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The thermal compression deformation behaviors of the hot extruded (HEX) FGH95 alloys were investigated systematically using the Gleeble 3800D thermal-mechanical simulator in the strain rate of 0.001~1.000 s−1 at the deformation temperature range of 1050~1120 ℃. The constitutive equations of the hot extruded FGH95 alloys were derived from the stress-strain curves obtained in the isothermal compression tests. Furthermore, the hot processing maps were established based on the dynamic models. In the results, the corresponding material constants of the constitutive equation are determined as Q=300.925 kJ·mol−1, α=0.01139 MPa−1, and n=1.86. Compared with the hot isostatic pressing (HIP) alloys, the activation energy of the hot extruded FGH95 alloys is declined by more than 50%. According to the energy dissipation efficiency and the microstructure analysis of the hot extruded FGH95 alloys, the processing safety zone and instability zone are identified during the hot extrusion process. Ultimately, the optimal processing conditions of the FGH95 alloys are proposed as the strain rate of 0.010~0.100 s−1 and the deformation temperature of 1050~1120 ℃.

In this paper, TWL12 + TWL20 inorganic salt aluminum coating was sprayed on the surface of Ni-based P/M superalloy. The microstructure changes of inorganic salt aluminum coating and P/M superalloy after high temperature oxidation at 700, 750 ℃ and 800 ℃ were studied by XRD, SEM, EPMA and TEM. The results show that after high temperature oxidation, the surface structure of the coating peels off, and the aluminum in the coating diffuses with the substrate to form a transition layer composed of oxidation zone, diffusion layer and interdiffusion zone. The oxidation zone is the outermost layer, where is mainly enriched with O and Al elements to form Al2O3 layer. The diffusion layer mainly contains Ni and Al elements, forming NiAl phase and α-Cr phase dispersed in it. Finally, the interdiffusion zone rich in Ti, Cr, Co, Ta and other elements exists between the diffusion zone and the matrix, which is mainly composed of Ni2AlTi phase matrix and σ phase dispersed in it. The analysis shows that the thickness of transition layer changes with the increase of oxidation temperature, it is mainly manifested by the increase of the width of the interdiffusion zone, the increase of the size of α-Cr phase in the diffusion layer and σ phase in the interdiffusion zone, and the growth trend of σ phase along the vertical transition zone is intensified. The oxidation weight gain curve shows that the transition layer exhibits good oxidation resistance during high temperature oxidation at 750 ℃ and 800 ℃ after the surface structure of the coating falls off, it indicates that the TWL12 + TWL20 inorganic salt aluminum coating has the potential to provide high temperature oxidation coating protection for advanced P/M superalloy used in aeroengines.

Effect of local interference (holding devices) on the surface microstructure of FGH96 alloys in quenching process was studied by optical microscope (OM), scanning electron microscope (SEM), electron probe microanalysis (EPMA), microhardness, and numerical simulation. The results show that, the holding devices used in quenching produce the chilling effect at the surface of FGH96 alloys, which changes the cooling mode of the connect points and then affects the precipitation and growth behavior of γ′ phases. After the low corrosion, the γ′ phases with the different size and morphology exhibit the visual color difference, leading to the corrosion circles. The maximum diameter of corrosion circles is ~10 mm, and the section depth is ~3 mm. The secondary γ′ phases inside the corrosion circles are fine and spherical, and the average size is ~100 nm. The secondary γ′ phases on the corrosion circles are polygonal distributed, and the average size is ~350 nm. The secondary γ′ phases outside the circles are spherical in distribution, and the average size is ~150 nm. The different size and morphology of the secondary γ′ phases result in the fluctuation of microhardness inside and outside the corrosion circles. The average microhardness is ~HV 450 outside the circles, and that of ~HV 485 inside the circles. The grain structure difference and the element segregation near the corrosion circles are not found.

In order to study the oxidation corrosion protection of key components of carrier aero engine, the high temperature oxidation behavior of the Ni-based powder metallurgy(PM) superalloys FGH4095 and FGH4096 in air at the temperature range of 750-1100 ℃ was investigated. The oxidation kinetics curves were determined by static weighing method. The structure, composition, and morphology of the surface oxide scale as well as its cross section were analyzed by metallographic microscope, scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and X-ray diffractometer (XRD). The results show that two kinds of alloys belong to the grade of full oxidation resistance at 750-900 ℃ and oxidation resistance at 1000-1100 ℃. The actual service temperature of both alloys is below 900 ℃, so they exhibit excellent oxidation resistance in their operating temperature range.At 750-900 ℃, the oxidation resistance of the two alloys is similar without obvious difference. The oxide film is intact without peeling.The oxide films of FGH4095 and FGH4096 are composed of two layers after high temperature oxidation. The internal layer is mainly Al2O3.The outer layer of FGH4095 is composed of Cr2O3, Nb2O5 and TiO2. The outer layer of FGH4096 is composed of only Cr2O3 and TiO2.At 1100 ℃, both alloys are obviously oxidized and a large number of oxide scales crack. Due to the different compositions of the alloys, the oxidation resistance of the two alloys varies greatly at this temperature.In contrast, FGH4095 alloy exhibits better oxidation resistance.

The effect of forging temperature, strain rate and strain state on grain microstructure during solid solution heat treatment of extruded Ni-based Powder Metallurgy Superalloy was studied based on the heat-compression experiment of double cone (DC) specimens. The corresponding relationship between forging temperature, strain rate and grain microstructure in the range of forging temperature from 1060~1120 ℃ and strain rate from 0.003 s–1 to 0.3 s–1 was obtained. The results show that in the Double Cone compression experiment, at the same strain and temperature, the flow stress increases with the strain rate. And at the same strain rate, the higher the temperature, the lower the flow stress is. After supersolvus heat-treatment, at the same strain rate, the grain microstructure is inhomogeneous. And at the same forging temperature, the average grain size of the specimens under the three strain rates of three strain rate is 18~20 μm, but the specimens with higher strain rate clearly show inhomogeneous grain microstructures. In order to obtain the homogeneous grain structure, the more appropriate forging parameter is: the temperature of 1120 ℃, and the strain rate of 0.003 s–1. At the same forging temperature and strain rate, with the decrease of local strain, the average grain size is increased gradually.

Foreign non‐metallic inclusions can significantly reduce the low‐cycle fatigue (LCF) life of powder metallurgy (PM) superalloy and greatly affect the safety and reliability of aeroengines. In this paper, LCF experiments on PM FGH96 superalloy with and without inclusions were conducted. Effects of inclusions on the fatigue life and damage mechanism of FGH96 alloy are discussed by fractography analysis. Parameters related to inclusion characteristics such as strength–inclusion coefficient are proposed. By introducing characteristic parameters of inclusions, the LCF life prediction models are established based on the Manson–Coffin relationship, which significantly improved the prediction accuracy and reduced the scattering band by a factor of 2. Highlights: By performing strain‐controlled low‐cycle fatigue tests at different strain amplitudes, statistical analyses were carried out to assess the influences of size, location, and type of inclusions on LCF life, with a quantitative relationship established between them.The change law of LCF life of powder metallurgy superalloy under different inclusion conditions is summarized. Effects of inclusions on the LCF life of FGH96 alloy were clarified and applied to the establishment of fatigue life prediction models considering characteristic parameters of non‐metal inclusions.A modified Manson–Coffin life prediction model considering inclusion characteristics is proposed to support the safety and reliability assessment of turbine disks made of PM superalloy FGH96.By introducing characteristic parameters of inclusions, the LCF life prediction models are established based on the Manson–Coffin relationship, which significantly improved the prediction accuracy and reduced the scattering band by a factor of 2. [ABSTRACT FROM AUTHOR]

Herein, few‐layered graphene nanosheets (GNS) with approximately 3, 6, or 9 layers are used to reinforce high‐performance nickel‐based superalloy metal matrix composite. A powder metallurgy method comprising solution‐mixing, hot isostatic pressing, and thermal processing is used to prepare GNS‐FGH96 composites with different numbers of GNS layers as well as referential FGH96 matrix materials. Compared with those of unreinforced FGH96, the mechanical properties of the GNS‐FGH96 composites are enhanced, specifically, the ≈6 layers GNS‐FGH96 composite exhibits an ultimate tensile strength of 1660 MPa and a yield strength of 1229 MPa, which are 9.79% and 6.87% higher, respectively, than those without the addition of GNS. Furthermore, the ≈6 layers GNS‐FGH96 composite exhibits the highest elongation‐at‐fracture of 29.9%. The as‐prepared GNS‐FGH96 composites show a good balance of strength and ductility owing to the increased dislocation density between the FGH96 matrix and GNS reinforcement interface area, as well as the high structural integrity of the GNS. Thus, this study provides a novel approach for designing and creating high‐performance graphene‐reinforced FGH96 metal matrix composites that exhibit exceptional strength and toughness. [ABSTRACT FROM AUTHOR]

This study focuses on the effects of adding low content of graphene nanosheets (GNSs) (≤0.3 wt%) on microstructure and tensile property of GNSs‐reinforced nickel‐based superalloy (FGH96). The GNSs‐reinforced FGH96 composite is fabricated by wet‐chemical mixing and hot isostatic pressing. Microstructures and mechanical properties of the composites are studied by scanning electron microscope, transmission electron microscope, and static tensile tests. Results indicate that, when the addition amount of GNSs is less than 0.1 wt%, the tensile properties of the GNSs‐reinforced FGH96 composites improve without losing their plasticity. The largest reduction of area (R/A) ratio of the composite is observed when 0.005 wt% GNSs is added, which is 81.4% higher than the unreinforced alloy. As the cross‐sectional area of tensile sample reduces during necking, the GNSs/unit area increases, leading to the delayed fracture with an increase in R/A ratio and true fracture strength. Theoretical analysis suggests that the strengthening mechanism of GNSs–FGH96 belongs to the modified shear lag model. The interfacial shear stress caused by the presence of GNSs is the main reason for the increase in yield strength. [ABSTRACT FROM AUTHOR]

A pre-hot-deformation process was applied for a polycrystalline nickel-base superalloy to active deformation twins and dislocations, and subsequent slow cooling treatment was used to achieve grain refinement and microstructure homogenization. The microstructural evolution of the alloy was investigated, and the corresponding underlying mechanism was discussed. It was found that twinning mainly occurred in large grains during pre-hot-deformation owing to the stress concentration surrounding the large grains. High density dislocations were found in large grains, and the dislocation density increased approaching the grain boundary. The average grain size was refined from 30 μm to 13 μm after slow cooling with a standard deviation of grain size decreasing from 10.8 to 2.8, indicating a homogeneous microstructure. The grain refinement and microstructure homogenization during cooling process could be achieved via (i) static recrystallization (SRX), (ii) interaction of twin tips and γ' precipitates, and (iii) grain coarsening hindered by γ' precipitates in grain boundaries. [ABSTRACT FROM AUTHOR]

Effect of the cooling rate on the morphological evolution of the γ′ precipitate in a powder metallurgy Ni-base superalloy FGH96 was studied. Results showed that as the cooling rate decreased from 600 °C/min to 2 °C/min, the precipitate morphology transformed from sphere to cuboid, octet and dendrite. As the cooling rate increased, the mean size of the γ′ precipitates decreased while the nucleation density increased. When the cooling rate was lower than 6 °C/min, the precipitate would evolve from a sphere to cuboids with concave faces, octets, dendrite to final split-off branches. The morphological transition was driven by energy balance between coherent strain induced by the γ′/γ lattice misfit and interface energy. The dendritic shape γ′ exhibited an eight-primary-branch pattern with preferred growth direction along 〈111〉, and secondary arms along 〈100〉. [ABSTRACT FROM AUTHOR]

The creep properties of a nickel-based superalloy at 700 °C and 690 MPa resulting from differing aging heat treatments have been investigated. The heat treatments gave rise to significantly different tertiary γ′ precipitate distributions which in turn influence the propensity for precipitate shearing. The creep life was found to decrease with an increase of volume fraction of tertiary γ′. It is shown that the γ′ precipitates undergo shearing by matrix dislocations resulting in residual stacking faults which have been identified from TEM studies. A physically-based crystal slip model for creep deformation in FGH96 has been developed. A critical γ′ precipitate size is found to exist above which precipitate shearing occurs by strongly-coupled dislocations pairs. The critical size for FGH96 superalloy is less than 15 nm, which is smaller than most of the γ′ precipitates in the aged treated samples. A bimodal precipitate hardening model has been presented from which the slip strength resulting from differing heat treatments may be determined. Creep strain rate is found to decrease with increasing slip strength. The crystal slip model successfully captures the effect of heat treatment (in terms of γ′ precipitate distributions) on resulting creep behaviour in alloy FGH96. [ABSTRACT FROM AUTHOR]

• Hot isostatic pressing eliminated weakly bonded area and microvoid at interface. • Hot isostatic pressing promoted migration of grain boundaries at the interface. • Dislocation density at the interface was decreased by hot isostatic pressing. • Tensile properties of the bonded joints were improved by hot isostatic pressing. Tensile properties of an advanced powder metallurgy (PM) Ni based superalloy diffusion bonded joints were significantly improved by hot isostatic pressing (HIP) and the improvement mechanism was studied. Results showed that HIP treatment eliminated the weakly bonded areas and microvoids at the interface. In addition, HIP treatment also significantly promoted the migration of grain boundaries and decreased the dislocation density at the interface. After HIP treatment, the ultimate tensile strength of the joint bonded using a 2-μm-thick Ni coating reached 98.63% of the base alloy following the same thermal cycle. [ABSTRACT FROM AUTHOR]