Influence of the Heterogeneous Structure Formed During the Cellular Reaction on the Mechanical Behavior and Structural Evolution of the TNM–B1 Alloy During Hot Deformation

Abstract

In the present work, using the three‑phase (α₂ + β + γ) alloy TNM–B1 (Ti–43.5Al–4Nb–1Mo–0.1B) as an example, a comparative study was carried out of the effect of the heterogeneous structure formed during the cellular reaction on the mechanical behavior and structural evolution during hot deformation in the upper part of the (α₂ + β + γ) phase field at T = 1050°C. The alloy in the as‑cast condition with a fine lamellar structure (interlamellar spacing λ ~ 100 nm) and after preliminary heat treatment (HT) (oil quench from the (α + β) region at T = 1290°C) were subjected to isothermal uniaxial compression at an initial strain rate of 0.001 s^–1 to ε = 70%. The as‑cast alloy exhibited the typical microstructural evolution of cast alloys, characterized by preferential localization of plastic deformation at colony boundaries, leading to the development of dynamic recrystallization/spheroidization. In the pre‑heat‑treated condition, a cellular reaction was found to occur already upon heating to the deformation temperature (T = 1050°C); prior to deformation a heterogeneous microstructure had formed, consisting of nano‑lamellar and differently oriented coarse lamellar colonies (λ ~ 10 nm and 0.3–1.0 µm, respectively) in a ~70/30 ratio. The study showed that formation of this heterogeneous structure leads to a reduction in peak stresses (from 350 MPa for the as‑cast condition to 220 MPa after HT) and to a more extended steady‑state flow stage, where stresses decreased from 280 MPa (as‑cast) to 190 MPa (HT). In the alloy after preliminary heat treatment, deformation was localized in the coarse lamellar colonies formed during the cellular reaction, in which recrystallization/spheroidization of α₂/β/γ grains/particles developed sequentially. Application of the preliminary heat treatment increased the volume fraction of recrystallized/spheroidized grains/particles at ε = 70% from 35% (as‑cast) to 50% (HT).