Effects of B,C, and Zr on the structure and properties of a P/M nickel base superalloy

The boron and carbon levels of a P/M nickel base superalloy were systematically varied in order to determine the mechanisms by which these elements strengthen the alloy, and their optimum concentration. Carbon levels were reduced to 20 ppm while the boron level was varied from 0.02 to 0.10 wt pct. Carbon levels of 0.002 and 0.05 wt pct were also studied, while maintaining a boron concentration of 0.02 wt pct. Zirconium levels were maintained at 0.06 wt pct. The resulting alloys were subjected to identical heat treatments and examinedvia SEM, TEM, and STEM microscopy. The alloys were also subjected to tensile, creep, stress-rupture, and fatigue crack growth tests. Results show that both carbon and boron have a strong influence on the formation of grain boundary precipitates, as expected. Carbon was present as the MC and M₂₃C₆ type carbides, while boron combined to form an intergranular M₃B₂ boride. Boron and zirconium were observed to be critical to the alloys' mechanical properties, although boron levels above the solubility limit resulted in no further improvement or debit in strength. Carbon additions resulted in no improvement in properties, indicating the feasibility of a carbon-free P/M superalloy. The role of the minor element additions is discussed in terms of both microstructural features and related strengthening mechanisms.