Superplasticity of titanium aluminides

In the present paper, processing methods and superplastic properties of fine-grained γ TiAl+α2-Ti3Al based intermetallic alloys with varying alloy composition have been considered. It has been demonstrated that superplastic behavior is feasible in single-phase (γ-TiAl and α-Ti3Al), two-phase (γ-TiAl+α2-Ti3Al) and three-phase (γ-TiAl+α2-Ti3Al+β(B2)) alloys. Superplastic properties in the single-phase alloys were reached due to the fact that diffusion processes occur slower in intermetallic alloys in contrast to pure metals; this impeded dynamic grain growth during superplastic flow and promoted superplastic elongations. It has been revealed that processing required for manufacturing of a fine-grained condition is dependent on the alloy composition and can be simplified at transition from peritectically solidifying γ-TiAl+α2-Ti3Al based alloys to β-solidifying ones. The use of β -solidifying alloys, which are known to have better chemical homogeneity in contrast to peritectically solidifying γ-TiAl+α2-Ti3Al based alloys, provided enhanced superplastic characteristics. Highest superplastic elongations (up to δ=1000% at T=1000°C and εʹ≈10-3 s-1) were obtained for the β-solidifying alloy (Ti-45Al-5Nb-1Mo-0.2B, at. %) containing a small amount of the β(B2)-phase and subjected to single-step hot forging. Interestingly, superplastic properties were even attained in the as-cast β-solidifying alloy containing around 20 vol. % of the β(B2) phase without any prior hot working.

In the present work, superplastic behavior of γ-TiAl+α 2 -Ti 3 Al-based alloys depending on the grain size, the chemical and phase composition, the temperature-strain rate conditions are considered and the mechanisms underlying the superplastic behavior are discussed.
The alloy designing thus led to development of processing methods for producing fine-and ultrafinegrained microstructures in γ+α 2 alloys. Initially, multistage isothermal hot forging was used for grain refinement of cast intermetallic alloys. In so doing, the forging temperature gradually decreased from one forging stage to another. The strain rate during forging was chosen in the range of ε´=5×10 -4 −5×10 -3 s -1 . The microstructural refinement occurred as a result of dynamic recrystallization. The forging procedure at a certain temperature was fulfilled with a change of the forging direction by 90° (abc forging) till achievement of a fully recrystallized microstructure because non-recrystallized areas were inherited and retained during subsequent forging at lower temperatures. The microstructural refinement achieved at each forging temperature improved the hot workability of the intermetallic alloys that allowed decreasing the forging temperature from stage to stage. At the same time, the forging temperature was decreased only to a certain temperature, below of which the intermetallic alloy was embrittled. Particularly, in the Ti-25Al alloy the embrittlement was associated with a partial disordering of the α 2 phase [28]. Using the described forging processing, the workpieces with a homogeneous ultrafine-grained microstructure with a grain size as small as d=0.1-0.4 µm were manufactured in Ti-45Al-5Nb-1Mo-0.2B / 60 1 forging step at 1200-1100°С 5-10 + LR Ti-43Al-4.5Nb-2Mo-0.2B / 5-50 As-cast condition 5-50 (G/LS) the stoichiometric and «conventional» γ+α 2 alloys ( Fig. 1 and Table 1). An achievement of ultrafine-grained microstructures was of great interest for understanding of a ductile capability of the intermetallic alloys.
Alloying by chromium and niobium in the ingotmetallurgy Ti-48Al-2Cr-2Nb alloy led to an increase in the forging steps number required for attaining fully recrystallized microstructure. This can be ascribed to the effect of chemical inhomogeneities caused by dendritic segregation, which is characteristic of peritectically solidifying alloys like Ti-48Al-2Cr-2Nb. The microstructure inhomogeneities caused by dendritic segregations promoted localized occurrence of dynamic recrystallization during hot forging that was a reason why so many hot forging steps was applied to produce fully recrystallized ultrafine-grained condition. The use of a powder metallurgy alloy (Ti-46Al) facilitated an achievement of ultrafine-grained microstructure owing to initial relatively fine-grained microstructure and twostep hot forging was found to be enough for producing fully recrystallized ultrafine-grained microstructure [8,11]. For stress relief, the forged workpieces were annealed for 2 hours at a temperature by 50°C lower than the last forging step temperature.
Two-step hot working processing with intermediate globularization annealing was used in the case of β-solidifying γ+α 2 alloys such as Ti-45.2Al-1.7Nb-1.5Cr-0.5B and Ti-44.2Al-0.7Nb-1.9Cr-0.4B (Table 1) [18,19]. On the first step, the ingots were canned using low-carbon steel, preheated and forged at a nominal strain rate of ε´=10 -2 -10 -1 s -1 in the temperature range of the α+γ phase field. The die tool was preheated, its temperature was T=950°C. After the first step forging, the workpieces were decanned and subjected to intermediate globularization annealing. The second hot working step was conducted in the same working direction under isothermal conditions at a strain rate of ε´~10 -3 s -1 using an isothermal die unit and a glass lubricant. Finally, the forgings were aged at T=900°C for τ=2 hours, followed by furnace cooling. The produced microstructures were mainly fine-grained although some lamellae remnants were still retained in the forged workpieces.

Superplastic behavior of γ-TiAl+α 2 -Ti 3 Al based alloys
As was mentioned, there are some features in superplastic behavior of intermetallic alloys as compared with that of conventional alloys. First of all, it is worth noting high superplastic characteristics obtained for the single-phase Ti-25Al alloy with the grain size d≈ 0.1 µm [13,16]. It is well known [17] that pure metals are not prone to superplastic behavior because of a rapid grain growth, which occurs during superplastic flow at elevated temperatures. Most probably, diffusion processes occur slower in the intermetallic Ti-25Al alloy as compared with pure metals that impeded dynamic grain growth during superplastic flow and provided high superplastic properties [16]. The stoichiometric Ti-50Al alloy demonstrated higher superplastic properties than pure metals but appreciably lower than Ti-25Al that can be associated with a higher ordering energy of the γ-TiAl phase as compared with that of the α 2 -Ti 3 Al phase. Partial disordering occurring in Ti-25Al promoted dislocation mobility and apparently grain boundary sliding as well. Another reason why the Ti-50Al alloy showed lower superplastic properties in comparison with those of the Ti-25Al alloy was the fact that a low stacking fault energy in the γ phase facilitated formation of annealing twins, which impeded grain boundary sliding and promoted dislocation accumulation in the grain interior [16]. Superplastic properties of the γ-TiAl+α 2 -Ti 3 Al based alloys are improved with increasing the volume fraction of the α 2 phase. This is probably due to a stabilizing effect of the α 2 phase on grain growth and formation of annealing twins both during preheating of sample and in the course of superplastic flow [6,8,16]. Note that the same was observed in conventional alloys [17]. At the same time, high-temperature mechanical behavior of the ultrafine-grained γ+α 2 alloys significantly depended on both uniformity of the α 2 phase distribution and the size of the α 2 phase particles. The γ+α 2 alloys solidifying through peritectic reaction (s) have non-uniform distribution of aluminum and the α 2 phase and, therefore, are prone to strain localization at elevated temperatures, shown by the formation of macroscopic deformation bands enriched or depleted with the α 2 phase particles [11,18]. This strain localization is avoided in the case of β-solidifying alloys, which are characterized by a more uniform aluminum distribution than peritectically solidifying alloys [16,19]. High superplastic characteristics are reached in the β-solidifying γ+α 2 alloys even if lamellae remnants are retained along with the fine-grained microstructure [19]. As a rule, the lamellae remnants completely transform into fine equiaxed grains during superplastic flow as a result of dynamic recrystallization and globularization processes, which occur in the course of superpalstic deformation. In this case, the size and morphology of the α 2 particles have an influence on superplastic properties: the presence of coarse and non-equiaxed α 2 particles promote the void formation and premature failure. Therefore, globularization annealing deteriorated superplastic properties of the γ+α 2 alloys if microstructural globularization was accompanied by an appreciable growth of the α 2 particle size leading to higher porosity and lower ductility [19].
The use of the β-solidifying γ+α 2 +β alloys instead of twophase γ+α 2 alloys leads to an increase of the hot workability in as-cast condition due to the refinement of as-cast structure and the presence of the ductile β (B2) phase. The presence of the β (B2) phase facilitated significantly the hot working and resulted in higher elongations under superplastic conditions (Fig. 2) [20].
If the β (B2) phase content is relatively high, as in the Ti-43Al-4.5Nb-2Mo-0.2B alloy, superplastic properties can be reached in as-cast condition without using any hot working or heat treatment [22,23]. The presented results have shown that the γ+α 2 +β (B2) alloys can be used for superplas-β (B2) alloys can be used for superplas-(B2) alloys can be used for superplastic forming processing at relatively moderate temperatures (T=900…1100°C).

Conclusions
The following conclusions can be drawn from the present work: 1. Superplastic properties were reached in the singlephase Ti-25Al alloy and in the near single-phase Ti-50Al alloy in the ultrafine-grained conditions obtained by multiple hot forging. It was suggested that diffusion processes occur slower in the intermetallic alloys as compared with pure metals and this impeded dynamic grain growth during superplastic flow promoting superplastic elongations.
2. The transition from «conventional» to β-solidifying alloys containing a large amount of the β (B2) phase allows reducing a number of hot working steps during fabrication of fine-or near fine-grained products. As has been demonstrated, superplastic properties are even attained in the as-cast β-solidifying alloy containing about 20 vol.% of the β (B2) phase without any prior hot working.