Hot deformation behavior and processing maps of B and Gd containing β-solidified TiAl based alloy

Abstract

High-temperature mechanical behavior and microstructure evolution during hot deformation of an as-cast β-solidified Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B (at.%) alloy were studied. Phase transformation temperatures and associated phase fields (α2 + γ, α + γ, α, and α + β) were established by Thermo-Calc calculations, dilatometry, differential scanning calorimetry and microstructure analysis of specimens annealed at different temperatures. The as-cast alloy was subjected to uniaxial compression at temperatures from 900 to 1250 °C and strain rates in the range 0.001–1 s−1. Two temperature intervals with different mechanical behavior were found: the first corresponded to the α2+γ phase field (900–1100 °C), where the microstructure was mainly lamellar, and the second covered the α + γ, α, and α + β phase fields (1100–1250 °C), in which the α phase dominated. In the first interval, mechanical behavior was typical for lamellar structure and was associated with transformation of lamellar structure into globular one via dynamic recrystallization/spheroidization processes. In the second interval, the mechanical behavior was similar despite the changes in phase composition and the controlling process of the microstructure evolution was dynamic recrystallization. In the α + γ phase field deformation accelerated the γ→α transformation. Deformation in all phase fields resulted in microstructure refinement which, in turn, induced superplasticity under certain temperature-strain rate conditions. To determine the most suitable conditions for working of the alloy, processing maps were constructed. Two domains with high power dissipation efficiency (η > 0.5) were found. The first domain was located at the top of the α2+γ and the bottom of the α+γ phase fields with the peak efficiency of 0.57 at ∼1100 °C and 0.05 s−1. The second domain belonged to the α + β phase field with the maximum efficiency of 0.74 at ∼1250 °C and 0.5 s−1. The relationships between mechanical behavior and microstructure after deformation were discussed.