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)