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 were carried out at high temperature on samples of superalloy Inconel 718 containing some boron. Following these a specific pattern was evidenced: spots observed the oxide film, being discontinuously distributed along grain boundaries. Whether outside or inside spots, film exhibited same three layers which likely rutile (Cr,Nb,Ti)O2, chromia Cr2O3 and spinel (Ni,Fe)(Cr,Fe)2O4. However, characterized by thicker layers, slightly different chemical composition layer, vitreous oxides top. Moreover, all intergranular (Nb,Mo)2CrB2 borides microstructure involved in pattern, thus leading to suspicion boron concentrations within spots. FEG-EPMA analyses confirmed that, during process, atoms diffused from matrix especially Actually, revealed areas where highly concentrated just before oxidation. Such originated non-equilibrium segregation near boundaries, is classical phenomenon when cooling and/or reheating boron-containing alloy. Thus, highlighted boundary segments segregated most, confirming that not uniform network.