On the Dominance of Grain Size Over Grain Boundary Character in Determining the Oxidation Behavior of Ni-Based Superalloy IN740H

Influence of grain size and grain boundary character distribution on the short-term oxidation behavior of IN740H was evaluated using thermogravimetric analysis test at 760 °C for 50 h. The oxidized specimens were critically examined through electron microscopy. Specimen with fine grain size recorded the lowest mass gain. This is attributed to higher grain boundary surface area that assists in rapid migration of Cr³⁺ ions toward the surface. As a consequence, growth of a thin and compact oxide layer has taken place, which restricts the substrate from further oxidation. On the other hand, formation of an inhomogeneous and defective oxide layer in the coarse-grained specimen fails to inhibit the ingress of O²⁻ ions into the substrate resulting in lower oxidation resistance. Further, easier penetration of O²⁻ ions through defective oxide layer in coarse-grained specimen resulted in significant internal oxidation reaction beneath the outer Cr₂O₃ oxide layer. Inferior oxidation resistance of the coarse-grained specimen even in the presence of a large fraction of special boundaries and low grain boundary connectivity indicates the dominant role of grain size over grain boundary character on the oxidation resistance of Ni-based superalloy IN740H.