Effect of Mo substitution by Nb on the high temperature oxidation behavior of Beta 21S titanium alloy

Full text: The β-21S titanium alloy that has a nominal composition Ti-15Mo-2,7Nb-3Al-0,2Si (wt. %) is one of the main titanium alloys for structural applications at elevated temperatures. The β-21S alloy is a metastable titanium alloy characterized by good mechanical properties and elevated resistance to corrosion by hydraulic fluid (Skydrol) and hydrogen absorption [1]. The alloy has been used to substitute Ni based superalloys in the BOEING 777 airplane for plug-and-nozzle assemblies which allowed for a considerable weight reduction of the exhaust structure [2]. The oxidation phenomena of titanium and its alloys are strongly dependent on the composition of the oxidizing gases and the time of exposure at high temperatures. During high temperature oxidation, dissolution of interstitials in titanium alloys are responsible for the degradation of their mechanical properties [3]. The present work aims to evaluate the effect of Mo substitution by Nb on the oxidation resistance at temperatures in the range 600-800°C. Alloys with compositions Ti-(15-x)Mo-(2,7+y)Nb-3Al-0,2Si were arc melted using high purity elements in cooper cooled crucible under inert atmosphere. The obtained ingots were machined to produce cylinders, heat treated at 1150°C to eliminate chemical segregations, cold forged and solution treated and aged. Samples were cut and their surfaces prepared with conventional metallographic methods to perform oxidation tests. Preliminary results from mass variation of the oxidized samples showed that there are no significant differences on the oxidation behavior of the different alloys. Further characterization of the oxidation products using XRD and EDS techniques along with the kinetics results will allow a detailed description of the oxidation mechanism of these alloys. References: [1] R.W.Schutz, Journal of Metals 46, 24-29 (1994) [2] R.R.Boyer, Journal of Metals 46, 20-23 (1994) [3] C.Leyens and M.Peters, Titanium and Titanium Alloys, 187–223 (2003). (author)