Behavior of grain boundary chemistry and precipitates upon thermal treatment of controlled purity alloy 690

Grain boundary composition and carbide composition and structure were characterized for various microstructures of controlled purity alloy 690. Heat treatments produced varying degrees of grain boundary chromium depletion and precipitate distributions which were characterizedvia scanning transmission electron microscopy (STEM). Convergent beam electron diffraction revealed that the dominant carbide is M23C6, and energy dispersive X-ray analysis (EDAX) determined that the metallic content was about 90 at. pct chromium. A discontinuous precipitation reaction was observed and is attributed to a high degree of carbon supersaturation. Grain boundary composition measurements confirm that chromium depletion is controlled by volume diffusion of chromium to chromium-rich grain boundary carbides in the temperature range of 873 to 1073 K. Grain boundary chromium levels as low as 18.8 at. pct were obtained by thermal treatment at 873 K for 250 hours and 973 K for 1 hour. A thermodynamic and kinetic model developed for alloy 600 was modified to describe the development of the chromium depletion profile in alloy 690 during thermal treatment. Experimentally measured chromium profiles agree well with the model results for the dependence of the chromium depletion zone width and depth on various input parameters. The establishment of the model for alloy 690 allows the chromium depletion zone width and depth to be computed as a function of alloy composition, grain size, and temperature. The chromium depletion profiles and the precipitate structure and composition of controlled purity 690 are compared to those of controlled purity 600. A thermodynamic analysis of the carbide stability indicates that other factors, such as favorable orientation relationships, play an important role in controlling the precipitation of Cr23C6 in nickel-base alloys.

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