TiAl合金因其低密度、优异的高温强度、抗蠕变和抗氧化性能而在航空航天和汽车工业中具有广阔的应用前景，但其本质脆性和变形能力差的缺点严重限制其进一步发展。本工作通过交叉包套轧制(CHPR)和一步退火处理实现了800℃下超高塑性Ti-44Al-5Nb-1Mo-2V-0.2B合金板材的制备。利用SEM、EBSD、TEM和拉伸等实验方法考察了TiAl合金的热变形行为、不同轧制和热处理工艺对微观组织和力学性能的影响。结果表明，与单向包套轧制(UHPR)相比，CHPR板材沿厚度方向和板面方向均表现出更高的组织均匀性，微观组织由残余α₂/γ片层及其晶界的等轴γ、α₂和B2相组成，但残余片层的尺寸较小且含量明显降低，其原因是在CHPR的双向剪切力和压应力的作用下大量残余片层破碎并发生了完全再结晶。CHPR TiAl合金的高温流变软化机制主要包括片层弯曲扭折变形、β/B2相协调变形、α₂/γ片层的相变分解、初生和二次孪晶诱导的γ相动态再结晶。随后对CHPR合金进行1200~1340℃的退火热处理，1200℃条件下获得了等轴片层和等轴晶粒(α₂ + γ + B2)的多相组织，部分晶粒达到了纳米级。1340℃条件下得到了近全片层组织。对UHPR和CHPR板材的横/纵向和1200℃退火热处理试样的拉伸性能进行比较，CHPR板材横纵向的拉伸性能更加均匀，再经1200℃退火热处理后获得的多相等轴组织具有最佳的强度-塑性匹配，室温下抗拉强度为624 MPa，延伸率为1.32%；800℃下抗拉强度为515 MPa，延伸率可达107.0%。分析试样的断裂行为可知，室温下3种试样的断裂方式为穿片层和解理断裂。800℃下断裂方式转变为韧性断裂，断裂失效机制以微孔联结机制为主，其中经过1200℃退火热处理的板材断口具有更小而深的韧窝，塑性最佳。精细的片层结构和均匀细小的等轴晶组织能够阻碍裂纹的扩展，有利于综合力学性能的提高。

TiAl alloys are highly promising for high-temperature structural applications in the aerospace and automotive industries because of their low density, excellent high-temperature strength, and resistance to creep and oxidation. Nevertheless, low-temperature brittleness and poor deformability are the main factors severely restricting the widespread application of TiAl alloys. The process of β-solidifying γ-TiAl alloys results in alloys that consist primarily of α₂, γ, and B2 phases, and have superior hot workability. Further thermomechanical treatments are applied to achieve a fine microstructure and enhance the inherent ductility of γ-TiAl alloys. In this work, Ti-44Al-5Nb-1Mo-2V-0.2B alloy sheet with ultrahigh plasticity at 800^oC was achieved by cross hot-pack rolling (CHPR) and one-step annealing processes. SEM, EBSD, TEM, and tensile methods were used to investigate the hot deformation behavior, and the effects of different rolling processes and heat treatments on the microstructural evolution and mechanical properties of the alloy. The results show that the CHPR sheet had a more highly uniform deformation microstructure along the thickness direction and sheet plane compared with that of a unidirectional hot-pack rolled (UHPR) sheet, which consisted of residual lamellar colonies and equiaxed γ, α₂, and B2 grains at colony boundaries. The size of the residual lamellar colonies was significantly smaller and the content was lower in the CHPR sheet compared with the UHPR sheet. This was due to a large number of broken residual lamellae and complete recrystallization under the combined action of a bidirectional shear force and compressive stress during the CHPR process. The high-temperature flow-softening mechanisms of TiAl alloy in the CHPR process mainly included bending and kinked lamellae, β/B2 coordinated deformation, phase-transformation decomposition of α₂/γ lamellar, and dynamic recrystallization induced by primary and secondary twinning. To achieve further grain refinement, subsequent annealing of the CHPR-processed TiAl alloy was performed at 1200-1340^oC. A multiphase equiaxed microstructure with fine lamellar colonies was obtained at 1200^oC and a nearly complete lamellar microstructure was obtained at 1340^oC. Moreover, the room-temperature and high-temperature tensile properties of UHPR and CHPR sheets in the horizontal and vertical directions were compared with samples annealed at 1200^oC. The tensile properties of the CHPR sheets were more uniform in both directions. The multiphase equiaxed microstructure obtained in the CHPR alloy annealed at 1200^oC had the best strength-plasticity balance with a tensile strength of 624 MPa (515 MPa) and elongation of 1.32% (107.0%) at room temperature (800^oC). According to the fracture behavior, the fracture mode of these alloy sheets was translamellar or cleavage fracture at room temperature. Conversely, the fracture mode changed to ductile fracture at 800^oC, and the failure mechanism was mainly via microhole coupling. The fractures in the annealed sheets (1200^oC) had small and deep dimples, indicating optimal tensile elongation. The uniform and fine lamellar structure and equiaxed microstructure can hinder crack propagation and achieve enhanced mechanical properties.