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.

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）.

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.

Based on the hot-ended casing of K4169 superalloy as the research object, aiming at the defects such as liquid splashing, oscillation and entrained air in the linear filling process of traditional superalloy counter-gravity casting, particularly, considering the complex variable cross-section structure of the casing castings, the work explores the influence of pressurization speed on filling of such structure through hydraulic simulation experiments. The results reveal that for the variable cross-section structure, a lower pressurization speed leads to more stable liquid filling. Orthogonal experiments are conducted to ascertain the optimal filling process parameter of the casing model such as a pouring temperature of 1460 ℃, a shell temperature of 900 ℃ and an average pressurization speed of 4 kPa/s. Based on the casing model’s structure, linear and nonlinear filling pressure curves are designed, and both numerical simulations and experimental studies are performed to compare two filling methods. When comparing two filling processes with the same filling time, the nonlinear filling exhibits a 16.77% decrease in average gate speed compared to the linear filling, resulting in a more stable filling process and production of fewer overall defects in the thin wall areas of the casing. Casings filled using the linear method exhibit numerous crack defects across various regions, leading to a relatively high overall defect rate. In contrast, casings filled using the nonlinear method devoid of crack defects and contain only a few micro-pores. Non-destructive testing results also support the notion that casing castings filled by the nonlinear method have fewer defects, which validates that the nonlinear filling effectively reduces the type and quantity of defects in casing castings.