Effect of Nb Content and Temperature on the TiN Formation During the Oxidation of Ti 3 Al
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ABSTRACT Nb addition to Ti3Al has been found to be effective to improve its oxidation resistance. Recently, the beneficial effect of nitrogen in the oxidation behaviour of Nb contained T 3AI-based alloys has been noticed. In this paper, the oxidation behaviour of five alloys with different Nb content from 0 to 20 at. % exposed in room air at different temperatures (from 600 to 950,C) has been examined with the focus on the TiN formation in the oxide scale. No TiN is detected in the scales of binary Ti3AAunder all temperatures tested. On the contrary, in alloys containing Nb, TiN is identified in the scales irrespective of the oxidation temperature. The amount of TiN formation differs with different Nb content. EPMA analysis of the cross section of the scales is performed and discussion is made accordingly. INTRODUCTION Ti 3A1 is an attractive material for application in aerospace engine. One of the drawbacks of
Ti3Al for high temperature application is its insufficient oxidation resistance since no protective scale is formed on the binary Ti3AI. It has been found that Nb addition to Ti 3A1 increases its oxidation resistance. Recently, the beneficial effect of TiN in the oxidation behaviour of Nb contained Ti 3AI-based alloys has been noticed. Our previous work a few years ago [1] indicated the improvement of oxidation resistance in Ti3A1 alloys with 11 at. % Nb addition in association with the formation of TiN. More recently, Roy et al [2] further demonstrated that the improved oxidation resistance of Ti-24Al-15Nb in oxygen was further improved in nitrogen containing gases. TiN was detected in the scales in Ti3A1 containing 11 or 15 at. % Nb after exposure at 1200K or above. TiN was also found in Ti-25Al-0ONb-
3V-lMo oxidised in air between 973 to 1273K [3]. However, there are still some discrepancies regarding to the effects of Nb content and exposure temperature to the formation of TiN in the scale in liter-
atures. In this paper, the oxidation behaviour of five Ti 3Al-based alloys with Nb content from 0 to 20 at. % at different temperatures (from 600 to 950°C) has been examined with the focus on the TiN for-
mation in the oxide scale. EXPERIMENTAL PROCEDURE One binary Ti 3A1 alloy (Ti-25A1) and four Ti 3Al-based alloys with different Nb contents (Ti25A1-5Nb, Ti-25Al-1ONb, Ti-25A1-15Nb, and Ti-25A1-2ONb) were prepared by arc melting in a laboratory furnace under an argon atmosphere. All compositions are in atomic percent unless mentioned otherwise. Specimens with the dimension of lOxlOx2 mm 3 were cut from the ingots and exposed in the open air at temperatures from 600 to 950'C respectively. After oxidation, the phase constitutions of the scales were characterised by X-ray diffraction (XRD) using Cu Ke radiation. The cross sections of the scales formed on the alloy were investigated by electron probe micro-analyser (EPMA).
KK8.8.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
RESULTS To our previous experience [1], TiN initially forms at the matrix/scale interface. During the long t
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