The influence of microstructure evolution on the tensile properties of the novel nickel-based powder metallurgy superalloys after long-term aging at 800 ℃ for 100~5000 h was studied. The results show that the grain size of the alloys does not change significantly with the increase of aging time, and the grain boundaries are coarsened and appear the discontinuous “sawtooth”. Secondary γ′ phase particles obviously coarsen which conforms to the LSW theory. The content of TCP phases precipitated increases with the increase of aging time. After aging for 1000 h, the intermittent long white precipitates are observed at the grain boundary. After aging for 2000 h, a large number of elongate needle-like precipitates appear in the grain. The content of TCP phases reaches the maximum after aging for 5000 h. Due to the coarsening of the secondary γ′ phase, the tensile strength and plasticity of the alloys at 700 ℃ gradually decrease with increasing the aging time. The tensile fracture has the characteristics of necking and dimple, and the fracture shows the dimple fracture.

The microstructure evolution of FGH96 alloys during the heat treatment was studied. The effects of solution temperature and holding time on the grain size, γ′ precipitate size, area fraction, and grain distribution were quantified. The influences of γ′ precipitate and strain storage energy on the grain evolution were analyzed. The results show that, the deformed grains containing a large amount of strain storage energy during heat treatment undergo the static recrystallization and refine the grains, while the dynamic recrystallization grains grow. The uniform fine grain structure can be obtained as the two processes are balanced. After the alloy is held at1060 ℃ for 120 min, the distribution of γ′ precipitate size and grain size are more uniform, and the average grain size is 7.37 μm. The area fraction of the γ′ precipitate decreases with the increase of solution temperature and holding time, while the coarsening and splitting of the primary γ′ precipitate during the subsolvus solution make the area fraction and size of the primary γ′ precipitate increase first and then decrease. The inhomogeneous dissolution of γ′ precipitate leads to the rapid growth of the local grains, forming the mixed crystals.

Powder metallurgy superalloy is an advanced high-temperature material, which has been widely used in the aerospace fields. Hot isostatic pressing (HIP) is one of the preparation methods for powder superalloy components. However, the presence of prior particle boundaries (PPBs) adversely affects the performance of the components significantly. The research status of PPBs for powder metallurgy superalloys prepared by HIP was reviewed in this paper, the formation mechanism and influence of PPBs in components were summarized, the removal PPBs methods in powder superalloys were proposed, and the feasibility and effectiveness of these methods were analyzed and prospected, including adding Hf, Nb, and other strong carbide forming elements to the powder, preheating the powder, using the vacuum dynamic degassing treatment or plasma droplet re-fining (PDR) treatment, optimizing the powder preparation process, selecting powders with higher purity and more uniform size distribution, selecting the appropriate HIP process parameters and methods, and applying hot extrusion, annealing, solution treatment and HIP post-treatment on the components.

Spherical GH3536 alloy powders were prepared by vacuum induction-melting gas atomization method at the different atomization pressures (7, 8, 9 MPa). The region below the nozzle was numerically simulated by multiphase flow model and discrete phase model, and the primary and secondary atomization processes at the different atomization pressures were reproduced. In the results, the flow velocity and stagnation pressure in the recirculation zone increase with the increase of atomization pressure. With the increase of atomization pressure, the powder particle size decreases continuously. The simulation results are similar with the experimental results, verifying the reliability of the atomization model. The increase of atomization pressure can increase the yield of fine powders, but the decrease of particle size and the change of particle morphology may directly affect the powder flowability. The powders prepared at the atomization pressure of 8 MPa show the best flowability and the optimum apparent density, which are 14.34 (s·50g^-1) and 4.728 g·cm^-3, respectively.

The fatigue crack initiation and short crack propagation behavior of a 3rd generation powder superalloy with the different microstructures were studied by in-situ scanning electron microscope (SEM) observation. The effects of microstructures at the wheel rim (coarse grain structure), wheel centre (fine grain structure), and grain transition zone (gradient grain tructure) of the dual performance powder turbine disk on the fatigue short crack propagation were investigated. The results show that, the crack prefers to nucleate from the first hardening phase at the notch. The gradient microstructure exhibits the multiple cracking at the grain boundaries. For the specimens with the gradient microstructure, the short crack growth behavior at room temperature is significantly affected by the microstructures, and the fatigue crack growth rate shows the large fluctuation. At the lower stress intensity factor range, the crack propagation rate of the coarse grains is higher than that of the fine grains. With the increase of stress intensity factor range, the crack propagation rate of the fine grains is increased faster and finally higher than that of the coarse grains, which mainly attributes to the competition mechanism of the long slip path, the enhanced slip reversibility, the few grain boundary barriers, and the reduced discontinuity.

The grain growth behavior of the fourth generation powder metallurgy (PM) superalloys was studied. The results indicate that the grain growth range is small when the heat treatment temperature is below the γ′ solution temperature, which is similar to the initial microstructure (as-forged, 3~4 μm). However, the grain size greatly increases to 30~40 μm when the heat treatment temperature exceeds the γ′ solution temperature, and there is little difference in grain size at the several temperatures over the γ′ solution temperature. The grain size increases significantly at the initial stage of heat treatment, and no longer changes after a certain holding time. The influence of temperature and time on grain size is related to the pinning effect of γ′ on grain boundary migration. A new model is established by modifying the parameters such as the activation energy for boundary migration (Q), the time exponent (n), and the generalized mobility constant (A₀) based on the traditional grain growth model. The determination coefficient (R²) and the mean-square error (MSE) between the predicted and the experimental values are 0.9997 and 0.12 μm, respectively, showing the high prediction accuracy, and the various characteristics of the grain growth curves can also be predicted accurately.

A novel nickel-based powder metallurgy superalloy FGH4113A (WZ-A3) was used to manufacture the full-size turbine disks by the process route of “vacuum induction melting + argon atomization + hot isostatic pressing + hot extrusion + isothermal forging”. The microstructure and mechanical properties of the forged FGH4113A alloys under the different heat treatment conditions were systematically studied. The results show that, the full-size turbine disks prepared by FGH4113A alloy have the good macro morphology and homogeneous grain structure. After the subsolvus heat treatment, the average grain size is ASTM 11~13, the yield strength at room temperature and 550 ℃ are 1249 and 1185 MPa, the tensile strength are 1674 and 1656 MPa, and the elongation after fracture are 23.5% and 19.5%, respectively. The mean fatigue life under the conditions of temperature 700 ℃, strain range 0~0.8% and loading frequency 0.33 Hz is 35000 cycles. After the supersolvus heat treatment, the average grain size is ASTM 6~8, the yield strength at 700 ℃ and 800 ℃ are 1063 and 966 MPa, the tensile strength are 1403 and 1112 MPa, and the elongation after fracture are 17.5% and 12.0%, respectively. The mean creep life under the conditions of temperature 800 ℃, stress 330 MPa and creep elongation 0.2% is 384 h. The crack propagation rate under the conditions of temperature 700 ℃ and stress intensity factor range 30 MPa·m^0.5 is less than 5×10^-4 mm·cycle^-1.

The porous superalloy materials were prepared by loose packing sintering using the atomized K418 nickel-based superalloy spherical powders as the raw materials. The microstructure, permeability, capillarity, and compressive strength of the sintered porous material samples were analyzed, and the effects of sintering temperature and original powder particle size on the microstructure and properties of the porous K418 superalloys were investigated. The results show that, the average pore size and porosity decrease with the increase of sintering temperature. At the same sintering temperature, the average pore size and porosity of the sintered samples increase with the increase of the original powder particle size. At the sintering temperature of 1230 ℃ and the powder particle size of 53~150 μm, the comprehensive performance of the porous material samples is the best, the permeability is 13.69×10^-15 m², the capillary pressure is 22.1 kPa, and compressive strength is 86 MPa.

The formation mechanism and process control of the common defects in selective laser melting GH4169 alloys were briefly introduced, such as spheroidization and holes. The effects of laser power, scanning rate, and powder thickness on the microstructure and mechanical properties of the GH4169 alloys during selective laser melting were emphatically analyzed, and the influences of heat treatment and particle reinforcement on the microstructure and mechanical properties of GH4169 alloy were investigated. Finally, the prospect of the selective laser melting GH4169 alloys was presented from the aspects of process control trend and material strengthening design. It was considered that the design and forming of the particle-reinforced GH4169 composites by selective laser melting were the effective way to further improve the performance of the GH4169 alloys.

The FGH4095 superalloys prepared by hot isostatic pressing + extrusion + isothermal forging were quenched with the different quenching transfer time, and the microstructure and mechanical properties of the treated superalloys were analyzed. The results show that, the quenching transfer time has little effect on the grain size, primary γ′ phase, and tertiary γ′ phase of the FGH4095 superalloys, but influences the size distribution of the secondary γ′ phase. The average size of the secondary γ′ phase with the quenching transfer time of 30 s is 142.9 nm, and that of the secondary γ′ phase is 161 nm with the quenching transfer time of 40 s. The shorter the quenching transfer time, the faster the quenching cooling rate of the FGH4095 superalloys, and the smaller the average size of the secondary γ′ phase. The yield strength of the FGH4095 superalloys at room temperature with the quenching transfer time of 30 s is better than that of FGH4095 superalloys with quenching transfer time of 40 s, and the tensile strength at room temperature is similar to that of FGH4095 superalloys with quenching transfer time of 40 s. The yield strength, tensile strength, endurance life, and endurance ductility of the FGH4095 alloys with the quenching transfer time of 30 s at 650 ℃ are higher than those of the FGH4095 superalloys with the quenching transfer time of 40 s. The shorter the quenching transfer time, the greater the number of the secondary γ′ phase, the smaller the size, the higher critical shear stress impeding the dislocation movement, the higher the tensile strength and the longer the endurance life of the FGH4095 superalloys.

The effect of high temperature oxidation on the surface state of FGH96 superalloy powders was studied by simulating the high temperature heating process during hot isostatic pressing (HIP) consolidation. The results indicate that the surface morphology, surface element distribution, and precipitate composition of the FGH96 superalloy powders are significantly changed by high temperature oxidation at near HIP temperature. With the increase of heating temperature and the extension of holding time, the surface solidification structure of cell crystals and dendritic crystals are covered by oxide/carbide layers. The high temperature oxidation process promotes the diffusion of Ni, Ti, Zr, Nb, Al, and C atoms inside the powder matrix to the powder surface. The pre-existing oxides (ZrO₂) on the powder surface provide the structural conditions for the nucleation of MC carbides (Ti, Nb)C. Controlling the formation of oxides on the powder surface can effectively limit the formation of prior particle boundaries (PPBs) defects in the alloys.

The Cu–9.3Cr–9.3Mo (mass fraction) powders were prepared by mechanical mixing and mechanical alloying, respectively, and the Cu–Cr–Mo alloys were pressed by hot isostatic pressing. The phase composition, organizational structure, and particle size of the powders were characterized by X-ray diffraction and laser particle size analysis. The relative density, hardness, conductivity, and microstructure of the alloys were measured. The results show that, the mechanical alloying process can induce the formation of Cu–Cr–Mo supersaturated solid solution, improve the degree of lattice distortion, and reduce the grain size and powder particle size. The prepared alloy blocks by mechanical alloying have the high hardness, ideal relative density, and electrical conductivity, showing the excellent comprehensive properties.