Estimation of Interdiffusivities of the Pseudo NiAl Binary Phase Formed in a Nickel-based Superalloy by Pack Cementation
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The pseudo NiAl binary phase was formed in a nickel-based superalloy by pack cementation. Scanning electron microscopy, transmission electron microscopy, x-ray diffraction, electron probe microanalysis, and positron annihilation technique were used to characterize the pseudo NiAl binary phase. Based on reasonable assumptions, the chemical interdiffusivities of the pseudo NiAl binary phase were then assessed by means of the modified Wagner’s method. The results showed that the chemical interdiffusivities of the pseudo NiAl binary phase were about two orders of magnitude lower than those reported by others. The analysis indicated that the change in thermodynamic properties due to the additions of the microalloying atoms originally present in a superalloy could be responsible mainly for a decrease in chemical interdiffusivities.
I. INTRODUCTION
The ordered intermetallic phase NiAl is of interest for high-temperature coatings for nickel-based superalloys due to the good oxidation resistance up to 1573 K. Knowledge of its fundamental physical and chemical properties is the necessary prerequisite for the development of NiAl coating. The previous studies1,2 indicate that the lifetime of the NiAl coating is not controlled directly by oxidation but by interdiffusion of the protective scale-forming element Al between the coating and the substrate. It is obvious that diffusion is an important factor to be considered in this application. Thus, a quantitative knowledge of the interdiffusivities is of great importance when dealing with the NiAl coating design issue. It should be noted, however, that the diffusion behavior of NiAl phase as coatings for nickel-based superalloys is more complicated resulting from the additions of many alloying elements initially present in the substrate, in comparison to the simple binary NiAl phase so that it is difficult to extrapolate the experimental data acquired by the Ni/Al diffusion coupling to an engineering applications. Unfortunately, the thermodynamic model for micro-alloying NiAl phase has been not until now set up, so it is impossible to estimate directly the thermodynamic properties of the micro-alloying NiAl
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0312 2340
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 14 Mar 2015
phase, which is important for the correlation between the chemical and self-diffusion coefficients. In this study, the interdiffusivities of the micro-alloying NiAl phase in a nickel-base superalloy can be evaluated based on the reasonable assumptions and the study on the conventional binary NiAl phase. II. EXPERIMENTAL
The K465 Ni-based superalloy was used as the substrate in this study. The NiAl phase on the substrate could be deposited by pack cementation (PC) was referred to as aluminization. When aluminizing, the specimens were embedded into the semi-permeable pipe filled with a powder consisting of 98 wt% FeAl powder as supplier of the Al element and 2 wt% NH4Cl as activator
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