| Ti Al alloys are one of promising candidates to replace Ni-based superalloys in aero-engines and space-engines due to their low density, high strength, excellent creep resistance and oxidation resistance. However, the development of Ti Al alloys was hindered by poor hot workability and intrinsic brittleness. Thermomechanical treatment is an effective method to refine microstructure and improve mechanical properties of Ti Al alloys. Thus, beta-gamma Ti Al alloys with excellent hot workability have become hot research topics. In this paper, the composition design, the hardness of constituent phases, the hot deformation behavior and the constrained forging of beta-gamma Ti Al alloys were investigated systematically.The effects of various β stabilizers on room temperature β0 phase and high temperature β phase were studied. A room temperature Mo equivalent([Mo]eq-RT) was proposed. The relation among alloy compositions, [Mo]eq-RT and the content of β0 phase was constructed. [Mo]eq-RT can be used to estimate the content of β0 phase in Ti Al alloys. The β0 phase would precipitate when the value of [Mo]eq-RT reaches 1. The value of [Mo]eq-RT of as-cast Ti Al alloys should be below 1 to avoid the appearance of β0 phase. A high temperature Mo equivalent([Mo]eq-HT) was established. The relation among alloy compositions, [Mo]eq-HT, the content of β phase and hot workability was built. The hot workability of the Ti Al alloy can be evaluated by the [Mo]eq-HT. When the value of [Mo]eq-HT is higher than 1.5, the peak stress of the alloy should be below 100 MPa during hot compression, which ensures that the Ti Al alloy exhibits excellent hot workability at 1200oC/0.01s-1. Two Mo equivalents provide scientific basis for the composition design of beta-gamma Ti Al alloysThe influences of different β stabilizers on the hardness of constituent phases in Ti Al alloys were studied. Nano-indentation tests show that the hardness of constituent phases in the Ti Al alloy can be ordered as β0>γ/α2>γ. Alloying elements have remarkable effect on the hardness of β0 phase, whereas, they have small effect on the hardness of γ phase and γ/α2 lamellae. The β0 phase induced by the addition of Cr, Mn and V exhibits low hardness, while the β0 phase induced by the addition of Nb, Mo and W has high hardness. The hardness of β0 phase can be decreased by alloying, which can enhance the deformability of β0 phase. The coordinated deformation among constituent phases can also be improved, which is beneficial to mechanical properties of Ti Al alloys.A Ti-43Al-2Cr-2Mn-0.2Y alloy was developed based on research results about the composition design of beta-gamma Ti Al alloys. The hot deformation behavior of Ti-43Al-2Cr-2Mn-0.2Y alloy was studied by isothermal compression tests in the temperature range of 1100°C-1225°C and strain rate range of 0.01s-1-0.5s-1. The results show that the present alloy exhibits excellent hot workability. The apparent activation energy for hot deformation was calculated as 377KJ/mol according to the Arrhenius equation. The microstructural evolution of the alloy depended closely on the temperature and the strain rate. The bending and coarsening of lamellae occurred when the alloy deformed at low temperature and high strain rate. Dynamic recrystallization took place with increasing temperature and decreasing strain rate. Coarse lamellae gradually decomposed into fine γ grains. The effect of can on the hot deformation of Ti Al alloys was also studied. The results show that the deformation of Ti Al alloys can be restrained effectively by the can. The processing map of Ti-43Al-2Cr-2Mn-0.2Y alloy was constructed. The optimum deformation condition of the present alloy was obtained according to the processing map, microstructure observation and the actual processing conditions. The temperature and the strain rate were determined to be 1200°C and 0.05s-1, respectively.The microstructural evolution of lamellae in Ti-43Al-2Cr-2Mn-0.2Y alloy was investigated during hot deformation. Coarse lamellar microstructure with hard orientation can be broken completely when the deformation amount reaches 60% at 1200°C/0.01s-1. Cracks tended to appear near β0 phase when the alloy deformed below the ordering temperature of β to β0. The interfaces of γ laths bulged when the alloy deformed at high temperature. Moreover, the decomposition of α2 and γ lathes also occurred. The dislocation density in lamellaes increased sharply with increasing deformation amount. Dynamic recrystallization tended to occur in the area with high density dislocations. Dynamic recrystallization is the main mechanism for the decomposition of lamellae. Moreover, a large amount of sub-grain boundaries would formed due to the movement of dislocations, which also promoted the decomposition of lamellaes.A Ti-43Al-2Cr-2Mn-0.2Y pancake was produced through near-isothermal one-step canned forging. The total deformation amount of the edge and the core are 80% and 90%, respectively. Relatively fully dynamic recrystallization occurred for the as-cast alloy during hot forging. The as-forged microstructure consisted of equiaxed γ phase, some β0 phase and few α2 phase. The pancake has good microstructure uniformity. Both the core and margin of the pancake exhibits fine and equiaxed microstructure. The tensile properties of the alloy were improved significantly by forging. The ultimate tensile strength(UTS) and the elongation increased to 657 MPa and 0.86%. The tensile properties of as-forged alloy also depend on temperature. When the deformation temperature is 650°C, the UTS and the elongation are 549 MPa and 3.6%, respectively. The UTS decreased to 496 MPa, and the elongation increased to 10% when the temperature increased to 700°C. The UTS value remained above 400 MPa but the elongation sharply increased to 42% when the test temperature was further increased to 750°C. Moreover, the heat treatment of Ti-43Al-2Cr-2Mn-0.2Y alloy and its effect on tensile properties of the alloy were also studied. Fully lamellar microstructure with dimensions of less than 200μm can be obtained after heat treatment at 1320°C/10min/FC. Duplex microstructure with equiaxed γ grains and lamellaes can be produced after heat treatment at 1250°C/4h/FC. The elongation of the alloy with duplex microstructure reached 1%. |
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