Effects of Nb addition on oxidation behavior of TiAl

Effects of yttrium addition on the formation of oxide scales and the oxidation resistance of elemental powder metallurgy (EPM) processed TiAl-Mn-Mo-C alloys were studied. The Y-containing TiAl-based alloys (0:1$0:6 at%Y) showed better oxidation resistance than Y-free alloy oxidized in air at 800C for 350 h. The Y-containing alloys showed a significantly reduced weight gain, especially for 0.6 at%Y-added alloy. In the Y-free alloy, the scale formed during extended air-exposure at 800 C consisted of TiO 2 and -Al 2 O 3 . For 0.6 at%Y-added alloy, however, the oxide scale was composed of a complex mixture of TiO 2 , -Al 2 O 3 , Y 2 O 3 and Al 5 Y 3 O 12 . The formation of multi-phase (Y, Al)O-type oxides layer in a transitional Y-rich subscale close to the substrate and an increase of the amount of -Al 2 O 3 in the mixture oxide layer in the Y-added alloys were the main contributor to improving the oxidation resistance of the alloys. The reduction of oxygen content in the substrate also had a beneficial effect on the oxidation resistance of the Y-containing TiAl-based alloys.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

High-Temperature Oxidation Behavior of Elemental Powder Metallurgy Processed TiAl-Mn-Mo-C Alloys with Yttrium Addition

Hwang

Umakoshi

2004

“…The improvement of the oxidation resistance of TiAl alloy by adding Nb has been reported and various mechanisms have been proposed to interpret the beneficial effect of Nb on oxidation. 5,[19][20][21][22][23][24] Two possible mechanisms, which are the suppression of rutile growth and the enhanced formation of alumina, can be used to account for the improvement of oxidation resistance by the addition of Nb in this study. The former mechanism is the socalled doping effect, by which oxygen vacancies in TiO 2 are reduced by the Nb cations which are incorporated to TiO 2 and substituted for Ti cations.…”

Section: Isothermal Oxidationmentioning

confidence: 99%

“…The oxidation resistance of the Nb-bearing TiAl alloys exceeds that of Nb-free TiAl alloys. 5) Moreover, TiAl alloys with high Nb contents exhibit improved capabilities for hotworking. 6,7) For industrial applications, the development of TiAl alloys that can be manufactured by conventional processes is necessary.…”

Section: Introductionmentioning

confidence: 99%

Effects of High Niobium Addition on the Microstructure and High-Temperature Properties of Ti-40Al-<I>x</I>Nb Alloy

Zhan

Koo

2006

The effect of Nb content on the microstructure and high-temperature properties of the TiAl alloy with low Al content has been investigated. The as-cast Ti-40Al-10Nb alloy consists of a Widmanstätten lath and a phase in a B2 matrix. The increased addition of Nb to the alloy inhibits the solidification path ! þ . Therefore, the as-cast Ti-40Al-xNb (x ¼ 15; 16) alloys are composed of primary /B2 phases as the major constituent. Following heat treatment, the microstructure of the heat-treated Ti-40Al-10Nb alloy resembles that of the as-cast Ti-40Al-10Nb alloy. The homogenized Ti-40Al-15Nb alloy has a two-phase microstructure of B2 þ , while the homogenized Ti-40Al-16Nb alloy has a fourphase microstructure of B2 þ þ þ . The creep response of the studied TiAl-Nb alloy is correlated closely with the tertiary creep, and is affected strongly by its microstructure. The creep resistance of the alloy depends on the resistance to the propagation of the cracks in the B2 matrix. In the Ti-40Al-15Nb alloy, the resistance to the extending of cracks is the most apparent among these three alloys. Therefore, the Ti40Al-15Nb alloy has the highest creep life of the three tested alloys. The oxidation resistance of the investigated TiAl-Nb alloy is independent of the addition of Nb. The differences among the oxidation resistances of these three alloys arise from their various microstructures. The oxide scales of the Ti-40Al-10Nb alloy are composed mainly of TiO 2 , while the oxide scales formed on the Ti-40Al-15Nb alloy are dense Al 2 O 3 -rich oxides. The Ti-40Al-15Nb alloy has the lowest mass gain among these three alloys due to the existence of the Al 2 O 3 -rich oxides. The formation of Al 2 O 3 -rich oxides appears to be related to the high Nb content of the alloy. The fact that the Ti-40Al-16Nb alloy has the largest weight gain of these three alloys is attributed to the formation of oxide scales of phase.

“…1,2) Niobium is a -stabilizer and very effective additive for improving the performance of TiAl alloys, because of its high solubility in TiAl alloys, and having a great ability to increase the oxidation resistance and the strength of the phase. [3][4][5][6] However, the oxidation resistance and the creep strength of TiAl-based alloys are regarded as crucial in practical applications at high temperature. From this viewpoint, improving the high-temperature properties of conventional TiAl alloys is important for industrial applications.…”

Section: Introductionmentioning

confidence: 99%

Creep Behavior of Ti–40Al–16Nb Intermetallic Alloy and Strengthening Effects of Minor Sc

Zhan

Koo

2006

The creep behavior of the Ti-40Al-16Nb alloy and the effect of the minor addition of Sc are investigated by creep tests at 1073 K under constant load from 200 to 280 MPa. The creep curve of the alloy with high Nb contents does not exhibit a steady-state region, and the minor addition of Sc added to the alloy has no effect on minimum creep rate. The creep responses of the alloys with high Nb contents are strongly correlated with tertiary creep behavior. The effects of Sc addition are apparent on the properties of tertiary creep rate and rupture life. The tertiary creep rate of the Sc containing alloy is reduced and the creep fracture life is increased. Subgrain structures of dislocations are absent in the alloys with high Nb contents during secondary creep, and the deformation of creep converges mainly at the B2 phase. A stress exponent of 4.5 estimated indicates that the mechanism of controlling creep behavior is dislocation climb. The creep fracture of the alloys is dominated by cleavage fracture over the entire fracture surface. Sc 2 O 3 particles are an effective obstacle to the propagation of cracks and the motion of dislocations.