Abstract
To elucidate the hot deformation characteristics of TiAl alloys, flow stress prediction, microstructural evolution and deformation mechanisms were investigated in Ti-44Al-5Nb-1Mo-2V-0.2B alloy by isothermal compression tests. A constitutive relationship using the Arrhenius model involving strain compensation and back propagation artificial neural network (BP-ANN) model were developed. A comparison of two models suggested that the BP-ANN model had excellent capabilities and was more accurate in predicting flow stress. Based on the microstructural analysis, bending and elongation of colonies, γ and B2 grains were the main microstructural constituents at low temperature and high strain rate. Dynamic recrystallization (DRX) of γ and dynamic recovery (DRY) of β/B2 were the main deformation mechanisms. With the increase of temperature and decrease of strain rate, phase transformation played an important role. The flake-like γ precipitates in B2 grains, and a coarsening of γ lamellae via α lath dissolution during compression were observed. Additionally, the flow softening process commenced with dislocation pile-up and formation of sub-grain boundaries, followed by grain refinement, twins and nano-lamellar nucleation. Continuous DRX and phase transformation promoted the formability of Ti-44Al-5Nb-1Mo-2V-0.2B alloy.
Highlights
TiAl alloys are highly promising for high-temperature structural applications in aerospace with the potential to replace nickel-based superalloys because of their excellent properties [1,2], including being of low density, having high-temperature strength, high specific modulus and creep resistance, etc.Recently, TiAl alloys were studied from the perspective of using them for turbines of aircraft engines and gas-burning power-generation plants, which are believed to be on the verge of reaching the goal for industrial scale
The Arrhenius constitutive equation (ACE) model is widely considered to develop a relationship between flow stress, strain rate and deformation temperature, at a certain strain during hot deformation
The following are the conclusions: (1) The true-stress–true-strain curves of Ti-44Al-5Nb-1Mo-2V-0.2B alloy deformed in the temperature range of 1050–1250 ◦ C and in the strain rate range of 1–10−3 s−1 showed characteristics of work hardening, discontinuous yielding phenomenon and flow softening
Summary
TiAl alloys are highly promising for high-temperature structural applications in aerospace with the potential to replace nickel-based superalloys because of their excellent properties [1,2], including being of low density, having high-temperature strength, high specific modulus and creep resistance, etc.Recently, TiAl alloys were studied from the perspective of using them for turbines of aircraft engines and gas-burning power-generation plants, which are believed to be on the verge of reaching the goal for industrial scale. TiAl alloys are highly promising for high-temperature structural applications in aerospace with the potential to replace nickel-based superalloys because of their excellent properties [1,2], including being of low density, having high-temperature strength, high specific modulus and creep resistance, etc. The widespread application of TiAl alloys is limited because of their low-temperature brittleness, poor workability and narrow processing windows [3,4,5]. It is necessary to tune the thermo-mechanical processing of TiAl alloys in order to obtain better microstructure and performance [6]. In traditional TiAl alloys, only isothermal conditions with high manufacturing costs could be applied to prevent failure. TNM alloys (abbreviation for Nb and Mo containing TiAl alloys) have been developed, which possess better hot workability and are suitable for conventional thermo-mechanical processing
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