Surface treatment for improved high temperature environmental stability of Ti-alloys

Titanium alloys are widely used as light weight structural materials at low temperatures. Due to their high affinity towards oxygen a protective TiO2-passive layer is formed. This layer is only protective at temperatures below about 500°C. With increasing temperature this layer gets deteriorated and hence oxygen inward diffusion is accelerated. This inward diffusion leads to an oxygen enriched subsurface zone which is brittle because Ti has quite high oxygen solubility. The enriched zone can cause failure of Ti-components under thermocyclic and/or mechanical load. To prevent this failure mode a two step process was developed. The first step was enrichment of Al in a narrow surface zone to form the intermetallic TiAl-phase with low oxygen solubility and a high Al-content. The second step consisted of fluorination. Unfluorinated -TiAl-alloys usually form a mixed non protective oxide scale but a protective alumina layer develops after addition of small amounts of fluorine into the surface zone. This so called fluorine effect works for TiAl-alloys with an Al-content above 40at.%. The alumina layer suppresses the oxygen inward diffusion and slows down the oxidation kinetics. The intermetallic TiAl subsurface prevents oxygen uptake into the metal and, thus, embrittlement. In this work results of high temperature exposure tests of untreated and treated technical Ti-alloys will be presented and compared. The results will be discussed considering a use of protected Ti-components in high temperature environments.