对Ti-43Al-4Nb-1.5Mo合金进行包套锻造和后续热处理实验,考察了该过程TiAl合金的热变形行为、流变软化机制以及热处理参数对微观组织和力学性能的影响。结果表明,TiAl合金包套锻造过程的高温流变软化以β相协调变形、片层相变分解、γ相内位错滑移以及孪晶诱导的动态再结晶为主,最终组织为残余α₂/γ层片和等轴α₂、γ、B2相的混合组织。随热处理温度的升高,热变形组织由残余α₂/γ层片和多相混合组织转变为α₂/γ层片+γ相组织,在较高的温度下(1300 ℃)转变为全层片组织。其中,B2相随着溶质扩散程度的增加逐渐消失,残余层片组织发生分解转变为等轴α₂/γ层片团,同时发生γ→α转变,形成全层片组织。对热等静压、锻态和热处理试样的高温(800 ℃)拉伸性能进行比较,经热处理后获得的全片层组织具有最佳的综合性能,抗拉强度为663 MPa,延伸率达到26%。分析该样品的断裂行为可知,由于存在层片扭曲拉长、微孔钝化以及裂纹曲折延伸的断裂机制,全层片组织具有良好强度-塑性的综合力学性能。另外,热加工过程中(高温) bcc结构B2相能够协调变形,但服役条件下硬脆的B2相作为裂纹源容易引起裂纹萌生,对力学性能极其不利。因此,TiAl合金在热变形和服役过程中需要对组成相进行严格控制,从而获得良好的力学性能。

TiAl alloys are highly promising for high temperature structural application due to their excellent mechanical properties. However, the widespread applications of TiAl alloys have been limited for their low temperature brittleness and poor workability. The further thermo-mechanical treatments is applied for fine microstructures and improved ductility to promote the commercial applications, during which the investigations of hot deformation behavior and microstructural evolution are necessary for the improved microstructure and mechanical properties. The canned forging and subsequent heat treatments of Ti-43Al-4Nb-1.5Mo alloy have been conducted, during which the hot deformation behavior, flow softening mechanism, microstructure evolution and mechanical properties were investigated. The results show that the flow softening process of the canned forging TiAl alloy can be attributed to the soft β phase, α₂/γ lamellae decomposition and the dynamic recrystallization induced by dislocation slipping and twinning in γ phase, and the final microstructure is composed of remnant α₂/γ lamellae and equiaxed α₂, γ and B2 phases. With the increasing heat treatment temperature, the microstructure changes from the multi-phase structure (remnant α₂/γ lamellar, equiaxed α₂, γ and B2 phases) at 1250 ℃ to the α₂/γ lamellar and γ phase at 1285 ℃, and then the fully α₂/γ lamellar structure at 1300 ℃, during which the B2 phase is gradually dissolved due to the solution diffusion, and the remnant α₂/γ lamellae change to equiaxed α₂/γ colonies according to the α₂/γ→γ+α₂+B2 transition, and the final fully α₂/γ lamellar structure is promoted by γ→α transition at high temperature. Moreover, the tensile tests of the hot isostatic pressed (HIPed) samples, canned forged and heat treated samples at 800 ℃ are conducted, in which the fully lamellar structure shows the high properties with the ultimate strength of 663 MPa and the elongation of 26%. The deformation process of the fully α₂/γ lamellar can be strengthened by the lamellae twisting, microvoid inhibition and wavy growth of the cracks, leading to the optimal high temperature performance. Moreover, the disordered bcc β phase can promote the deformation during the hot working process at the high temperature (≥1200 ℃), while the hard-brittle B2 phase severely deteriorates the service properties, which should be controlled accurately for the high mechanical properties during the thermo-mechanical processing.