Ti3Al

The Ti-Al phase diagram shows the formation of a number of intermediate phases. Of special interest are Ti3Al, TiAl (47-67at.% Al, peritectic melting at 1476°C), andTiAl3 (75 at.% Al, peritectic melting at 1387°C). For the structures of these phases, see 7.4.4.4 and Fig. 7.47. 

A candidate interlayer consisting of dual coatings of Cu and Nb has been identified successfully for the SiC-Ti3Al-I-Nb composite system. The predicted residual thermal stresses resulting from a stress free temperature to room temperature (with AT = —774°C) for the composites with and without the interlayers are illustrated in Fig. 7.23. The thermo-mechanical properties of the composite constituents used for the calculation are given in Table 7.5. A number of observations can be made about the benefits gained due to the presence of the interlayer. Reductions in both the radial, and circumferential, o-p, stress components within the fiber and matrix are significant, whereas a moderate increase in the axial stress component, chemical compatibility of Cu with the fiber and matrix materials has been closely examined by Misra (1991). 

Misra, A. (1991). Chemical compatibility issues related to the use of copper as an interfacial layer for SiC fiber reinforced Ti3Al + Nb composite. NASA CR-187100. 

Whether a particular phase is a chemical compound or a solid solution can hardly be subject to any doubt in obvious cases such as in the Ni-Bi binary system with the intermetallics NiBi (homogeneity range HR < 0.3 at.%) and NiBi3 (stoichiometric phase) or in the Ti-Al binary system with the intermetallics Ti3Al (HR 12 at.% at 600°C), TiAl (HR 7 at.%), TiAl2 (HR < 1 at.%) and TiAl3 (stoichiometric phase).142 145 193... 

F.J.J. van Loo66 with artificially prepared Ti-Ti3Al-TiAl-TiAl2-TiAl3-Al specimens at 625°C becomes easily explainable. 

The melting point of titanium is 1670°C, while that of aluminium is 660°C.142 In kelvins, these are 1943 K and 933 K, respectively. Thus, the temperature 625°C (898 K) amounts to 0.46 7melting of titanium and 0.96 melting of aluminium. Hence, at this temperature the aluminium atoms may be expected to be much more mobile in the crystal lattices of the titanium aluminides than the titanium atoms. This appears to be the case even with the Ti3Al intermetallic compound. The duplex structure of the Ti3Al layer in the Ti-TiAl diffusion couple (see Fig. 5.13 in Ref. 66) provides evidence that aluminium is the main diffusant. Otherwise, its microstructure would be homogeneous. This point will be explained in more detail in the next chapter devoted to the consideration of growth kinetics of the same compound layer in various reaction couples of a multiphase binary system. 

It is now relevant to return to considering the formation of the Ti3Al layer between Ti and TiAl (see Section 3.7.1 of Chapter 3). If only Ti were the diffusing species, then this layer would be quite homogeneous in appearance because its growth would be a result of the single reaction... 

In fact, it has a duplex structure, with the sublayer adjacent to titanium being about two times thicker than that adjacent to TiAl.66 This means that the Ti3Al layer grows mainly at the expense of diffusion of aluminium through the reactions... 

The former reaction yields one molecule of Ti3Al at the Ti3Al-TiAl interface and releases two aluminium atoms which then diffuse across the bulk of the Ti3Al layer and enter the latter reaction producing two molecules of Ti3Al at the Ti-Ti3Al interface. 

Rare earth addition and rapid solidification processing might result in (i) grain refinement and (ii) development of fine incoherent dispersoids leading to dispersed slip. Addition of dispersoids such as Er203 and Ce2S3 to Ti3Al(Nb) alloys produces refinement... 

Fig. 7 High-resolution transmission electron microscopy. HRTEM micrograph of lamellar y/ot2 titanium aluminide. From top to bottom, first twin variant of tetragonal y-TiAl, hexagonal 2-Ti3Al, second twin variant of y-TiAl and again ot2-Ti3Al. Incident beam direction for the tetragonal phases is 1 1 0, for the hexagonal phase 1 1 0. (View this art in color at www.dekker.com.)...

Increase of zirconium content above 6 % results in formation of another eutectic composed not of (Ti, Zr)5(Si, Al)3, but of (Ti, Zr)2Si phase. This eutectic is much more disperse, and the volume fraction of the phase is a little bit higher, than in alloys without zirconium. Increase of aluminum content above 6 wt.% results in appearance of Ti3Al a2-phase structure, allowing expecting for additional increase of strength and thermal stability of such systems. 

The oxidation of Ti3A1 alloys would not be expected, in light of the above thermodynamic considerations, to form continuous alumina scales. Instead they form mixed rutile-alumina scales [49].The oxidation kinetics of Ti3Al between 600 and 950°C are reported to be essentially those expected for rutile growth [50,51]. These kinetics result from the development of a complex oxide layer which contains continuous paths of Ti02 through which rapid transport occurs. Typical oxide scales are shown schematically in Figure 14. 

An additional aspect of the oxidation of Ti3Al alloys is dissolution of oxygen into the alloy at the scale/alloy interface. The embrittlement associated with this phenomenon can be more damaging to the mechanical properties than the surface recession caused by scale formation in the temperature range where Ti-,A1 will likely be used (< 700°C) [58],This subject will be discussed in a separate section. 

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