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 application is hindered by insufficient oxidation resistance at temperatures above 700 degrees C in air. The oxidation resistance can be improved by the addition of ternary and quaternary alloying elements, such as niobium. In several studies it has been demonstrated that this element can reduce the oxidation rate dramatically although the underlying mechanism is not yet fully understood. In the present study the influence of niobium on the high temperature oxidation behavior at 800 degrees 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 the initially formed corrosion scale. Moreover, the implantation experiments revealed that the often suggested "doping mechanism" of the titania lattice by Nb5+ ions cannot play an important role explaining the beneficial effect of Nb. The morphology and composition of the scales formed during oxidation were studied by glow discharge optical emission spectroscopy (GDOES) and secondary neutral mass spectrometry (SNMS). The latter technique was also used in combination with two-stage oxidation experiments using the isotope tracers O-18 and N-15.