Microstructure and oxidation kinetics of intermetallic TiAl after Si- and Mo- ion implantation

Titanium aluminide alloys based on gamma-TiAl have a substantial potential for high-temperature applications. However, there are still problems concer ning the high-temperature oxidation behavior. Ion implantation is a promisi ng tool for improving the oxidation resistance without disturbing the mecha nical properties. A systematic investigation of the microstructure and phas e development of silicon- and molybdenum-implanted gamma-TiAl is presented. For Si, the fluence is varied from 2.5 x 10(16) to 8 x 10(17) cm(-2) at an implantation energy of 1 MeV resulting in a local concentration of Si betw een 1 and 35 at.% at a projected range of 1 mu m, measured by AES depth pro filing. Grazing incidence XRD and transmission electron microscopy show the formation of a buried Ti5Si3 enriched laver at 650 degrees C, which acts a s a diffusion barrier for oxygen. Long-term TGA oxidation tests at 900 degr ees C in air show a positive effect in the beginning of oxidation for the f luence of 8 x 10(17) Si cm(-2). After a few hours the oxidation kinetics is similar to unimplanted Ti50Al. but the mass gain after 100 h is still 30% smaller. The influence of the local Si concentration on the oxidation behav ior is discussed. Implantation of Mo at 180 keV (2 x 10(16) and 1 x 10(17) cm(-2)) has only a minor influence on the oxidation at 900 degrees C. After implantation, Mo precipitates are found for the high fluence. (C) 2000 Els evier Science S.A. All rights reserved.