Cyclic oxidation of Ti3Al-based materials

Abstract

The microstructure variation and oxidation behavior of a hot-rolled and quenched Ti-24Al-11Nb (at%) intermetallic alloy with and without protective Ni-20Cr (at%) coating were studied in the air at a cyclic annealing temperature of 600 °C and 900 °C. The phase transformations monitored up to 120 h of alloy oxidation and oxidation products were examined using different experimental techniques. The oxidation kinetics was determined by recording the mass gain vs. time data and oxidation activation energy was evaluated. It was found that the annealing temperature and deposited coating significantly affect the scale formation and growth. The alloy showed better oxidation resistance at 600 °C, irrespective of the protective coating application. The higher temperature promoted the formation of thicker and multi-layered scale predominantly composed of Nb-doped TiO2 and Al2O3 oxides, which cracked and spalled causing oxidation rate for an order of magnitude higher than that at 600 °C. The presence of AlN decelerated the oxide scale growth, which obeyed a parabolic rate law. It was also noted that a two-phase α2+β microstructure changed to a greater extent. With increasing temperature, the α2→β phase transformation occurred more intensive and new α2′′ and O-Ti2AlNb phase appeared. The Ni-20Cr coating reduced the oxidation rate at both temperatures and improved the scale-spallation resistance. The compact Cr2O3 layer formed on the coated alloy led to the preferential Al2O3 formation, increasing its amount in mixture with Nb-enriched TiO2 oxide. Thin TiN layer detected beneath the oxide scale also supported the formation of slow-growing Al2O3, leading to a decrease in the oxidation rate. The external scale suppression contributed to the greater diffusion zone enrichment, resulting in the faster O-Ti2AlNb formation. The experimentally observed O-Ti2AlNb formation was confirmed by ab initio modeling. Furthermore, additional structures were predicted and studied using first-principles calculations in the O-Ti2AlNb compound. © 2018