Combined Al- plus F-treatment of Ti-alloys for improved behaviour at elevated temperatures

Due to its low weight and good corrosion resistance at moderate temperatures, titanium is being currently used in a large number of applications. As a result of increased oxidation rate and environmental embrittlement the maximum operating temperature is only about 600°C while the melting point is much higher (1677°C). The oxidation behaviour can be improved by different methods e.g. Al-enrichment of the surface zone. This leads to an improvement which is, however, not sufficient. The combination of Al-enrichment in the surface zone so that a TiAl-layer is formed plus an additional F-treatment gives the best results because a protective alumina scale is formed. The fluorine effect is known for TiAl-alloys. An alumina scale is found on TiAl-alloys after F-treatment. This alumina scale prevents oxygen inward diffusion which causes embrittlement and protects the material against environ-mental attack. Now this effect is transferred to alloys with a very low Al-content or even no Al at all. These alloys can not form an alumina layer by themselves without any treatment. In this work results of oxidation tests of several Ti-alloys (-Ti, Ti3Al, etc.) are presented without any treatment and with Al-treatment, F-treatment and the combination of both. Aluminium was diffused into the samples by a powder pack process. Fluorine can be applied by several ways e.g. ion implantation or gas phase processes. The formation of a thinner oxide scale on treated samples is revealed by post experimental investigations like metallography. The results are discussed referring to the fluorine effect model for TiAl-alloys.