Boron effect on high temperature oxidation and spallation behavior of a Ni-based superalloy (Alloy 617)

Abstract

We report on the effect of minor boron additions to a Ni-based superalloy, Alloy 617. To analyze the effect of 200 ppm B, Alloy 617 and Alloy 617B were oxidized at $1050^\circ C$ in air for up to 100 h and compared based on weight gain measurement, SEM, TEM, EDS, XRD, and atom probe tomography (APT) investigation results. Both alloys showed similar microstructure of oxide layers and a trivial difference in the distribution of oxide phases. Oxide scale on both alloys were composed primarily of polycrystalline $Cr_2O_3 with (Mn, Co)Cr_2O_4$ spinel and Ti enriched grains on it. Two alloys were found to have similar oxidation resistance. Spallation of oxide scale upon cooling turned out to have occurred by intergranular cracking at $Cr_2O_3$ grain boundaries and Alloy 617B showed significantly reduced spallation. Segregation of about 0.04 at.% B at $Cr_2O_3$ grain boundaries was identified by APT investigation of Alloy 617B oxidized for 10 h. B segregation appears to be closely related to the reduced spallation. To speculate the origins for the difference in spallation between two alloys, partial covalent bond between Cr and B, and O and B, and properties of boron oxide were considered. However, without proper data for speculation, especially the exact boron sites at grain boundaries and the modified grain boundary structure by boron, the discussion section has been completed without reliable explanation.