Roles of Zr and Y in cast microstructure of M951 nickel-based superalloy

The influence of Zr and Y on the cast microstructure of a nickel-based superalloy was investigated by optical microscopy （OM）,scanning electron microscopy（SEM）,electron probe micro-analysis（EPMA）and X-ray diffraction（XRD）.Theγ＋γ′eutectic volume in the superalloy rises notably with the increase of Zr or Y content.Meanwhile,the morphologies of primary MC carbides change from needle and platelet-like to blocky shape with increasing Zr and Y doped.The XRD results show that the primary MC carbide lattice constant increases with Zr and Y additions,and EPMA investigation shows that the platelet-like MC carbides contain primarily Nb and C,while those carbides in blocky shape have 39.2%Zr and 39.6%Nb in average,.These influences on the cast microstructure can be attributed to the atomic size effects of Zr and Y.

Effect of Co on oxidation and hot corrosion behavior of two nickel-based superalloys under Na_(2)SO_(4)-NaCl at 900℃

Nickel-based superalloys with and without Co by partial replacement of W were prepared using double vacuum melting.A comparison of the oxidation in air and hot corrosion behaviors under molten 75 wt.%Na2 SO4+25 wt.%NaCl at 900℃were systematically investigated.The results showed that partial replacement of W with Co promoted the formation of chromia scale and consequently decreased the oxidation rate.Besides,the addition of Co also retarded the internal oxidation/nitridation of Al and consequently promoted the growth of Al_(2) O3 scale,which further decreased the scaling rate and improved the adhesion of scale.Moreover,the addition of Co also further improved the hot corrosion resistance under molten Na2 SO4-NaCl salts.

Determination of Aluminum in Nickel-Based Superalloys by Using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy （LIBS） was developed to detect aluminum in nickel-based superalloys （K417, GH4033, DZ125L, З ∏742y） using a non-intensified, non-gated, low-cost detection system. The precision of LIBS depends strongly on the experimental conditions. The calibration curves of Al（I）394.4 nm and Al（I）396.2 nm under the optimum experimental parameters are presented. Finally the limit of detection （LOD） for aluminum is calculated from the experimental data, which is in the range of 0.09% to 0.1% by weight.

HIGH-TEMPERATURE LOW CYCLE FATIGUE BEHAVIOR OF NICKEL BASE SUPERALLOY GH536

Low cycle fatigue tests on nickel base superalloy GH536 were performed at 600, 700 and 800℃. The strain-life and cyclic stress-strain relationship were given at various temperatures. The change in fatigue life behavior and fatigue parameters with tem- perature increasing was discussed. At low and intermediate total strain amplitudes, the fatigue life was found to decrease with increasing temperature.

Study on Ultrahigh Cycle Fatigue Performance of GH4169 Nickel⁃Based Alloy at 650℃

The fatigue test between 10^5—10^9 cycles of GH4169 nickel‑based superalloy commonly used in aircraft engines is carried out by ultrasonic fatigue machine at 650℃.The S‑N curve is obtained and the fatigue fracture morphology is observed.The fatigue S‑N curve presents a“step‑like”shape,with the first inflection point near 1×10^7 cycles and the second inflection point near 1×10^8 cycles.There is no engineering fatigue limit,and it still shows a downward trend after 107 or even 10^9 cycles.The crack initiation location is related to its life.Cracks are generated on the surface below 10^7 cycles,while it is inside above 10^7 cycles.The crack initiation source in the ultra‑high cycle fatigue at 650℃ is mainly the local intergranular fracture and casting defect of the matrix.In the phase of crack propagation,the mixed propagation of intergranular and cleavage is the main form.

Life Prediction Model for a Nickel-base Single Crystal Superalloy DD3

The possibility of a life prediction model for nickel base single crystal blades has been studied. The fatigue creep (FC) and thermal fatigue creep(TMFC) as well as creep experiments have been carried out with different hold time of DD3. The hold time and the frequency as well as the temperature range are the main factors influencing the life. An emphasis has been put on the micro mechanism of the rupture of creep, FC and TMFC. Two main factors are the voiding and degeneration of the material for the cre...

Effect of Melt Superheating on the Morphology of MC Carbide in a Cast Ni-base Superalloy M963

The effect of the melt superheating temperature on the as cast microstructure of a cast nickel base superalloy M963 has been investigated. The results show that the as cast microstructure of the alloy consists of γ solid solution matrix,γ′ precipitate in cubic shape, (γ+γ′) eutectic and MC carbide, and the morphology of MC carbide in the microstructure can be varied from coarse scriptlike, fine scriptlike to fine cubelike or discontinuous particles by increasing the melt superheating temperature. The mechanism of melt superheating is discussed by means of differential thermal analysis (DTA) technique.

R & D OF CAST SUPERALLOYS AND PROCESSING FOR GAS TURBINE BLADES IN BIAM

Research and development of cast superalloys and processing for turbine blades in BIAM during the last 35 years have been reviewed briefly in this paper.

Cracking Susceptibility After Post-Weld Heat Treatment in Haynes 282 Nickel Based Superalloy

This paper presents a study of the standard post-weld heat treatment （PWHT） behaviour of autogenous laser welded γ＇ age-hardenable precipitation strengthened nickel based superalloy Haynes 282 （HY 282）. The study involves a careful and detailed microstructural characterisation as well as an analysis of the weld cracking susceptibility during welding and Gleeble thermo-mechanical physical simulation. Various factors that could influence post-weld cracking in superalloys weld were experimentally examined. Our microstructural exami- nation of the as-solution heat treated （SHTed） material and the thermo-mechanically refined grain material shows that intergranular heat affected zone （HAZ） cracking is observable in only the as-welded SHTed material. There was no indication of post-weld heat treatment cracking in all welded materials. Our conclusion, in this study, is that the chemistry of superalloy HY 282 which aids the preclusion/formation of deleterious so- lidification microconstituents during welding as well as its relatively slow aging kinetics enhances its resistance to PWHT cracking.

Thermomechanical Fatigue Behavior of Powder Metallurgical Nickel Based Superalloy FGH96

The fracture behavior of the thermomechanical fatigue （TMF） of the powder metallurgical nickel based superalloy FGH96 was investigated under in-phase （IP） and out-of-phase （OP） loadings in the temperature range from 550 ℃ to 720 ℃ and the mechanical strain amplitude range from 0.3% to 0.8%. The results show that the FGH96 TMF fracture character is intergranular for the IP samples and transgranular cleavage-like for the OP samples, at the same strain amplitude, the fatigue life is shorter for the IP than that for the OP samples that is related to crack propagation along grain boundary on the IP samples, the γ′ size is larger in the IP than that in the OP sample, which is related to the bulk diffusion processes accelerated by the tensile strain during the high temperature portion of the IP cycle. Dislocation pairs and stacking faults are main microstructures induced by IP TMF, and they are hindered by the grain boundary, which likely resulted in the crack propagation along the grain boundary in the IP samples.

Experimental investigation for ultra-precision cutting of nickel based superalloy with the assistance of magnetic field

Nickel based superalloy is an important material because of its excellent properties under high temperatures.However,it is a difficult-to-machine material due to its low thermal conductivity,which can cause undesired localized high temperatures in the processing area.In this study,magnetic field-assisted end face turning experiments of nickel-based superalloy is carried out under the assistance of external magnetic fields of different strengths formed by permanent magnets.The experiment results show that,compared with ordinary machining,the chip morphology is improved,the oscillation of cutting force F_(c),F_(a),and Ffare significantly reduced by 90%,88%,and 78%,and the surface roughness Ra is improved from 23 to 13 nm,the P-V value of the fan-shaped area of the machined surface is reduced,and hardness and ductility are improved after the magnetic field is applied.The experiment results indicate that the application of a magnetic field is an efficient and convenient approach to improve the cutting performance of nickel based superalloy.

A numerical study on the influence of grain boundary oxides on dwell fatigue crack growth of a nickel-based superalloy

A theoretical treatment on the oxide-controlled dwell fatigue crack growth of aγ'strengthened nickelbased superalloys is presented.In particular,this study investigates the influence of an externally applied load and variations in theγ'dispersion on the grain boundary oxide growth kinetics.A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature.The viscoplastic model explicitly accounts for multimodalγ'particle size distributions.A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip,where stress-assisted vacancy diffusion is assumed to operate.The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme.Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement.Simulations indicate that the presence of a fineγ'size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge.It is shown that a unimodal dispersion leads to reduced oxide growth rates(parabolic behavior)when compared to a bimodal one.Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip,which become progressively negative as the oxide wedge develops.However,mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge,where failure of the oxide is predicted.The time taken to reach this critical stress for oxide failure has been calculated,from which dwell crack growth rates are computationally derived.The predicted rates are shown to be in good agreement with available experimental data.

Site occupation evolution of alloying elements in L1_2 phase during phase transformation in Ni_(75)Al_(7.5)V_(17.5)

Correlation between site occupation evolution of alloying elements in L12 phase and growth of DO22 phase in Ni75Al7.5V17.5 was studied using microscopic phase field model. The results demonstrate that the growing process of DO22 phase can be divided into two stages. At the early stage, composition in the centre part of L12 phase almost remains unchanged, and the nucleation and growth of DO22 phase is controlled by the decrease of interface between L12 phases. At the late stage, part of V for growth of DO22 phase is supplied from the centre part of L12 phase and mainly comes from Al sublattice, the excess Ni spared from the decreasing L12 phase migrates into the centre part of L12 phase and occupies the Ni sublattices exclusively, while the excess Al mainly occupies the Al sublattice. At the late stage, the growth of DO22 phase is controlled by the evolution of antisite atoms and ternary additions in the centre part of L12 phase.

Effect of cold rolling and first precipitates on the coarsening behavior of γ″-phases in Inconel 718 alloy

The coarsening behaviors of γ″-phase particles in Inconel 718 alloy aged at 750, 800, and 850℃ were investigated by scanning electron microscopy（SEM）. Detailed observations and quantitative measurements were conducted to characterize the coarsening behavior of the γ″-phase under various aging conditions. The experimental results indicate that the existence of the δ-phase retards the formation and coarsening of the γ″-phase, without influencing its final particle size or amount. Moreover, when cold rolled with a reduction of 50%, the dimensions of the γ″ particles in Inconel 718 alloy decrease with increasing aging time. Furthermore, the coarsening behavior of the γ″-phase in the Inconel 718 alloy after a normal aging treatment（sample A） and that of the primary δ-phase（sample B） follow the Lifshitz–Slyozov–Wagner（LSW） diffusion-controlled growth theory; the thus-obtained activation energies for the γ″-phase are 292 k J·mol^-1 and 302 k J·mol^-1, respectively.

Oxidation and Thermal Shock Behavior of a Glass-Alumina Composite Coating on K38G Superalloy at 1000℃

The glass-alumina composite coatings were successfully prepared on the K38G superalloy substrates.Their isothermal oxidation and thermal shock behavior at 1000 ℃ were characterized.With a post-annealing process at 850 ℃,the composite coatings possessed an improved protective effect for the alloy substrates from isothermal oxidation and a higher resistance to thermal shock.Crystallization from the glass matrix and interfacial reaction between the matrix and alumina inclusions,which caused the composites more refractory and tough,accounted for this improvement.The micromechanisms for the formation of oxidation results of spinel ZnCr_2O_4 were also discussed.

Nonequilibrium Grain Boundary Segregation of Phosphorus in Ni-Cr-Fe Superalloy

In virtue of Auger electron spectroscopy, the grain boundary concentrations of phosphorus in Ni-Cr-Fe superalloy are measured after solution treatment at 1 180 ℃ for 45 min. The results show that a peak of phosphorus concentration occurs at about 180 min during isothermal ageing at 500 ℃, and a maximum concentration of phosphorus appears also at about 500 ℃ for all specimens aged for 20 min at temperatures of 200, 400, 500, 700 and 800 ℃. The results are analyzed with the laws of nonequilibrium grain boundary segregation. It is found from the analysis that peaks are related to critical time for nonequilibrium grain boundary segregation of phosphorus.

Microstructure and Mechanical Properties of Graphene Reinforced K418 Superalloy by Selective Laser Melting

In order to obtain the superalloy with excellent properties, graphene reinforced K418 nickel base superalloy(GNPs/K418 composite) was prepared by selective laser melting technique in this study. Through systematically comparing and analyzing the microstructure and mechanical property of K418 superalloy and GNPs/K418 composite, it is found that the percentage of small-diameter grain(≤ 15 μm) increased from 84% to 90%, and the max strength of grain orientation(<001>) reduce from 5.76 to 4.17 due to the addition of GNPs. And GNPs can also improve the height and the full width at the half peak of the strong diffraction peak of GNPs/K418 composite. Besides, GNPs/K418 composite is a kind of sandwiched structure, which is consist of GNPs, carbides, and K418 matrix. Therefore, the hardness of the GNPs/K418 composite is 4.1% and 6.9% higher than that of the K418 matrix in the transverse and vertical direction, respectively. The room temperature tensile strength of the GNPs/K418 composite is 9% higher than that of the K418 matrix. And the 600℃ and 900℃ tensile strengths of the GNPs/K418 composite are 7.6% and 10.4% higher than that of the K418 matrix, respectively. It is inferred that the effect of graphene on K418 matrix strengthening is mainly fine grain strengthening and Orowan strengthening. However, the elongation rate of the composite material is reduced, which is attributed to crack sprouting at the interface between the matrix and GNPs under high stress.