SNMS AND GDOES STUDIES OF THE OXIDATION BEHAVIOR OF TIAL MODIFIED BY NIOBIUM ADDITION

TiAl-based intermetallic alloys are promising candidates as structural materials for high temperature applications. However, industrial appl ication is hindered by insufficient oxidation resistance at temperatur es above 700 degrees C in air. The oxidation resistance can be improve d by the addition of ternary and quaternary alloying elements, such as niobium. In several studies it has been demonstrated that this elemen t can reduce the oxidation rate dramatically although the underlying m echanism is not yet fully understood. In the present study the influen ce of niobium on the high temperature oxidation behavior at 800 degree s C of Ti-48Al-2Cr was investigated. Niobium was added by alloying as well as by ion implantation. Some specimens were pre-oxidized prior to ion implantation. Thus, it could be demonstrated that niobium is not only active when present in the bulk alloy, but also when located in t he initially formed corrosion scale. Moreover, the implantation experi ments revealed that the often suggested ''doping mechanism'' of the ti tania lattice by Nb5+ ions cannot play an important role explaining th e beneficial effect of Nb. The morphology and composition of the scale s formed during oxidation were studied by glow discharge optical emiss ion spectroscopy (GDOES) and secondary neutral mass spectrometry (SNMS ). The latter technique was also used in combination with two-stage ox idation experiments using the isotope tracers O-18 and N-15.