Internal nitridation of nickel-base alloys. Part II. Behavior of quaternary Ni-Cr-Al-Ti alloys and computer-based description

Whereas in Part I of this study the process of internal nitridation was described for binary and ternary alloys within the Ni-Cr-Al-Ti system, this part focuses on quaternary Ni-Cr-Al-Ti alloys, which are similar to commercial Ni-base alloys used in high-temperature applications regarding their chemical compositions. These alloys can simultaneously form two different nitride-precipitation zones consisting of TiN and AIN. In order to quantify the nitridation process, thermogravimetric measurements in an oxygen-free nitrogen atmosphere in the temperature range 800-1100 degrees C were carried out and supplemented by extensive microstructural studies. While single-nitride internal nitridation can easily be described by Wagner's theory of internal oxidation, modeling of the more complex internal-precipitation reactions that involves more than one nitride requires a numerical treatment of both the diffusion and the thermochemical processes in the alloy. For this purpose, a computer simulation was developed in which the commercial thermodynamic software ChemApp is combined with a finite-difference diffusion calculation. It was shown that this calculation technique can be applied successfully to quantitatively describe the internal-nitridation process of the Ni-Cr-Al-Ti model alloys used in this study.