Oxidation titanium aluminides

There are two important titanium aluminides Tig A1 which has a hexagonal structure with a density of 4.20 g/cm and a melting point of 1600°C and Ti A1 which has a tetragonal structure with a density of 3.91 g/cm and a melting point of 1445°C. As do all aluminides, they have excellent high temperature oxidation resistance owing to the formation of a thin alumina layer on the surface. They have potential applications in aerospace structures. 

Effect of Complex Environments on the Oxidation of Titanium Aluminides... 

There has been relatively little work published on the reaction of titanium aluminides in atmospheres other than air or oxygen. Niu et al. [96] studied the reaction of Ti-25Al-llNb in a simulated combustion atmosphere (N2+1%02+ 0.5%SO2) with and without surface deposits of Na2S04-t- NaCl at temperatures between 600 and 800°C. Exposures in the absence of surface deposits resulted in reaction rates similar to those described above for simple oxidation. The rates in the presence of the deposits at 600 and 700 °C were initially rapid and then slowed markedly after 25 to 50 hours exposure. The rate at 800°C remained rapid with the kinetics being essentially linear. The major difference in the corrosion morphology at 800 °C was the presence of copious amounts of sulfides below the oxide scales. The authors postulate a mechanism of attack involving a combination of sulfidation-oxidation and scale-fluxing. 

D.S. Appolonia The Oxidation of Gamma-Titanium Aluminides". Bachelor of Science Thesis, University of Pittsburgh, Pittsburgh, PA (1988). 

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

In the niobium containing alloy which shows a better oxidation resistance the doping of titania with niobium may reduce the dissolution of AlON. By this means a thin layer AlON is formed at the interface leading to a reduced oxidation rate. Thus it is assumed that the oxidation behaviour of titanium aluminides could be improved by stabilizing the aluminium oxide at the metal/oxide interface either by prevention of aluminium depletion of the metal subsurface zone or by reduction of A1203 dissolution in Ti02. 

N.S. Choudhury, H.C. Graham, J.W. Hinze Oxidation Behavior of Titanium Aluminides, Z.A. Foroulis, F.S. Pettit, Eds., Proceedings of the Symposium on Properties of High Temperature Alloys (Tlte Electrochemical Society, Princeton, N.J.), (1976), pp. 668. 

In the present study the oxidation behaviour of three binary titanium aluminidcs, Ti45 Al,Ti48Al and Ti50Al (additions given in at.%), as well as a number of ternary alloys with Nb contents between 2 and 10 % were studied at 900°C in order to clarify the apparent contradictory effect of nitrogen on the oxidation resistance of titanium aluminides. 

In industrial applications the environments usually contain more than one reactant. For example high temperature oxidation occurs in air by the combined attack of oxygen, nitrogen and quite frequently water vapour. However, most of the studies concerning the oxidation resistance are performed in dry oxygen or dry air. The oxidation behaviour of the intermetallic phases of theTi-Al system has recently received considerable attention. The influence of water vapour on the oxidation of titanium aluminides has not been studied intensively. There are only a few studies of the high temperature corrosion of titanium and its alloys. 

A great deal of research effort has been directed towards explaining the effect of niobium alloying on the oxidation behaviour of the titanium aluminides y- IiAl and 2-Ti3Al in air. A comprehensive survey is given in [14]. Among the possible mechanisms currently discussed are ... 

AljTi is being considered for use as an oxidation-resistant coating on Ti alloys and titanium aluminides (Subrahmanyam... 

The much advanced nickel aluminides and titanium aluminides can be used only up to about 1000 °C because of their limited strength or oxidation resistance or both at higher temperatures, as has been stated before (Sauthoff, 1994). For applications significantly above 1000 °C other less-com--mon phases with higher melting temperatures have to be used. Such phases are available, and examples are shown in Fig. 34 (Sauthoff, 1992). In comparison to the nickel aluminides and titanium alu-... 

The selective oxidation of an alloy component, e.g., A1 or Si, requires the alumina or silica to be more stable than the oxides of the other components in the alloy. Figure 2.5 indicates this condition would be met for compounds such as nickel aluminides and molybdenum silicides. However, in the case of Nb- or Ti-base compounds the oxides of the base metal are nearly as stable as those of A1 or Si. This can result in conditions for which selective oxidation is impossible. This situation exists for titanium aluminides containing less than 50 at% A1 as illustrated in Figure 5.27. In this case a two-phase scale of intermixed AI2O3 and I1O2 is generally observed. It should be emphasized that the determination of which oxide is more stable must take into account the prevailing metal activities. 

Recently, a great interest has been placed on intermetallic titanium aluminides the two systems under development are based on Tis A1 and TiAl compounds, which promise temperature capabilities of 800 °C and 980 C, but great efforts have to go into the achievement of ductility, toughness and oxidation resistance above 650 C. 

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