Surface oxidation and ductility loss in boron-doped Ni 3Al at 760°C

Abstract

The authors' previous study revealed that the tensile ductility of preoxidized Ni{sub 3}Al specimens tested at 760{degrees}C in vacuum is much more sensitive to grain size than that of the so-called bare-specimens that had not been preoxidized. The ductility remained unchanged at a level of 24% for both bare and preoxidized specimens with fine grain sizes ({lt} 20 {mu}m), whereas preoxidation caused a sharp drop of ductility from 15 to 1% in a large-grained specimen (193 {mu}m). A loss in the ductility was caused by oxygen penetration along grain boundaries during the preoxidation. It is clear from the evidence that oxygen penetration occurred only in the large-grained specimens and not in the fine-grained ones. This difference in behavior is associated with the change in surface oxidation product from aluminum-rich to nickel-rich oxides with increasing grain size: a continuous aluminum-rich oxide layer forms in the fine-grained specimens and protects the underlying alloy from oxygen penetration, whereas the nickel-rich oxide which forms in the large-grained specimens allows oxygen to penetrate along the grain boundaries. The purpose of this study is to elaborate the factor responsible for the ductility of preoxidized Ni{sub 3}Al with the large grain size. This work has demonstrated thatmore » preoxidation does not embrittle large-grained Ni{sub 3}Al at 760{degrees}C if protective oxide scales are first produced on its surface through complex heat treatments.« less