- 中国科学引文数据库(CSCD)核心期刊
- 中国核心期刊
- 文摘与引文数据库(Scopus)
- 剑桥科学文摘(CSA)
- 英国INSPEC数据库
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.
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.
The creep properties of a corrosion resistant single crystal superalloy at 760 ℃/800 MPa, 980 ℃/250 MPa and 1120 ℃/130 MPa are investigated. The creep fracture microstructure, fracture characteristic and dislocation morphology under different conditions are analyzed using scanning electron microscopy（SEM）and transmission electron microscopy（TEM）. The results indicate that the alloy exhibits good creep performance at 760 ℃/800 MPa, 980 ℃/250 MPa and 1120 ℃/130 MPa, with its creep curves showing similar three-stage creep characteristics. As temperature increases and stress decreases, the creep lives of the initial and acceleration stages becomes shorter, while the creep life of the steady-state stage increases. Compared to 980 ℃/250 MPa and 1120 ℃/130 MPa, the creep rate during the initial stage is faster at 760 ℃/800 MPa. Under 760 ℃/800 MPa conditions, the γ′ phase retains its cuboidal morphology. The dislocation tangles forming in the matrix channels and stacking faults forming from some dislocation cutting γ′ phase have a reinforcing effect. The creep fracture morphology of the alloy involves cleavage-like and ductile dimple mixed fracture. At 980 ℃/250 MPa and 1120 ℃/130 MPa, the alloy exhibits obvious rafting behavior and topological inversion of γ′ phase and γ phase has finished. A high-density dislocation network forming at the γ/γ′ interfaces have a reinforcing effect. No stacking faults are observed to form. Dislocations cut into the γ′ phase during the later stage of creep.The creep fracture morphology is dominated by ductile dimple fracture. At 1120 ℃/130 MPa, a small amount of lamellar σ phases precipitate along specific directions in the alloy, indicating good microstructural stability.
To meet the lightweigh requirements, the structure of castings is evolving towards thin-walled designs. Therefore, it is necessary to study the evolution characteristics of the microstructures and mechanical properties of thin-walled structures made of nickel-based superalloys. Firstly, a thin-walled casting with wall thicknesses of 1, 1.25 mm and 1.5 mm is designed. Gravity casting experiments are conducted under two different process conditions, and the microstructural analysis and mechanical property tests are carried out for two types of castings, respectively. The values of microstructural characteristics are determined, including secondary dendrite arm spacing（SDAS）, grain morphology and average grain size, as well as the size and volume percentage of the γ′ phase at different wall thicknesses of the castings under different cooling conditions. The corresponding hardness and tensile strength also are measured by experiments. The results show that SDAS increases by more than 29.9% as the wall thickness of the casting increases from 1 mm to 1.25 mm and 1.5 mm. The tensile strength of the casting fluctuates with the increase in wall thickness when the flask temperature is 900 ℃. However, the tensile strength of the casting increases as the wall thickness increases when the flask temperature is 25 ℃. The variation range of the castings cooling rates are determined through numerical simulation. The cooling rate range of castings with a sand mold temperature of 900 ℃ ranges from 16.0 ℃/s to 28.2 ℃/s while those produced with a sand mold temperature of 25 ℃ exhibit a cooling rate range of 26.2 ℃/s to 58.5 ℃/s.
The directionally solidified superalloy DZ125 is widely used as turbine blades in aero-engines. This work investigates the influence of phosphorus（P）on the microstructure, mechanical properties and crack susceptibility of DZ125 alloy. The results indicate that P primarily segregates at grain boundaries in DZ125 alloy and has little effect on γ′ phases, γ+γ′ eutectic and carbides in the alloy. When P content reaches 0.008% （mass fraction, the same below） P-rich phases form in the interdendritic regions during casting, which subsequently dissolve back into the matrix during heat treatment.When the P content is no more than 0.0039%, P shows no obvious effects on the room-temperature tensile properties or the stress rupture life at 980℃/235 MPa. However, it has a significant impact on the stress rupture life at 760℃/805 MPa: the alloy with 0.0039% P exhibits a 37% decrease in stress rupture life at 760℃/805 MPa compared to the alloy with 0.0013% P, due to the segregation of P at grain boundaries weakens the grain boundaries. When the P content reaches 0.011%, the intergranular cracks appear in the DZ125 alloy hollow turbine blades during directionally solidified process. The main reason for the increase of crack susceptibility is excessive enrichment of P at grain boundaries and precipitation of P-rich phases nearby grain boundaries, which leads to grain boundary weakening and crack initiation.
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