficients in b-NiAl are D Ni ¼D A l ¼ 9.15$10 À10 ; in Cureach diffusion couple side are D Ni ¼ 5.29$10 À14 [23], D Al ¼ 2.91$10 À13 [24] and the intrinsic diffusion coefficient of Cu (for the whole range) will be used from previous calculations e D Cu ¼ 10 À19:37þ1:38NCuþ1:19N 2 Cu cm 2 s À1 .…”

Modeling of Surface Modification of Stainless Steel by Halide Activated Pack Cementation Method

“…At the elevated temperature, the master alloy reacts with the halide salt activator to produce volatile metal halides that diffuse through the gas phase of the porous pack, such that they deposit and diffuse into the substrate to be coated. The processing temperature is so chosen that both adequate halide vapor pressures are generated and solid-state diffusion occurs (Bianco and Papp 1996, Ravi 2003, Wierzba et al 2014, Sahoo et al 2017, Tian and Guo 2009. The diffusion elements modify the surface chemical composition resulting in phase transformations along the depth of the material (Casteletti et al 2009).…”