Effect of phosphorus content and grain size on the long-term phase stability of Ni-base superalloys

The effect of grain size on the phase stability of two nominally identical high refractory content powder-processed Ni-base superalloy with varying levels of P additions (0.013 wt% for alloy P1 and 0.026 wt% for alloy P3) was studied. Solutioning of the alloys at either sub-solvus or super-solvus temperatures were used to vary the grain size of the starting microstructure prior to long-term thermal exposures at 800 °C for up to 1000 h. EBSD analyses revealed that the sub-solvus solutioned samples had an average grain size of 10 μm and possessed a high angle grain boundary length density that was approximately 2.5 times greater than that found in the super-solvus solutioned samples with an average grain size of 16 μm. Differences in both the initial grain boundary character distribution and P content present in these alloys resulted in varying behavior with respect to the grain boundary precipitation of Laves and sigma phase during aging. Since P additions to Ni-base superalloys are known to promote the formation of Laves phases and segregate predominately to grain boundaries, sub-solvus heat treated samples exhibited a lower susceptibility of Laves phase precipitation compared with super-solvus heat treated samples for both P1 and P3 alloys. Sub-solvus heat treated P1 samples were found to be resistant to the formation of Laves phase after 1000 h exposure while the coarser grained super-solvus heat treated P3 samples exhibited extensive formation of intertwined Sigma-Laves along the grain boundaries just after 100 h exposure. The improvement in phase stability could be attributed to the reduced concentration of P along the grain boundary as increasing the length density of random high-angle grain boundary resulted in a more dilute spatial distribution of segregated P atoms. •The length density of random high-angle grain boundaries that accommodate P atoms was quantitatively compared between the subsolvus and supersolvus solutioned microstructure.•Susceptibility variation of Laves phase precipitation associated with the grain size adjustment was found.•The effect of grain structure change on the phase stability variation was also found to be dependent on the P addition content.