Effect of small amounts of nitrogen on properties of a Ni-based superalloy

The microstructural, tensile, and stress-rupture lives of conventionally and directionally solidified (DS) cast specimens of a nickel-based superalloy have been investigated using materials of normal commercial composition and two melts with additions of 22 and 30 ppm nitrogen. It was found that the ductility, tensile life, and stress-rupture life of the conventional castings dramatically decreased with the increase of nitrogen content, but, in the longitudinal direction of DS samples, these properties were independent of nitrogen within the contents studied. The transverse tensile rupture life of DS castings had no significant change with nitrogen, except for some decrease in elongation of the sample containing 30 ppm N. The stress-rupture life and elongation in the transverse direction of the DS samples were improved at the 22 ppm nitrogen level, which might be attributed to the dendrite refining. Increasing the nitrogen content resulted in a change in carbide morphology from that of a "Chinese script" to a blocky form and a change in microporosity formation, especially in the conventional castings. The increased nitrogen content resulted in a significant reduction in the volume fraction of eutectic microconstituents and the in dendrite refinement in DS samples, but nitrogen content did not affect the morphology of gamma' precipitates in the heat-treated alloys.