Flash oxidation tests to evidence the segregation of boron at the grain boundaries of superalloy Inconel 718

In this study very short oxidation tests, called flash oxidation tests, were carried out at high temperature on samples of superalloy Inconel 718 containing some boron. Following these tests, a specific oxidation pattern was evidenced: spots were observed on the oxide film, these spots being discontinuously distributed along the grain boundaries. Whether outside or inside the spots, the oxide film exhibited the same three oxide layers which were likely rutile oxide (Cr,Nb,Ti)O-2, chromia oxide Cr2O3 and spinel oxide (Ni,Fe)(Cr,Fe)(2)O-4. However, inside the spots the oxide film was characterized by thicker oxide layers, a slightly different chemical composition of the spinel layer, and vitreous oxides at the top. Moreover, all the intergranular (Nb,Mo)(2)CrB2 borides of the microstructure were involved in this specific oxidation pattern, thus leading to the suspicion of high boron concentrations within the spots. FEG-EPMA analyses confirmed that, during the oxidation process, boron atoms diffused from the matrix to the oxide film, especially within the spots. Actually, the spots revealed areas where boron was highly concentrated just before oxidation. Such areas originated from the non-equilibrium segregation of boron near grain boundaries, which is a classical phenomenon when cooling and/or reheating a boron-containing alloy. Thus, the spots of the oxide film highlighted the grain boundary segments where boron segregated the most, confirming that the segregation of boron was not uniform within the grain boundary network.

Automated summary

The study found that after high temperature flash oxidation tests on Inconel 718 samples containing boron, the oxide film exhibited unevenly distributed spots, indicating a specific influence of boron during oxidation, especially near grain boundaries. The non-equilibrium segregation of boron near grain boundaries was found to be the cause of the uneven distribution of boron in the oxide film.