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

Abstract

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)0 2 , chromia oxide Cr 2 O 3 and spinel oxide (Ni,Fe)(Cr,Fe) i 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) i CrB 2 borides of the mi crostructure were involved in this specific oxidation pattern, thus leading to the suspicion of high boron concen trations within the spots. FEG-EPMA analyses confumed that, during the oxidation process, boron a toms diffused fr001 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<ontaining al loy. 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 bounda,y network.

Quantitative analysis of susceptibility to intergranular corrosion in alloy 625 joined by friction stir welding

“…Binder et al [27] evidence that the increase of boron concentration in Cr–Ni–Co–Fe alloys accelerates the development of complex spinels at the protective oxide Cr 2 O 3 , improving the corrosion resistance. Vernier et al [28] found boron atoms – initially in the solid solution in the matrix – incorporated in the protective oxide layer of the Inconel 718. Besides, the literature presents that the addition of B in Ni–Cr superalloys can reduce the agglomeration of M23C6 carbides [29], diminishing the sensitisation.…”