Oxidation of TiAl intermetallic

Abstract

The oxidation kinetics of TiAl intermetallic at 500-900°C in air is studied using a gravimetric method, and the phase composition of the scale is studied using an x-ray phase analysis. At t > 600°C, the kinetics of oxidation is described by a parabolic equation. The oxides TiO 2 (rutile), γ-Al 2 O 3 , α-Al 2 O 3 , Ti 2 O 3 are found in the scale. It is shown that at the first stage the γ-Al 2 O 3 and low-titanium oxides form on the sample surface at t < 70°C. At t ≥ 850°C, the Ti 2 O 3 forms on the external surface of the scale, TiAl 3 is found in the sublayer at the alloy/scale interface. It is shown that at t ≤ 800°C the process is controlled by oxygen diffusion. At t > 800°C, the oxidation mechanism changes: counterdiffusion of titanium ions through interstitial sites in TiO 2 lattice occurs.There is a need for data on the properties of titanium-aluminum alloys to create high-temperature oxidationresistant structural alloys of relatively low density [1]. Particular emphasis is placed on intermetallic compounds, including TiAl whose properties can be changed by alloying [1,2]. Of practical importance is to study the oxidation resistance of titanium-aluminum intermetallics [3].Our objective is to study the isothermal oxidation of TiAl alloy in air at temperatures 500-900°C. That this intermetallic compound exists over a wide concentration range is indicated by the phase diagrams of the Ti-Al system in many publications [1,4,5]. However, different authors specify different limits for this range. For the purpose of this study, we prepared alloys of titanium with 50 and 56 at.% (36 and 40 wt.%) of aluminum by smelting in an arc furnace with nonconsumable tungsten electrode in argon followed by homogenizing annealing in a vacuum furnace at 1000°C for 50 h. The charge was iodide titanium and electrolytic aluminum. A chemical analysis of the prepared samples revealed no significant deviation from the charge composition (±0.1 %).The annealed ingots were analyzed in molybdenum radiation using a DRON-3 x-ray diffractometer. Then, the ingots were turned into powders, which were annealed in vacuum at 700°C for 2 h to relieve stresses. After that, x-ray diffraction pattern were recorded with a Debye camera (diameter 150 mm) with copper radiation. The calculated data for plotting a Debye powder diagram are collected in Table 1, which includes, for each reflection, the values of sin 2 ϑ (ϑ is the Bragg reflection angle) and their relative intensities I/I 0 (on a 10-point scale). The x-ray patterns were indexed as in [6,7].The studies confirmed that the smelted alloy with 56 at.% Al is single-phase and has a CuAu-type tetragonal face-centered lattice with a = 0.390 ± 001 nm and c = 0.408 ±0.002 nm, which is in agreement with published data [1,[4][5][6][7]. The x-ray diffraction patterns of the Ti-50 at.% Al alloy show, except for the intensive reflections of TiAl, weak reflections that, according to [6], may belong to Ti 3 Al (Table 1). A metallographic analysis shows that this phase amounts to approximately 5%.To study the ai...

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