Ti3Al Alloys

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. 

Ti3Al alloys have lower oxidation resistance than TiAl alloys owing to the lower Al content, and the former have lower mechanical strengths than the latter. Therefore, their potential application temperature seems lower than that for TiAl alloys, i.e. up to around 1000 K, or they would be substitutes of current high-temperature Ti alloys. Since stoichiometric TisAl is very... 

Researches on the influence of alloying additions have been rather few for Ti3Al alloys. A possible reason is that most candidate alloys contain Nb which improves the oxidation resistance to some degree, and thus there is little room for further improvement by a further addition. Additions to Ti-25Al-10Nb (at%) were tried (Koo et a]., 1994) with results that 0.5 at% Si or 0.3 at% Cr improves the oxidation resistance slightly at 973 and 1073 K, but 0.5 at% Y is effective only at 1173 K. 

Gauer L, Alperine S, Steinmetz P and Vassel A (1994), Influence of niobium additions on high-temperature oxidation behavior of Ti3Al alloys and coatings , Oxid Met, 42 (1/2), 49-74. 

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... 

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. 

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. 

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)... 

Pn >erty H-based alloys Ti3Al-based intermetallic materials HAl-based intermetallic materials Ni-based superalloys... 

Alloys based on y-TiAl, also called gamma titanium aluminides, excel due to their high strength per unit density. These alloys contain the Q 2-phase Ti3Al as a second phase and are further alloyed with other elements for property optimization. The composition range is Ti-(45 8) Al-(0-2)(Cr, Mn, V)-(0-5)(Nb, Ta, W)-(0-2)(Si,B, Fe, N) (at.%). Further components such as Hf,... 

Mechanical Properties. Elastic properties of TiAl-based materials are compiled in Table 3.1-32. Strength, ductility, and toughness of the TiAl-based alloys are sensitive functions of both composition and microstructure which is controlled by prior processing [1.59]. Characteristic data are shown in Table 3.1-33 for various alloys. Different property data for the same alloy composition indicate the effect of different prior thermo-mechanical treatments. It is noted that TiAl-based alloys are prone to hydrogen/environmental embrittlement depending on the amount of q 2 from Ti3Al [1.60]. Creep and fatigue data are available [1.59,61]. 

Usually y-TiAl-based alloys contain Q 2-Ti3Al as a second phase and are, therefore, subject to hydrogen uptake and hydrogen/environmental embrittlement depending on the amount of Q 2-Ti3Al. 

Figure 6-21. Through-thickness cracks in a pack-aluminized Ti3Al-r Nb alloy (Ti-25A1-1 INb at.%) after 200 1 -h cycles at 982 °C in air (coating D from Smialek et al. (1990b) micrograph courtesy of J. L. Smialek).

Ti-Al-Cr alloys, in which the Ti(Cr,Al)2 Laves phase is formed at the alloy-scale interface instead of the Ti3Al and/or TJX phases observed in the oxidation of binary TiAl (refer to Section 6.5.5 for further details). Despite the relatively low Al content of the Laves phase (approximately 40-42 at.% Al), speculated by these authors to be of the type Nb(Cr,Al)2, an alloy consisting primarily of this phase was found by Doychak and Hebsur (1991) to be capable of AI2O3 scale formation at 1200°C in air. 

---