Oxidation Resistance of TiAl Improved by Ion Implantation of Beta-Former Elements

Abstract

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 surfacemodification 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 10 2 1 ios.m - 2 at acceleration voltages ranging from 50 to 340 kV. The acceleration voltage has a small influence. The oxide scales consist predominantly of α-Al 2 O 3 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-ΤiAl which has an ordered structure. The enhanced Al diffusion results in the formation of a thin but continuous Al 2 O 3 -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 Al 2 O 3 -rich layers by retarding TiO 2 growth.