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

Recently the target temperature of components manufactured from gamma-TiAl alloys like turbine blades, turbocharger rotors or automotive valves has been increased to 900 °C. However, there is an insufficient oxidation resistance above 750 °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 °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 °C (1 h/air) in steps of 100 °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 °C and 500 °C. At temperatures above 800 °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 °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.