Investigation of the Oxidation Behavior of Orthorhombic Ti2AlNb Alloy

Journal of Materials Engineering and Performance, Feb 2015

The results of investigation on the oxidation behavior of orthorhombic Ti2AlNb alloy are presented. Oxidation was carried out in static air atmosphere at two temperatures: 700 and 800 °C. The investigation of the material structure of the specimen and chemical composition of oxidation products was performed. It was determined that the alloy shows a sufficient high-temperature corrosion resistance only at 700 °C.

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Investigation of the Oxidation Behavior of Orthorhombic Ti2AlNb Alloy

Journal of Materials Engineering and Performance Investigation of the Oxidation Behavior of Orthorhombic Ti AlNb Alloy 2 Joanna Maecka 0 0 Joanna Maecka, Faculty of Mechanical Engineering, Opole University of Technology , Opole , Poland. Contact The results of investigation on the oxidation behavior of orthorhombic Ti2AlNb alloy are presented. Oxidation was carried out in static air atmosphere at two temperatures: 700 and 800 C. The investigation of the material structure of the specimen and chemical composition of oxidation products was performed. It was determined that the alloy shows a sufficient high-temperature corrosion resistance only at 700 C. corrosion and wear; oxidation; titanium 1. Introduction In recent years, high-temperature titanium alloy development for aerospace applications has focused on c-TiAl alloys. These alloys are a new type of materials and represent a very attractive structural material for operation at elevated temperatures and in aggressive chemical environments, mainly due to particularly favorable combination of mechanical properties and low density. Good creep resistance and relatively good oxidation resistance are their main assets which add to their versatile application (Ref 14). The research carried out so far by research centers have concerned the issue of high-temperature oxidation resistance of c-TiAl-based alloys (Ref 511). Recent efforts to improve high-temperature properties have been directed toward the optimization of the Nb (Ref 1214). However, the tendency to brittle fracture and low flow is their main shortcomings which limit their use. Recently, there has been a considerable interest in Nb-rich Ti3Al alloys due to the discovery of an orthorhombic (O) phase based on the compound Ti2AlNb (Ref 15). This orthorhombic phase was first found in a Ti-25Al-12.5Nb (at.%) alloy. The O phase is similar in nature to a2 phase (Ti3Al, DO19 structure), yet differs in the lattice arrangement of Nb with respect to Ti (Ref 15, 16). In Ti-alloy compositions ranging from 20 to 30 Al and 11 to 30 Nb the O phase has been identified (Ref 17, 18) and such alloys are often referred to as O alloys. Orthorhombic Ti2AlNb-based alloys have outperformed a2 alloys in terms of creep, tensile strength, ductility, toughness, and thermomechanical fatigue (Ref 1922). These alloys appear to be quite promising for this application, but will find increased attention only if they offer a unique set of properties, not provided by competing alloys. For demanding applications in elevated temperatures, e.g., jet engines, this new class of alloys competes with conventional near-titanium alloys, the almost mature c-TiAl alloys and, as all high-temperature titanium base alloys, with nickel-based materials (Ref 23). In the Ti-Al-Nb system, the orthorhombic alloys based on Ti2AlNb are generally constituted of some of the following phases: the a2 phase (based on Ti3Al, hexagonal DO19 structure), the O phase (based on Ti2AlNb, orthorhombic distortion of the a2 phase), and the b0 (or B2) phase (based on Ti2AlNb, ordered from the bcc b-phase) (Ref 24). For mechanical optimization several alloying elements are added, which show specific phase stabilization properties: Mois reported to reduce the oxygen solubility and thus to inhibit internal oxidation (Ref 25). Nb is a b-forming element so it causes the formation of b-Ti(Nb) phase in a2 and c phases. It reduces the solubility of the oxygen thus preventing internal oxidation of these alloys (Ref 2628). Introducing niobium as an alloying additive makes niobium ions replace Tileading to reduction of vacancies in oxygen, which limits the diffusion of oxygen (Ref 29). Niobium improves the resistance to oxidation if it forms a solid solution with the scale. If, however, it occurs in the form of TiNb2O7 or AlNbO4 phase, it decreases the oxidation resistance. The effect of niobium in Ti-Al also consists in hindering the mass transfer of TiO2 (Ref 13). Vpromotes the improvement of the plasticity of alloys (Ref 30). Relatively few works have been reported on the oxidation behavior of orthorhombic alloys (Ref 3133). This work aims to shed light on the oxidation and scale formation mechanisms in orthorhombic Ti-Al-Nb alloys (Ti-22Al-25Nb alloy). In the present paper the isothermal oxidation behavior of orthorhombic alloy (Ti-25Al-12.5Nb) was studied in static air atmosphere for two temperatures: 700 and 800 C. 2. Experimental Procedures The tests were performed on O-Ti2AlNb-based alloy Ti25Al-12.5Nb (at.%) with the content of b-stabilizing elements: Mo (3.0 at.%) and V (0.48 at.%). Isothermal oxidation experiments were performed in static air atmosphere at the temperature 700 and 800 C during 500 h. Oxidation tests were carried out on rectangular coupons of 20 9 15 9 2 mm. The samples were ground on abrasive paper of 800 grade paper and subsequently degreased in acetone. The computer-aided acquisition system was used to verify the actual test temperature. After (...truncated)


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Joanna Małecka. Investigation of the Oxidation Behavior of Orthorhombic Ti2AlNb Alloy, Journal of Materials Engineering and Performance, 2015, pp. 1834-1840, Volume 24, Issue 5, DOI: 10.1007/s11665-015-1449-6