The time behaviour of surface applied fluorine inducing the formation of an alumina scale on gamma-TiAl during oxidation at 900 deg C in air

• Published in: Intermetallics Vol. 14; no. 10-11; pp. 1136 - 1142
• Main Authors: Zschau, Hans-Eberhard, Schutze, Michael, Baumann, Horst, Bethge, Klaus
• Format: Journal Article
• Language: English
• Published: 01.10.2006
• ISSN: 0966-9795
• DOI: 10.1016/j.intermet.2006.01.063

Recently the target temperature of components manufactured from gamma-TiAl alloys like turbine blades, turbocharger rotors or automotive valves has been increased to 900 deg C. However, there is an insufficient oxidation resistance above 750 deg C. One method used to improve the gamma-TiAl oxidation behaviour is the so-called fluorine microalloying effect. After application of fluorine to the TiAl surface by ion implantation or treatment with diluted HF and oxidation at 900 deg C in air a dense alumina layer is formed. The aim of this work was firstly to study the short time development of the fluorine concentration during heating up to 400-1000 deg C (1 h/air) in steps of 100 deg C. Using ion beam analysis the depth profiles of F, Al, Ti and O were obtained simultaneously and non-destructive. A distinct loss of fluorine was found between 400 deg C and 500 deg C. At temperatures above 800 deg C an alumina layer was formed with fluorine maximum located at the metal/oxide interface. Secondly the long time behaviour during oxidation of up to 500 h/900 deg C/air was investigated showing a slow fluorine decrease. The alumina layer acts as a diffusion barrier for fluorine, whereas fluorine diffuses into the metal. The diffusion coefficient was calculated. The results fit into the theoretical model assuming a selective transport of gaseous aluminium fluorides at the metal/oxide interface.