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Tuesday, May 16, 2006 - 2:10 PM
SECP3.3

Role of Niobium in the Oxidation Mechanisms of an Alpha2-Ti3Al Alloy

A. G. Noumet, P. Josso, D. M. P. Bacos, ONERA, CHATILLON CEDEX, France; V. Maurice, D. S. Zanna, D. P. Marcus, Ecole Nationale Supérieure de Chimie de Paris, Paris Cedex 05, France; B. Lefez, IUT de Rouen,, Mont Saint Aignan, France

  The Ti-Al based intermetallics exhibit many outstanding properties including high melting point, low density, intrinsic strength retention and high stiffness but their high temperature degradation is fast. Niobium is well known to increase the oxidation resistance of titanium aluminide. Moreover during oxidation, the mass gain of the niobium doped titanium aluminide decreases as the niobium percentage increases up to a limit value. However if this effect is well known, the involved mechanisms are far from being identified.

 

In the present work, two aspects of the hot oxidation behaviour of various niobium doped titanium aluminides Ti3Al have been studied.

 

Firstly an XPS (X-Ray Photoelectron Spectroscopy) study of the early stages of oxidation of  a2-Ti3Al at 650°C under low oxygen pressure is reported. It showed that niobium promotes a faster stage of oxygen absorption, favors a faster formation of a protective pure alumina layer and delays the formation of the titanium oxide. For niobium concentration higher than a limit value, depending on the composition of the alloy, TiO2 quickly appears, leading to the material’s ruin. Further aspects, as pressure or interstitial elements, will be also discussed.

 

Secondly, results for long term oxidation under laboratory static air at 650°C obtained by X-Ray diffraction, Infrared Spectroscopy, Glow Discharge Spectroscopy and Scanning Electron Microscopy are presented. According to the percentage of niobium, different types of modified layers enriched with oxygen or nitrogen are developed in the substrate at the metal/oxide interface.

 

The correlations between the modifications induced during the initial stages of oxidation and the observed layers obtained after long term oxidation are described. An optimal percentage of niobium is proposed to obtain the best oxidation resistance.

Summary: Two aspects (initial stages and long term oxidation) of the hot oxidation behaviour of various niobium doped titanium aluminides have been studied. One of the objectives is to explain the correlations between the modifications induced during the initial stages of oxidation and the observed layers obtained after long term oxidation.