PM Nickel Base Superalloy Research Articles

The evolution of γ′ precipitates and hardness response of a novel PM Ni-based superalloy during thermal exposure

The effects of elemental repartitioning on the evolution of γ′ precipitates and the hardness of the phase of a novel PM nickel-based superalloy after long-term thermal exposure at 750 °C were investigated in this study. The results showed that the splitting of γ′ particles during exposure was related to the γ′/γ lattice mismatch caused by the partition changes in the elements. The increase in the concentrations of Ta, Hf, and W in the γ′ particles and the decrease in the concentrations of W and Mo in the γ matrix were the main reasons for the increase in the γ′/γ mismatch and the promotion of secondary γ′ splitting. A hardness contribution model of the nanoindentation hardness was established, and the hardness changes in γ′ and γ during thermal exposure were obtained. In addition, the dominant roles of Ta, W, Hf, and Mo on the hardness of the γ′ phase and the γ matrix after long-term exposure were revealed.

Effect of hot working on the microstructure and tensile properties of a novel PM Re-bearing nickel base superalloy

The work is devoted to study of an experimental PM nickel-based superalloy Ni-16(Al, Ti, Nb, Ta)-30(Cr, Co, Mo, Hf, W, Re) (wt.%). The initial as-HIP’ed material was prepared as cylinders with a size of ∅100 × 200 mm. The hot workability of the superalloy was evaluated by compression tests of small samples under isothermal conditions. The tests were performed at temperatures of 1150 –1240°C with a strain rate of έ =10−4 –10−1 s−1. Some samples were subjected to preliminary homogenization annealing. Examination of the deformed samples by SEM made it possible to define the hot working conditions providing development of recrystallization processes and formation of a uniform fine-grained structure. It was established that preliminary homogenization of as-HIP’ed material followed by fractional forging with intermediate recrystallization annealings in the temperature range of (Ts− 40) – (Ts−15), where Ts is the γ' solvus temperature, provided a uniform development of recrystallization. The obtained forgings were subjected to ageing or solid solution treatment and ageing. In the fine-grained condition obtained by hot working and heat treatment, the material exhibited tensile properties comparable to the best properties achievable in disc nickel-based superalloys.

Effects of temperature and microstructure on low cycle fatigue behaviour of a PM Ni-based superalloy: EBSD assessment and crystal plasticity simulation

The macro and mesoscale deformation and damage of the PM nickel-based superalloy FGH4098 under low cycle fatigue loading at elevated temperatures were investigated using both SEM/EBSD characterization and crystal plasticity finite element (CPFE) simulations. The results indicate that higher temperature leads to more severe macroscale damage i.e. larger areas of hysteresis loops and shorter fatigue life with transgranular failure mode at both 650 °C and 750 °C. The misorientation parameters derived from EBSD characterization show similar deformation at both temperatures, but more intense localized deformation in the crack initiation region at 750 °C. Based on the accumulated shear-strain energy dissipation density and Fatemi-Socie fatigue indicator parameter, the distribution and evolution of fatigue damage are shown to be strongly affected by local stress–strain state and temperatures, and more severe and homogeneous damage is found at 750 °C. The CPFE simulation results also show higher damage propensity, as indicated by the fatigue indicator parameters used here, in the vicinity of annealing twin boundaries with high difference in elastic modulus.

Influence of γ’ precipitate on deformation and fracture during creep in PM nickel-based superalloy

FGH96 is a powder metallurgy nickel based superally used for turbine disk of aero-engines. In the present study FGH96 alloy with four different γ′ precipitate microstructures were produced via solution heat-treatment with different cooling rates, and the maximum cooling rate reached 400 ​°C/min which was a super cooling rate for Nickel-based superalloy. The creep tests were conducted for PM FGH96 alloy under the testing condition of 704 ​°C and 690 ​MPa. The relationship between the creep properties and the distribution of γ’ precipitate was established. The creep mechanism was analyzed by using TEM and ACTEM, and the dislocation movement was studied at the atomic scale. The creep strain rate was calculated through a physically based crystal slip model established based on crystal plasticity. The calculated results were consistent with the test ones, illustrating the validity of the model. The fracture mechanism was also investigated, and the results showed that the creep cracks generated on the surface due to the oxidation. It was observed that the cracks propagated in different ways depending on the different average diameters of ​γ′ ​precipitate. With the decrease of the average ​γ’ ​precipitate size, the critical shearing stress increased and the resistance of the dislocation slipping increased. The fracture mechanism for the primary stage transformed from intragranular to intergranular due to the change of dislocation slipping.

Hot working characteristics of HEXed PM nickel-based superalloy during hot compression

Abstract To study the hot deformation behavior of a new powder metallurgy nickel-based superalloy, hot compression tests were conducted in the temperature range of 1020–1110 °C with the strain rates of 0.001–1 s−1. It is found that the flow stress of the superalloy decreases with increasing temperature and decreasing strain rate. An accurate constitutive equation is established using a hyperbolic-sine type expression. Moreover, processing map of the alloy is constructed to optimize its hot forging parameters. Three domains of dynamic recrystallization stability and instability regions are identified from the processing map at a strain of 0.7, respectively. The adiabatic shear band, intergranular crack and a combination of intergranular crack and wedge crack are demonstrated to be responsible for the instabilities. Comprehensively analyzing the processing map and microstructure, the optimal isothermal forging conditions for the superalloy is determined to be t=1075–1105 °C and =10−3–10−2.8 s−1.

Novel powder packing theory with bimodal particle size distribution-application in superalloy

Powder packing behavior plays an important role in determining sintering ability of powder and the resultant performance of materials. In this study, a novel powder packing theory with bimodal particle size distribution is proposed by considering the loosening effect, wall effect and wedging effect. This theory is applied in PM nickel base superalloy by using mixture of coarse particles and fine particles. Microstructures of alloy sintered by vacuum hot pressing (HP) are observed by optical microscope (OM) and electron backscatter diffraction (EBSD). The prediction result by this theory is in good agreement with the experimental results. The enhanced sintering ability of powder containing appropriate fractions of coarse particle and fine particle is ascribed to the filling of fine particles to the voids between coarse particles, which enhanced the density of sample after sintering. Tensile behavior and the fracture morphology of alloys with various particle distributions are analyzed in details, suggesting the higher reliability of the present theory.

Direct powder forging of PM nickel-based superalloy: densification and recrystallisation

Powder metallurgy nickel-based superalloys have been widely used in high temperature applications. For these materials, a fully dense and fine-grained microstructure is important. Full densification can be achieved by a suitable processing technique, while the latter can be achieved through recrystallisation for which valuable guidance is provided by the information about recrystallisation nuclei. In this study, a fully dense powder metallurgy nickel-based superalloy component has been produced by a new manufacturing method—direct powder forging—using a single acting hydraulic press under normal atmosphere. Boundary misorientation of the produced material has been analysed to determine the degree of recrystallisation nucleation. A finite element model for direct powder forging has been developed in DEFORM-2D/3D and validated by comparing experimental and simulated load curves. The relationship of stress and strain state with densification and recrystallisation nucleation degree has been analysed. It was found that the direct powder forged FGH96 alloy has a much higher recystallisation nucleation degree and more recrystallised sub-grains, compared with those of the hot isostatic pressed material. Within the forged component, a higher recystallisation nucleation degree resides in the material near the container wall where greater values of shear strain rate have operated.

Plastic Flow and Microstructure Evolution during Thermomechanical Processing of a PM Nickel-Base Superalloy

Plastic flow and microstructure evolution during sub- and supersolvus forging and subsequent supersolvus heat treatment of the powder-metallurgy superalloy LSHR (low-solvus, high-refractory) were investigated to develop an understanding of methods that can be used to obtain a moderately coarse gamma grain size under well-controlled conditions. To this end, isothermal, hot compression tests were conducted over broad ranges of temperature [(1144 K to 1450 K) 871 °C to 1177 °C] and constant true strain rate (0.0005 to 10 s−1). At low temperatures, deformation was generally characterized by flow softening and dynamic recrystallization that led to a decrease in grain size. At high subsolvus temperatures and low strain rates, steady-state flow or flow hardening was observed. These latter behaviors were ascribed to superplastic deformation and microstructure evolution characterized by a constant grain size or concomitant dynamic grain growth, respectively. During supersolvus heat treatment following subsolvus deformation, increases in grain size whose magnitude was a function of the prior deformation conditions were noted. A transition in flow behavior from superplastic to nonsuperplastic and the development during forging at a high subsolvus temperature of a wide (possibly bi- or multimodal) gamma-grain-size distribution having some large grains led to a substantially coarser grain size during supersolvus annealing in comparison to that produced under all other forging conditions.

Effect of cutting speed on surface integrity and chip morphology in high-speed machining of PM nickel-based superalloy FGH95

High-speed machining is being recognized as one of the key manufacturing technologies for getting higher productivity and better surface integrity. FGH95 powder metallurgy superalloy is a kind of nickel-based superalloy which is produced by near-net-shape technology. With increasing demands for high precision and high performance of FGH95 components in aerospace industry, it is essential to recognize that the machined surface integrity may determine machined part service performance and reliability. Then, little is known about the machined surface integrity of this superalloy. Thus, the surface integrity in high-speed machining of FGH95 is investigated in this paper. Experiments are conducted on a CNC milling center with coated carbide tools under dry cutting conditions. The surface integrity is evaluated in terms of surface roughness, microhardness, and white layer. The influence of cutting speed on chip morphology is also investigated. Experiment results show that surface integrity and chip morphology of FGH95 are very sensitive to the cutting speed. When cutting speeds are below 2,400 m/min, the values of surface roughness have little variation, while when cutting speeds are in the range of 2,800–3,600 m/min, the values of surface roughness are higher than that of other cutting speeds. Severe work hardening is observed resulting from high-speed machining of FGH95 superalloy. The higher the cutting speed, the higher the surface hardness. When cutting speeds are in the range of 2,800–3,600 m/min, the white layer thickness is slightly higher than that of other cutting speeds. In high-speed machining of FGH95, the chip is segmented and has a typical sawtooth shape. The degree of serrated chip increases with the cutting speed. When the cutting speeds exceed 2,400 m/min, serrated chips change into fragment chips.

Hot Deformation Behaviors and Microstructure Evolution in a New PM Nickel-Base Superalloy

The hot deformation behaviors and the effect of flow stress on the microstructure were investigated for the as-HIPed and as-extruded materials in a new PM nickel-base superalloy. Under most test conditions, the as-extruded material exhibits superplastic flow because of grain growth corresponding to work hardening during deformation process, activation energy for deformation of which is 211 kJ/mol. However, activation energy of deformation for the as-HIPed material is 717 kJ/mol, because grain size is refined due to dynamic recrystallization, the reason being considered to be associated with the initial microstructure before the deformation. Based on the results of compression tests, two constitutive equations for the two materials were established using the mathematical regression; these will provide the valuable helps for predicting and controlling the deformed microstructure as well as optimizing hot-working process.

Model of viscoplastic behaviour of a PM nickel base superalloy at 650 °c : V. Aubourg et al. (University of Dayton, Dayton, Ohio, USA)

Model of viscoplastic behaviour of a PM nickel base superalloy at 650 °c : V. Aubourg et al. (University of Dayton, Dayton, Ohio, USA)

The effect of directional recrystallization on the LCF characteristics of PM nickel-based superalloy APK-6: Godfrey, A. W. and Martin, J. W. Proc. Conf. Superalloys 1992, Champion, Pennsylvania, USA, 20–24 Sept. 1992 (The Minerals, Metals & Materials Society, 420 Commonwealth Dr, Warrendale, PA 15086, USA) 1992 pp 757–764

The effect of directional recrystallization on the LCF characteristics of PM nickel-based superalloy APK-6: Godfrey, A. W. and Martin, J. W. Proc. Conf. Superalloys 1992, Champion, Pennsylvania, USA, 20–24 Sept. 1992 (The Minerals, Metals & Materials Society, 420 Commonwealth Dr, Warrendale, PA 15086, USA) 1992 pp 757–764

Dual structure PM nickel-based superalloy turbine disc : K. Iwai et al, (Kobe Steel Ltd., Kobe, Japan)

Dual structure PM nickel-based superalloy turbine disc : K. Iwai et al, (Kobe Steel Ltd., Kobe, Japan)

PM nickel-base superalloy dual-property disks produced by superplastic forging

A manufacturing process for dual-structured turbine disks made from PM nickel-base superalloys has been specially developed. The disks are composed of two metals: AF115, which has superior stress rupture properties, was used in the rim region, and TMP-3, which has good tensile properties and a low cycle-fatigue life, was used in the bore region. A 400mm diameter dual-property disk was successfully produced through double-stepped HIP treatment and superplastic forging, in which gas atomized powders composed of the above alloys were consolidated into a well-designed preformed structure. After heat treatment, the microstructure of the product was very fine and homogeneous in the bore region, and coarse grains were obtained in the rim region. The mechanical properties were of such high quality as to enable practical use as a turbine disk.

Effect of Boron on the Structure and Properties of a PM Nickel Base Superalloy

Effect of Boron on the Structure and Properties of a PM Nickel Base Superalloy

Effect of Minor Chemistry Variations on the Structural and Mechanical Behaviour of a PM Nickel Base Superalloy

Addition of 0.7% Hf to a heavily alloyed low carbon Ni base PM superalloy has been found to significantly reduce the alloy solidus in contrast to another chemisty variant free of Hf but with increased alloying. The Hf-bearing alloy showed major microstructural changes with increase in HIPing temperature. Reduction in C to < 0.02% severely affected its elevated temperature strength. The Hf-free alloy containing 0.03% C offered a good combination of tensile and stress rupture properties after a suitable heat treatment.

Grain coarsening of PM nickel-base superalloy by critical strain annealing

Critical strain annealing has been investigated for the of development of coarse grain structures in PM Astroloy. The need for this type of treatment arises because normal grain coarsening is severely retarded by carbides in the prior particle boundaries (PPBs). It was found that a strain of ∼ 2.2% produced coarse grain structures (∼ 360 μm) on subsequent annealing for 2 h at 1250°C. Smaller amounts of deformation could also initiate the nucleation of recrystallization but here the driving force was insufficient to cause the migration of the recrystallization front through the PPBs, resulting in no increase in grain size beyond the size of the prior powder particles.