Alloying titanium aluminides

Table 15. Properties of Titanium Aluminides, Titanium-Base Conventional Alloys, and SuperaHoys ...

P/M processing of titanium aluminides results in more consistent product quaHty than the conventional casting process, and offers novel alloy/microstmcture possibiHties and improved ductiHty. Processing trends include use of high (1200—1350°C) temperature sintering to improve mechanical properties of steel and stainless steel parts. 

Saunders, N. (1997a) in Light Metals, ed. Huglen, R. (TMS, Warrendalc, PA), p. 911. Saunders, N. (1997b) Phase diagram modelling of TiAl alloys , presented at the Symp. Fundamentals of y Titanium Aluminides, TMS Armual Meeting, Orlando, Florida, 10-13 February 1997. 

Clement. T.P., Parsonage, T,Band M.B, Kuxhaus Ti2AlNb = Based. Alloys Outperform Conventional Titanium Aluminides, Advanced Materials < Processes, 37 (March 1992). 

Titanium aluminides are ordered intermetallics and hence have lower diffusivity and high elastic modulus. These compounds are stronger than the conventional titanium alloys and are suitable for high temperature applications. But these compounds have low ductility due to the planarity of slip in these compounds. 

The NMR parameters for titanium metal deduced in earlier studies by field sweeping techniques (Narath 1967, Ebert et al. 1986) have been confirmed by more recent room temperature FT NMR (Bastow et al. 1998a). The value of Xq deduced from the ( /2, 2) satellite transitions was used in an accurate simulation of the central transition, which required an axial Knight shift of 70 10 ppm. The Ti NMR spectra of a number of titanium aluminide alloys and TiAg have also been reported... 

P. R. Subramanian, M. G. Mendiratta, D. M. Dimiduk and M. A. Stucke, Advanced intermetallic alloys - beyond gamma titanium aluminides Mat. Sci. Engg., A239-240 (1997) 1-13. 

Gao, M., Dunfee, W., Wei, R. P., and Wei, W., Environmentally Enhanced Thermal-Eatigue Cracking of a Gamma-Based Titanium Aluminide Alloy, Proceedings of 124th International Symposium on Gamma Titanium Aluminides VII Microstructure and Mechanical Behavior, Las Vegas, NV, Y.-W. Kim, et ah, eds.. The Minerals, Metals and Materials Society, Warrendale, PA (1995), 911-918. 

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 ... 

Titanium aluminide alloys with two compositions, and differing structures, were used as substrates. The a-2 Ti3Al was a commercial alloy (Heat T8991), fabricated as 1 mm thick sheet, by the Titanium Metals Corporation of America and contained (as w/0)... 

Table 2. Properties of alloys based on the titanium aluminides TijAl and TiAl compared with conventional titanium alloys and nickel-base superalloys (Morral, 1980, Lipsitt, 1985a Kim, 1989 Kim and Froes, 1990 Froes et al., 1991).

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