In situ oxidation of Ni3Al Alloys

Ni3Al is subject to dynamic embrittlement when tested in an oxidizing environment; whereas pre-oxidation under stress does not embrittle the alloy. In situ HVEM oxidation studies of several ordered intermetallic (LI2 structure) Ni3Al alloys with small additions of boron and hafnium are reported. AEM studies of the oxidized specimens were also made to fully characterize the microstructure. Specimens were annealed and then electropolished to penetration just prior to insertion in the hot stage of the HVEM. Oxidations were carried out at an oxygen pressure of 13-60 mPa at 750°C. Gas pressure and composition were monitored with a residual gas analyzer. Specimens raised to the operating temperature in the base vacuum (∼0.2 mPa) showed no indication of oxide in the diffraction pattern and were almost entirely free of dislocations. Many of the features of different stages of the oxidation process are shown in Fig. 1. After two minutes exposure to oxygen, a thin, fine-grained, oriented, polycrystalline region of primarily γ-Al2O3 developed at the thinnest regions of the foil (Region A in Fig. 1 and Fig. 2).