FORMATION OF A HETEROGENEOUS STRUCTURE IN γ-TiAl-BASED ALLOY USING A CELLULAR REACTION

The structure of the Ti ‒ 43.7Al ‒ 3.4Nb ‒ 1.1Mo alloy (at. %) was studied, the temperatures of the phase transformations and their sequence during heating and heat treatment aimed at activating the cellular reaction were determined. The latter consists in the formation of new large-plate (λ₂) in nanoplatellar (λ₁) colonies (λ₂ >> λ₁). The temperature of the eutectoid (α₂ → γ) transformation (1137 °C (calculated temperature 1071 °C)) and the temperature of the dissolution of the γ-phase (1273 °C (calculated temperature 1262 °C)) were theoretically and experimentally determined using differential scanning calorimetry during heating, and the dependence of phase fractions on temperature is modeled. A metastable α₂-grain structure with β-phase interlayers was obtained by quenching from the two-phase (α + β)-phase region. As a result of subsequent aging, a heterogeneous structure formed during the cellular reaction was obtained. After aging at a temperature of 800 °C for a duration of 6 hours, the heterogeneous structure consisted of nanoplatellar (α₂ + γ) colonies with an interplate distance of 0.01 microns and large-plate (α₂/β + γ) colonies with an interplate distance of 0.3 microns and a volume fraction of 5 %. An increase in the aging temperature to 850 ° C led to an increase in the interplate distance of large-plate colonies to 0.5 microns and the absence of changes in the volume fraction of 5 %. The obtained states were subjected to subsequent annealing at a temperature of 1050 ° C, which led to an increase in the interplate spacing of nanoplatellar colonies to 0.5 microns and large-plate colonies to 3.2 microns, as well as an increase in their proportion to 23 vol. % in case of aging at 800 °C. As a result of annealing after aging at a temperature of 850 ° C, the interplate distance in nanoplatellar colonies was 0.7 microns, and in large-plate colonies 4.2 microns, the volume fraction of which reached 22 %. During annealing at 1050 °C, an active α → β transformation was observed, leading to the transformation of large-plate (α₂/β + γ) colonies into (β + γ) colonies, and nanoplate (α₂ + γ) colonies into (α₂/β + γ) colonies.

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