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 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 10 21 ionsÁm À2at acceleration voltages ranging from 50 to 340 kV. The acceleration voltage has a small influence. The oxide scales consist predominantly ofAl 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 -Ti phase layer in the modified area. Therefore, a possible explanation for the improved oxidation resistance is the formation of -Ti phase, which is a solid solution where diffusion of Al seems much faster than in -TiAl 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.

Influence of Ion Implantation of Several Elements on Oxidation Behavior of TiAl

“…The formation of β-phase by implantation is very effective to establish an Al 2 O 3 scale, as is confirmed for Nb, W or Ta in our previous study [7,8]. However, the improvement by Zn or Cu cannot be explained in terms of this mechanism, because the β-phase is not detected by GAXRD for the as-implanted specimens.…”

“…With the above background we have been concerned with ion implantation of more than 20 elements for the last few years to clarify the influence of them on oxidation behavior of TiAl as well as the mechanisms [4][5][6][7][8]. The ion implantation is a successful surface modification technique, and at the same time, a useful research tool to investigate the mechanisms of the influence of implanted elements.…”

Oxidation behavior of gamma alloys designed for high temperature applications

“…This can be attained through b-phase formation [15], enhanced Al diffusion, decreased solubility and diffusivity of oxygen in the substrate, and/or increased Al activity. The amount of the b-phase in the gamma alloys is quite small according to the XRD result, it seems to be difficult to form a protective Al 2 O 3 layer, and data is insufficient to discuss other mechanisms.…”

TEM Observation of the Initial Stage Oxidation of TiAl Based Alloys in a Simulated Combustion Atmosphere

“…This may result from the B2 phase formation during oxidation. Since the Al content in the B2 phase is lower than that in the alloy matrix, the B2 The diffusion of the species in the Ti-Al system is not well understood yet, however, Al 2 O 3 formation is significantly enhanced in a W-implanted specimen with the β-phase in the W-modified surface layer [12]. The diffusion coefficient of the iron of bcc structure, α-Fe, is much larger than that of fcc structure, γ-Fe.…”