TEM observations of the initial oxidation stages of Nb-ion-implanted TiAl

Coupon specimens of TiAl were implanted with Nb ions at an acceleration voltage of 50 kV with a dose of 10(21) ions . m.(-2) They were then slightly oxidized during heating to 900 or 1200 K, or at 1200 K for 3.6 ksec (1 hr) in a flow of purified oxygen under atmospheric pressure. The implanted specimens and oxidized specimens were characterized and observed by AES, X-ray diffractometry, SEM, TEM, EDS, and EPMA. Implantation improves the oxidation resistance significantly by forming virtually alpha-Al2O3 scales. The implanted layer is about 75 nm thick; the outer part of 30-nm thickness is beta-Ti phase and the rest of 45-nm thickness is amorphous. Heating to 900 K in O-2 results in partial crystallization of the amorphous layer to Ti5Al3O2 (Z-phase) and to 1200 K results in oxide scales of 270 to 400 nm thickness consisting mainly of Al2O3. The fraction of Al2O3 in the scale increases toward the substrate. Oxidation at 1200 K for 3.6 ksec results in Al2O3-rich scales of about 400-nm thickness. The oxide grain size is very fine, about 80 nm in size, and becomes smaller toward the outer scale surface. This implies that implantation enhanced the nucleation of Al2O3 grains relative to the growth of TiO2 grains. This finding and the formation of beta-Ti phase are thought to be responsible for the excellent oxidation resistance obtained.