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11/9/03

4:29 PM

Page 2253

Effect of Ion Implantation on the Oxidation Resistance of TiAl M. YOSHIHARA, S. TANIGUCHI, and Y.-C. ZHU Ion implantation was applied to a gamma TiAl alloy, and the oxidation behavior of the alloy implanted with B, P, Fe, or W was investigated through a cyclic oxidation test at 1200 K in a flow of purified oxygen under atmospheric pressure. Metallographic examinations were performed for the implanted specimens and the oxidized specimens using X-ray diffractometry (XRD), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The
phase was detected along with -TiAl in the Fe- or W-implanted specimens, while -TiAl was the predominant phase for the B- or P-implanted specimens. The implantation of P, Fe, or W significantly improved the oxidation resistance, provided the ion dose was sufficient, whereas the implantation of B enhanced scale spallation, resulting in deteriorated oxidation resistance. The AES profiles for the specimens oxidized for a very short period strongly suggested the incorporation of P or W in TiO2. The possible mechanisms for the improvement are discussed on the basis of the results obtained for the implanted alloys.

I. INTRODUCTION

THE gamma alloys based on a TiAl intermetallic compound have attractive properties as lightweight heat-resistant materials. However, their oxidation resistance is not sufficient at the supposed application temperatures above about 800 °C.[1,2] In spite of the efforts to improve the oxidation resistance of -TiAl alloys by element additions, there seems to be no single alloying element which can improve mechanical properties and oxidation resistance simultaneously. For instance, Nb, Mo, Ta, or W are known to improve the oxidation resistance of the alloy to some degree;[3–7] however, a large-amount addition results in embrittlement of the alloy and an increase of the alloy density. A possible solution for this kind of problem is a combination of alloying additions of suitable elements to improve mechanical properties and surface modification, which can provide sufficient oxidation resistance. The microalloying using the ion-implantation technique provides such a surface modification. At the same time, the ion implantation is a useful research tool to investigate the influence of the additional element on oxidation behavior and to elucidate the mechanisms responsible for the effect. However, studies dealing with the influence of ion implantation in TiAl are limited.[8–14] The present study is part of a continuation of a current project surveying the influence of a range of elements on the oxidation behavior of TiAl alloys.[15–24] In the present article, the implantation of B, P, Fe, or W was carried out, and the influence of these elements on the oxidation M. YOSHIHARA, Research Associate, is with the Department of Mechanical Engineering and Materials Science, Graduate School of Engineering, Yokohama National University, Kanagawa 240-8501, Japan. Contact e-mail: [email protected].