Effects of High Niobium Addition on the Microstructure and High-Temperature Properties of Ti-40Al-xNb Alloy
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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 γ phase in a B2 matrix. The increased addition of Nb to the alloy inhibits the solidification path β→β+α. Therefore, the as-cast Ti-40Al-<I>x</I>Nb (<I>x</I>=15,16) alloys are composed of primary β/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+γ, while the homogenized Ti-40Al-16Nb alloy has a four-phase microstructure of B2+γ+α+σ. 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 Ti-40Al-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<SUB>2</SUB>, while the oxide scales formed on the Ti-40Al-15Nb alloy are dense Al<SUB>2</SUB>O<SUB>3</SUB>-rich oxides. The Ti-40Al-15Nb alloy has the lowest mass gain among these three alloys due to the existence of the Al<SUB>2</SUB>O<SUB>3</SUB>-rich oxides. The formation of Al<SUB>2</SUB>O<SUB>3</SUB>-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 σ phase.
- Materials Transactions, JIM
Materials Transactions, JIM 47(10), 2588-2594, 2006-10-20
The Japan Institute of Metals and Materials