Development Of The B2-containing TiAl-based Intermetallics And Its Low-temperature Superplasticity

B2-containing TiAl alloy (the y-based TiAl alloy, which contains a certain amount of B2phase) has arisen worldwide concerns due to its excellent properties, including good workability and high-temperature performances. A novel B2-containing Ti-Al-Fe-Mo alloy has been developed in this research. And the relationship among composition, microstructure and properties of the Ti-Al-Fe-Mo system has been studied systematically. Models of the solidification process (SP) and discontinuous dynamic recrystallization (DDRX)-induced low-temperature superplasticity of this alloy have also been set up. Eventually thin-gage foils of the B2-containing TiAl alloy were successfully fabricated via hot rolling.A large number of experimental and theoretical analyses were performed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXA), electron probe micro-analysis (EPMA), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The following aspects of B2-containing TiAl alloy were studied:(a) the formation of B2phase——to understand the relationship between composition and microstructure by investigate the phase equilibrium and diffusion of Ti-Al-Fe-Mo system;(b) the controllability of the status of B2phase——to adjust and modify the content and distribution of B2phase by optimal designing the composition of B2-containing TiAl alloy and following heat treatment;(c) the effect of B2phase——to analyze the mechanism of thermal deformation and superplasticity of B2-containing TiAl alloy. Results are shown as below:1) The high-throughput diffusion multiples and EPMA has been used to analyze the Ti-Al-Fe-Mo diffusion system at900℃. The partial composition phase diagram and the database of Ti-Al-Fe-Mo phase equilibrium and diffusion have been established, which can be used for the design of new B2-containing TiAl alloy. There are different diffusion coefficients of the elements (including Ti, Al, Fe, and Mo) in different phases (B2, a2, and y phase). The distribution of the elements in various regions nearby y phase field has been discussed.2) The relationship between the content of β stabilizers (Mo and Fe) and the microstructure of the Ti-45A1based alloy has been understood. With the increasing amount of Mo or/and Fe content, the B2phase grows gradually. In the Ti-45Al-3Fe-2Mo alloy, the grains are significantly refined to about12μm, and this alloy shows a very good hot workability at elevated temperatures. Its solidification process (SP) can be described as:L→L+βprmary→βprimary+α→α→α+(α’+γ)→lamellar (α’+γ)+equiaxed (β’+γ)→lamellar (α2+γ)+equiaxed (B2+y).3) The hot deformation mechanism and the processing map (PM) of the B2-containing TiAl alloy were established. Ti-45Al-3Fe-2Mo alloy is sensitive to temperature and strain rate since its flow stress decreases with temperature and increases with strain rate. The relationship among the strain rates, flow stress, and temperature could be expressed by the function: ε=e23.118[sinh(0.0138σ)]3.54exp(-292.43/RT). The PM shows that the strain rate should be lower than0.056s-1when compressed at800℃, and not higher than0.18s-1at the temperature between800and1100℃. A higher temperature should be selected according to the PM when compressed at higher strain rates.4) The mechanism of DDRX-induced low-temperature superplastic deformation of the B2-containing TiAl alloy was discussed. The low-temperature superplastic deformation takes place during tensile deformation of Ti-45Al-3Fe-2Mo alloy at790℃. For the existence of B2phase, the reciprocating cooperation between strain hardening and softening by DDRX, kinking and elongating of lamellar, and grain boundary sliding (GBS) has been promoted. The reciprocating cooperation is conductive to reduce the stress concentration and release the accumulation energy, which is helpful for postponing the occurrence, aggregation and concatenation of cavitations. This cooperation results in the B2-containing TiAl alloy exhibiting large plastic strain, not breaking until the material is stressed beyond its strength limit.