2004 年 45 巻 5 号 p. 1693-1699
TiAl coupon specimens were implanted with Fe, Mo, Ta or W ions and then cyclically oxidised with temperature varying between room temperature and 1200 K in a flow of purified oxygen under atmospheric pressure. The surface modification by the ion implantation was characterised by glancing angle X-ray diffractometry (GAXRD), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The oxidised specimens were examined by AES, GAXRD, X-ray diffractometry, scanning electron microscopy and electron probe microanalysis. The oxidation resistance of TiAl is significantly improved by the implantation of Mo, Ta or W ions with a dose of 1021 ions·m&minus2; at acceleration voltages ranging from 50 to 340 kV. The acceleration voltage has a small influence. The oxide scales consist predominantly of α-Al2O3 and are very adherent to the substrates even after 20 cycles (400 h). On the other hand, the implantation of Fe has a little effect. The significant effect brought about by the implantation is attributable to the formation of a thin β-Ti phase layer in the modified area. Therefore, a possible explanation for the improved oxidation resistance is the formation of β-Ti phase, which is a solid solution where diffusion of Al seems much faster than in γ-TiAl which has an ordered structure. The enhanced Al diffusion results in the formation of a thin but continuous Al2O3-rich layer in the scale during the initial oxidation stages. The enrichment of Al relative to Ti by the implantation was thought playing some role. The so-called doping effect of Mo, Ta and W is also contributing to the early formation of Al2O3-rich layers by retarding TiO2 growth.
