Oxidation Properties of a Beta-Stabilized TiAl Alloy Modified by Rare Earth Elements.

This paper explores the effects of adding rare earth elements (lanthanum or erbium) on the oxidation properties of Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy. Isothermal oxidation tests were performed under air atmosphere at 900 and 1000 °C. Mass gain was measured in several steps during the oxidation test, and the oxidized specimens were characterized by XRD and FE-SEM. The results showed that while adding 0.1 at.% rare earth elements (REEs) reduced oxidation rate of the TNM alloy, 0.2 at.% REEs addition increased the mass gain of the alloys. The oxidation curves were fitted by a power-law equation; the results showed that the oxidation kinetic curves of all alloys obeyed parabolic growth kinetics (n = 2). Meanwhile, the activation energy of oxidation was in the range of 40-50 kCal/mol, thereby suggesting that the scale growth was controlled by mass transport in the TiO2 layer. Also, the results of the scale characterization showed that addition of REEs at low level (e.g., 0.1 at.%) could reduce diffusion rate in the scale. However, addition of the higher amounts of La or Er (e.g., 0.2 at.%) due to the lower valency (+ 3) of these elements, as compared with Ti (+ 4), could lead to the increased anion diffusion, the formation of hillocks in the scale and a rise of the oxidation rate. [ABSTRACT FROM AUTHOR]