Grain refinement can effectively enhance mechanical properties of materials at low and medium temperatures, however, it may weaken the stress rupture property above the equicohesive temperature. To study the effect of grain refinement on the stress rupture property of K447A alloy, the microstructure evolutions of alloys with three grain sizes and their corresponding stress rupture mechanisms under the conditions of 760 ℃/724 MPa, 815 ℃/600 MPa, 870 ℃/365 MPa and 980 ℃/210 MPa are investigated using scanning electron microscopy（SEM） and energy dispersive spectroscopy（EDS）. The results show that the equicohesive temperature of K447A alloy lies between 815 ℃ and 870 ℃. Grain refinement shows a temperature-dependent effect on the stress rupture life of K447A alloy. At 760 ℃/724 MPa, as the grain size decreases from 5.0 mm to 1.3 mm and then to 58 μm, the stress rupture life of K447A alloy increases from 83 h to 115 h and further to 194 h, respectively. At 815 ℃/600 MPa, the stress rupture life increases from 31 h to 84 h, as the grain size decreases, and then slightly drops to 76 h. At 870 ℃/365 MPa and 980 ℃/210 MPa, the stress rupture life shows a monotonic decreases with grain refinement. Therefore, grain refinement serves as an effective technology to improve the stress rupture property of K447A alloy below 870 ℃.The stress rupture process is dominated by intragranular deformation below 815 ℃, and grain refinement mainly extends the stress rupture life by shortening the slip band length and increasing the volume fraction of γ′ phase. Above 870 ℃, grain boundary sliding dominates the stress rupture process. The deterioration of the stress rupture property due to grain refinement can be attributed to the severe grain boundary slip at high temperatures, grain boundary oxidation and the formation of brittle AlN and a low-strength precipitation free zone（PFZ）.

In order to investigate the effect of low-angle grain boundary（LAGB） on the high-temperature creep behavior of a second-generation nickel-based single-crystal（SX） superalloy, the high-temperature creep fracture and interrupted experiments are carried out at 1100 ℃/137 MPa using plate-shaped samples with different grain boundary misorientations. The results show that after standard heat treatment, fine MC carbides are formed at the LAGB with the misorientation of 7° in alloy GB-7, while blocky M₆C carbides are formed at the LAGB with the misorientation of 12° in alloy GB-12. The high temperature creep life of the investigated alloys decreases with increasing the misorientation degree. The creep life of alloy GB-12 is only 40% of that of the single crystal alloy. Further investigation reveals that LAGB migration occurrs in both the GB-7 and GB-12 alloys during high-temperature creep, but the migration distance of the GB-12 alloy is lower than that of the GB-7 alloy. Blocky M₆C carbides in alloy GB-12 hinder the grain boundary migration, leading to strain concentrations at the LAGB region. Cracks tend to initiate at the low-angle grain boundary either inside GB-12 alloy or on its surface, leading to a significant reduction in its creep life. This study can provide guidance and data support for improving the tolerance of LAGBs in high-temperature creep.

Titanium alloy investment castings are widely used in the aerospace industry. During the manufacturing process, titanium is prone to reacting with the ceramic shell, which leads to defects such as shell cracking and casting deformation. Therefore, it is important to investigate the temperature distribution and deformation behavior during the sintering process of ceramic shell to improve the performance of the ceramic shell and enhancing the quality of casting. An advanced Monte Carlo method is employed to establish the radiative heat transfer model. Additionally, considering the impact of thermal damage, a coupled thermo-mechanical-damage constitutive model is established. A specialized simulation software is created through secondary development based on ABAQUS to investigate the sintering process of ceramic shell. Thermo-physical parameters of the ceramic shell are measured to provide data support for the simulations. The proposed models are experimentally validated using a flat-plate specimen, and experimental results agree well with the simulated outcomes. Using the developed software, a comprehensive study is conducted on the temperature distribution and deformation behavior of the ceramic shell in an annular-stepped specimen under various process schemes. The results indicate that a non-uniform temperature distribution during the sintering process is more likely to induce significant deformation and even cracking in the shells, particularly at structural protrusions. Moreover, as the sintering temperature rises, the decreased viscosity of the glassy phase in the ceramic shell will also intensify thermal stress accumulation and localized deformation. The simulation study on the temperature distribution and deformation behavior of the ceramic shell during the sintering process provides theoretical insights and technical support for optimizing the sintering process of the ceramic shell and improving the qualification rate of titanium alloy investment castings.

TiAl alloys have attracted much attention due to its excellent specific strength, specific stiffness, and high-temperature performance, which has great potential for application in the aerospace industry. With the development of aerospace technology, the performance requirements for its equipment and service materials have further increased. Thermomechanical treatment plays a very important role in the field of manufacturing technology of aerospace equipment. The mature preparation processes for TiAl alloys are mainly ingot metallurgy and powder metallurgy. TiAl alloys are obtained by both processes require subsequent thermomechanical treatment. Combining the processes of deformation with heat treatment, the microstructure of TiAl alloys can be effectively controlled, thereby improving the room-temperature brittleness and fracture toughness of alloys. On the basis of fully understanding the thermoplastic deformation behavior of TiAl alloys, further research on different hot working methods and processes, process parameter design and control of TiAl alloys are of great significance for reducing the processing cost of TiAl alloy products as well as promoting their production and application. This article mainly reviews the development status of thermomechanical treatment of TiAl alloys.The research progress in the thermoplastic deformation behavior as well as microstructure control of hotworking （hot forging, hot rolling, hot extrusion） and subsequent heat treatment of TiAl alloys is summarized. On the basis, this article proposes the development directions in thermomechanical treatment of TiAl alloys. The first is the research on thermomechanical treatment process of TiAl composite materials. On the basis of high-throughput material design, exploring the hot working and post-treatment process routes suitable for TiAl composites, is expected to develop a new type of TiAl material with excellent high-temperature comprehensive performance. The second is the optimization design of hot working process for large-sized TiAl alloy components. Combining machine learning methods to optimize the hot working parameters of large-sized TiAl alloy components, as well as predict the microstructure evolution during hot working, and developing new mold materials to effectively control the processing temperature, are expected to significantly improve the controllability and stability in the forming process of large-sized TiAl components. The third is the development of low-cost thermomechanical treatment technology of TiAl alloys, such as no package hot working technology and single-step heat treatment process. The fourth is the thermomechanical treatment control of new microstructures for TiAl alloys. On the basis of introducing nanostructures to refine the microstructure of TiAl alloys, a new type of TiAl alloy microstructure design is expected to carry out by thermomechanical treatment to further enhance the performance of TiAl alloys. The fifth is the efficient screening of thermomechanical treatment process parameters for TiAl alloys. Integrating multidisciplinary knowledge, constructing a large database of components, hot working/heat treatment parameters, microstructure, and properties, can reduce the costs and cycles of researches.