Grain boundary embrittlement of the iron-base superalloy IN903A

It is shown that a low coefficient of expansion, iron-base superalloy, IN903A, suffers severe tensile embrittlement following high temperature air exposure at 1000 °C. This embrittlement involves a transition to intergranular failure at low strains, with no reduction in yield strength, and is manifested in the room temperature to 800 °C range. In parallel with earlier observations on nickel-base superalloys, ductility is regained at 1000 °C. However, in contrast to these earlier results, air exposure enhances rather than hinders grain growth in the near surface regions, and, in addition, suppresses the occurrence of the jerky flow seen in vacuum-exposed material. Oxygen is demonstrated to be the damaging species, and it is show’n that boundaries are embrittled far ahead of any matrix internal oxidation. Small additions of boron are successful in eliminating the embrittlement, as they were in nickel-base alloys. The results of stress rupture tests are then reviewed, and it is concluded that the rapid failures which occur on air testing are a consequence of embrittled grain boundaries failing in tension, rather than the stress accelerated grain boundary oxidation mechanism previously proposed.