Powder Metallurgy Ni-based Superalloy Research Articles

Effect of Creep Process on the Microstructural Evolution and Deformation Behavior of a New Powder Metallurgy Ni-Based Superalloy

Effect of Creep Process on the Microstructural Evolution and Deformation Behavior of a New Powder Metallurgy Ni-Based Superalloy

Evaluation of Hot Workability of Powder Metallurgy Ni-Based Superalloy with Different Initial Microstructures

The effect of the initial microstructure on the hot workability of a powder metallurgy Ni-based superalloy was investigated in the high-temperature range of 950 °C to 1180 °C and strain rate range of 0.001 to 1.0 s−1. Six samples with different initial microstructures were fabricated by various hot isostatic pressing (HIP) conditions and subsequent treatments such as hot extrusion. The coarse-grained samples exhibited low hot workability regardless of the deformation conditions. In contrast, the hot workability of the fine-grained samples significantly varied depending on the deformation conditions. The hot workability exhibited a peak at the sub-solvus temperature of ~ 1100 °C and decreased at temperatures higher and lower than this temperature. In addition, the hot workability decreased monotonically with increasing the strain rate. The prior particle boundaries (PPBs) acted as cavity nucleation sites and crack paths, especially at lower temperatures and higher strain rates, resulting in early fracture and low hot workability. With decreasing the grain size, the hot workability at the peak temperature improved. The extruded sample with the smallest grain size exhibited the best hot workability, owing to the avoidance of PPB fracture and the acceleration of dynamic recrystallization.

Fatigue crack growth mechanisms in powder metallurgy Ni-based superalloys—A review

Powder metallurgy (PM) Ni-based superalloys are widely used for aeroengine turbine disc applications due to their excellent mechanical properties and good corrosion resistance at elevated temperatures. Understanding the fatigue crack growth (FCG) mechanisms of PM Ni-based superalloys is important for both disc alloy development and life prediction of disc components in these advanced aeroengines where damage tolerance design prevails. FCG in PM Ni-based superalloys is a complicated function of microstructure, temperature, loading conditions and environment and is usually a consequence of the synergistic effects of fatigue, creep and environmental damage. In this review, the mechanisms controlled by microstructural features including grain size, grain misorientation, γ′ size and distribution on short and long FCG behaviour in PM Ni-based superalloys are discussed. The contribution of creep and environmental damage to FCG has been critically assessed. The competing effects of mechanical damage (i.e. fatigue and creep) and environmental damage at the crack tip are microstructure-sensitive, and usually results in transition between transgranular, mixed-trans-intergranular and intergranular FCG depending on the contribution of environmental damage to FCG processes.

Effect of initial powder particle size on the hot workability of powder metallurgy Ni-based superalloys

The effect of the initial powder particle size on the hot workability of Ni-based superalloys was investigated. Herein, four powders with different particle size ranges (<53, 53–106, 106–150, and 150–500 μm) were prepared by sieving the same batch of gas atomized powder. It was found that the oxygen content of the powders decreased monotonically from 200 to ~50 ppm with increasing the particle size. The hot workability was evaluated by high-temperature tensile tests with different temperatures. The hot workability was found to be highly sensitive to the deformation temperature and was the highest at γ′ sub-solvus temperature of ~1100 °C. The initial powder particle size had a significant effect on the hot workability only at the peak temperature. The sample fabricated using the powder with relatively coarse particles (>106 μm) exhibited superior hot workability than that of the sample fabricated using the powder with finer particles (<53 μm). This was because of the high density of prior particle boundaries, which served as preferential cavity nucleation sites. These findings suggest that finer particles are not necessarily better with respect to the hot workability and provide novel insights into the powder metallurgy technology.

Surface Integrity and Oxidation of a Powder Metallurgy Ni-Based Superalloy Treated by Laser Shock Peening

Laser shock peening (LSP) is a mechanical surface treatment which can induce large compressive residual stresses and microstructural changes in a material by using repetitive shocks from laser pulses. In this work, the surface integrity (surface microstructure, topography, hardness and residual stress) of a LSP-treated powder metallurgy Ni-based superalloy was investigated for the first time. LSP treatment introduced large plastic deformation especially at a depth of about 100 μm from the surface, which increased the local hardness. The residual stress from the surface to the interior of the sample was investigated by synchrotron x-ray diffraction. The maximum compressive stress reached 400 MPa at the surface, while the depth of the compressive stress is about 0.7 mm. Lastly, the oxidation behavior of the treated and untreated samples was evaluated by thermal exposure at 700°C for 312 h. The LSP treatment decreased the thickness of the oxide layer, thereby showing improved oxidation resistance.

The sharp drop in fatigue crack growth life at a critical elevated temperature for a PM Ni-based superalloy FGH97

The fatigue crack growth (FCG) experiments for Ni-based powder metallurgy (PM) FGH97 superalloy were performed from RT to 800 °C in air. It has been found that the FCG life decreases in an accelerated manner with an increase in temperature in the service temperature range, and a sharp drop in FCG life was noticed at the inflection point temperature (Tc) of about 800 °C. The fractographic and microstructural analyses were carried out to investigate the primary reason for the phenomenon of observed sharp drop. The obtained results indicate that the fracture transforms from transgranular to intergranular mode during the crack propagation under elevated temperature, and the transition point of fracture mode (ΔKT) also declines rapidly with an increase in temperature. At 800 °C, ΔKT appears even from the crack starts to propagate, which is closely related to the significant decline of FCG life. Combined with the calculated apparent activation energies of FCG, it can be inferred that the sharp drop in FCG life is primarily attributed to the dynamic embrittlement induced weakening of grain boundary, which dominates in the intergranular FCG stage at 800 °C. Further, the evolution of the fracture mechanism governing the FCG process under the conditions of different temperatures and stress intensity factor range (ΔK) has been discussed. With an increase in temperature in the service temperature range, the dominant mechanism of the intergranular fractures transforms from stress assisted grain boundary oxidation to dynamic embrittlement, which results in the sharp decline of FCG life at Tc.

Effects of Original Microstructure on Hot Deformation Behavior and Microstructure of a P/M Ni-Base Superalloy

The hot deformation behavior and microstructure of a powder metallurgy (P/M) Ni-base superalloy with different original microstructure were studied by isothermal compression tests. The isothermal compression tests were conducted on Gleeble-3500D simulator with the temperature range of 1000°C~1100°C and the strain rate of 0.001s-1~0.1s-1. The results showed that the flow stress of the specimens with fine grains (10μm) and ultrafine grains (3μm) gained by hot extruding (HEX) were much less than the that with the average grain diameter of 30μm by hot isostatic pressing (HIP). At the strain rate of 0.001s-1，the as-HIPed specimens with the average grain diameter of 30μm showed steady-state deformation at 1100°C only， whereas the as-HEXed specimens with the average grain diameter of 10μm and 3μm showed steady-state deformation both at 1050°C and 1100°C. The flow stress showed decreasing trend as the average grain diameter decreasing. The activation energy of hot deformation decreased form 622.79 kJ·mol-1 to 302.36 kJ·mol-1 as the average grain diameter decreased from 30μm to 3μm. When the as-HEXed specimen with the average grain diameter of 3μm was deformed at the condition of (1050°C, 0.001s-1), the flow stress was lower than that at the condition of (1100°C,0.001s-1), and the former also gained much finer and uniform grain, the later gained mixed grains.

Residual stress distribution in Ni-based superalloy turbine discs during fabrication evaluated by neutron/X-ray diffraction measurement and thermomechanical simulation

Residual stress distribution induced by multistep fabrication processes is of great significance in reliable design of critical engineering components, such as powder metallurgy Ni-based superalloy turbine discs. By using neutron diffraction and 2D-detector X-ray diffraction, the through-thickness residual stress distribution was determined in fabricated turbine discs via isothermal forging, solution treatment and aging treatment. The experimental results demonstrated that residual stress exhibited typical thermal stress distribution while plastic deformation-induced surface stress was relieved surprisingly after solution treatment. After aging treatment, the reduction of matrix supersaturation after long-time diffusion contributed to increase of compressive radial stress in the rim of disc. By introducing temperature-dependent supplementary phase transformation latent heat into the thermo-mechanical model, overall residual stress distribution after solution treatment was simulated by the finite element method. Numerical simulated stress distribution is in agreement with the experimental results, with the deviation < 40 MPa.

The investigation of the of fatigue crack growth mechanism in powder metallurgy Ni-based superalloy

Nickel alloys with operating temperatures up to 650 °C are used for the manufacture of turbine disks in modern gas turbine engines. During an operation or testing the high centrifugal forces, which appear in the discs, could initiate fatigue cracks formation. If the cracks start from internal defects, there is no air access and their growth occurs in a vacuum conditions. The failure mechanism will correspond to the low-cycle fatigue. Various electron microscopy methods were used in the study of billets formed from the powder nickel alloys. The fractographic analysis demonstrated that in vacuum conditions a rugged rough surfaces of the crack consisted of spherical particles with the sizes in the range between 50 and 150 µm is formed at the beginning. When a crack reaches a billet surface, the crack surface relief changes to a quasi-faceted one, and that corresponds to the first stage of fatigue fracture development. It was found that nano-grains with the size of 100–200 nm are formed in front of the crack tip. The crack surfaces are covered with amorphous layers with a thickness of about 20 nm. The destruction occurs mainly along the boundaries of nanocrystals layer. A model is proposed to explain the appearance of such microstructure and the key factor is the air access to a crack and hence the oxidation of the crack surfaces.

A study of solution cooling rate on γ’ precipitate and hardness of a polycrystalline Ni-based superalloy using a high-throughput methodology

In powder metallurgy Ni-based superalloys, the mechanical properties such as strength, creep and fatigue resistance at the elevated temperature closely rely on the solution cooling rate during heat treatment. Understanding the precipitation mechanisms upon different quenching media can offer a bridge across cooling rate to mechanical response. To date, extensive works have been done to construct the relationship between cooling rate and mechanical performance only using a limited quantity of quenching media such as water, oil and air, resulting in the inadequacy of process-microstructure-property database. In this work, a high-throughput approach based on the Jominy end quenching technique was developed to produce a gradient distribution of γ’ precipitates within an individual specimen by introducing a wide range of cooling rates from 35 °C/min to 6346 °C/min. A transformation from unimodal to bimodal γ’ Particle Size Distribution (PSD) was observed along the end quenched specimen. The average size of γ’ precipitates is inversely proportional with cooling rate in logarithmic scale. The hardness measurements display a strong dependence on cooling rate, which is explained by phase field simulation to provide deeper insight into the interaction between dislocation and γ’ precipitates.

Low-temperature hot corrosion effects on the low-cycle fatigue lifetime and cracking behaviors of a powder metallurgy Ni-based superalloy

In this paper, low-cycle fatigue (LCF) experiments were conducted to analyze the influence of low-temperature hot corrosion on the LCF behavior of the powder metallurgy Ni-based superalloy FGH96 at 700 °C. The LCF lifetime of the salt-coated samples was drastically lower than that of bare samples. The low-temperature hot corrosion would change the crack initiation mode (from subsurface defects to corrosion pits) and early propagation modes (from transgranular to intergranular). Moreover, the degradation mechanism of hot corrosion in the LCF lifetime was connected with early crack initiation resulting both from the deteriorated surface because of corrosion pits and the accelerated fatigue crack propagation due to weakened grain boundaries.

On the Effect of Nb on the Microstructure and Properties of Next Generation Polycrystalline Powder Metallurgy Ni-Based Superalloys

The effect of Nb on the properties and microstructure of two novel powder metallurgy (P/M) Ni-based superalloys was evaluated, and the results critically compared with the Rolls-Royce alloy RR1000. The Nb-containing alloy was found to exhibit improved tensile and creep properties as well as superior oxidation resistance compared with both RR1000 and the Nb-free variant tested. The beneficial effect of Nb on the tensile and creep properties was due to the microstructures obtained following the post-solution heat treatments, which led to a higher γ′ volume fraction and a finer tertiary γ′ distribution. In addition, an increase in the anti-phase-boundary energy of the γ′ phase is also expected with the addition of Nb, further contributing to the strength of the material. However, these modifications in the γ′ distribution detrimentally affect the dwell fatigue crack-growth behavior of the material, although this behavior can be improved through modified heat treatments. The oxidation resistance of the Nb-containing alloy was also enhanced as Nb is believed to accelerate the formation of a defect-free Cr2O3 scale. Overall, both developmental alloys, with and without the addition of Nb, were found to exhibit superior properties than RR1000.

Effect of cooling rate on microstructure and tensile properties of powder metallurgy Ni-based superalloy

Abstract The effects of size distribution, morphology and volume fraction of γ′ phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods. Oil quenching and air cooling were adopted with cooling rate of 183 °C/s and 4–15 °C/s, respectively. The experimental results show that the average size of the secondary γ′ after oil quenching is 24.5 nm compared with 49.8 nm under air cooling, and corresponding volume fractions of γ′ are 29% and 34%, respectively. Meanwhile, the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens. The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen, but the difference approaches each other as the temperature increases to 650 °C. The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature, in contrast to the intergranular fracture at 650 °C or even higher temperature. These two mechanical responses indicate the strengthening effects of γ′ precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.

Morphology evolution of γ′ precipitates in a powder metallurgy Ni-base superalloy

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〉.

Microstructure evolutions and nucleation mechanisms of dynamic recrystallization of a powder metallurgy Ni-based superalloy during hot compression

Dynamic recrystallization (DRX) has been of great concern throughout the manufacturing processes, and it deeply affects in-service performance of powder metallurgy Ni-based disk components. Understanding the underpinning mechanisms of DRX is vital to produce the desired microstructure and mechanical properties of the superalloys. In this article, microstructure evolutions and nucleation mechanisms of DRX of an advanced Ni-based superalloy during hot deformation were studied using high resolution EBSD and TEM. The experimental results show that low angle grain boundaries were formed at low temperature and readily evolved to high angle grain boundaries with temperature increasing and/or strain rate decreasing. Effects of strain amount on DRX were examined and significant DRX was detected when the strain increased to 0.5 under conditions of 1050°C/0.01s−1. Three different nucleation mechanisms were found under different deformation parameters. Nucleation of DRX was strongly related to the bulged-original boundaries, which acted as the interaction barrier with mobile dislocations at relatively low temperature and strain rate of 0.1s−1. In contrast, twining boundaries were identified as the nucleation sites at a higher temperature and strain rate of 0.01s−1. At 1100°C and low strain rate of 0.001s−1, coalesced γ′ was encompassed by bulged-original boundary, where nucleation of DRX was detected.

An investigation of diffusion-mediated cyclic coarsening and reversal coarsening in an advanced Ni-based superalloy

The anomalous cyclic coarsening behaviour of γ′ precipitates after ageing at 1073 K has been investigated for the low misfit commercial powder metallurgy (PM) Ni-based superalloy RR1000. Using scanning transmission electron microscope (STEM) imaging combined with absorption-corrected energy-dispersive X-ray (EDX) spectroscopy, the elemental segregation as a function of coarsening behaviour has been experimentally observed for secondary γ′ precipitates. Elemental EDX spectrum imaging has revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the γ matrix close to the γ-γʹ interface. Our experimental results, coupled with complementary modelling and synchrotron X-ray diffraction analysis, demonstrate the importance of elastic strain energy resulting from local compositional variations for influencing precipitate morphology. In particular, elemental inhomogeneities, as a result of complex diffusive interactions within both matrix and precipitates, play a crucial role in determining the rate of coarsening. Our findings provide important new evidence for understanding the microstructural evolution observed for advanced superalloys when they are exposed to different heat treatment regimes.

Compositional variations for small-scale gamma prime (γ′) precipitates formed at different cooling rates in an advanced Ni-based superalloy

Size-dependent compositional variations under different cooling regimes have been investigated for ordered L12-structured gamma prime (γ′) precipitates in the commercial powder metallurgy Ni-based superalloy RR1000. Using scanning transmission electron microscope imaging combined with absorption-corrected energy-dispersive X-ray spectroscopy, we have discovered large differences in the Al, Ti and Co compositions for γ′ precipitates in the size range 10–300nm. Our experimental results, coupled with complementary thermodynamic calculations, demonstrate the importance of kinetic factors on precipitate composition in Ni-based superalloys. In particular, these results provide new evidence for the role of elemental diffusion kinetics and aluminium antisite atoms on the low-temperature growth kinetics of fine-scale γ′ precipitates. Our findings have important implications for understanding the microstructure and precipitation behaviour of Ni-based superalloys, suggesting a transition in the mechanism of vacancy-mediated diffusion of Al from intrasublattice exchange at high temperatures to intersublattice antisite-assisted exchange at low temperatures.

First-principles-aided design of a new Ni-base superalloy: Influence of transition metal alloying elements on grain boundary and bulk cohesion

A new approach to the design of Ni-base polycrystalline superalloys is proposed. In this approach, we assume that the creep–rupture characteristics of a superalloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of γ matrix. The ideal work of separation, Wsep, of a GB is used as a fundamental thermodynamic quantity that controls the mechanical strength of an interface, whereas the partial cohesive energy, χ, of an alloy component serves to characterize its contribution into the strength of the bulk. Using the Σ5 (210)[100] symmetric tilt GB as a representative high-angle GB in Ni, we calculate Wsep,χ, and GB segregation energies, Eseg, for the complete set of 4d and 5d transition metal impurities, to which we add B (a typical microalloying addition), S and Bi (notoriously known as harmful impurities in Ni-base superalloys). The purpose of the analysis is to identify the elements that demonstrate a high tendency to segregate to GBs, have positive (preferably high) partial cohesive energies in the bulk, and have positive impact on Wsep of GBs. We refer to these elements as low-alloying additions. Our study reveals Zr, Hf, Nb, Ta and B as the most promising low-alloying additions. Our next step is to introduce the elements found in the first step into a new powder metallurgy (P/M) Ni-base superalloy. The results of the subsequent testing confirm that the newly created P/M superalloy indeed demonstrates superior mechanical properties at high temperatures compared to the existing Russian P/M alloy EP741NP.

Creep behaviors and role of dislocation network in a powder metallurgy Ni-based superalloy during medium-temperature

By means of creep propertiesmeasurement, microstructure observation and contrast analysis of dislocation configuration, the creep behaviors and role of dislocation networks in FGH95 powder metallurgy Ni-based superalloy during creep have been investigated. The results show that the microstructure of alloy consists of the fine γ′ phase coherently embedded in the γ matrix, and a few coarser γ′ particles are distributed in the boundary regions. In the ranges of the applied temperatures and stresses, the alloy displays a better creep resistance and longer lifetime. The deformation mechanisms of alloy during creep are the dislocations slipping in the matrix and shearing into the γ′ phase, and the dislocations shearing into the γ′ phase may be decomposed to form the configuration of the partials plus the stacking fault. During creep, two groups of dislocations on different slip planes may knit to form the quadrangular dislocation networks; while two groups of moving dislocations on the same slip plane may encounter and react to form the hexagonal dislocation networks. The strength of boundaries is responsible for the creep resistance of alloy, and the one is related to the deforming behaviors of coarse γ΄ particles distributed in the boundaries. Thereinto, the dislocation networks distributed in the interfaces of coarse γ′/γ phases may release the lattice-misfit stress and relax the stress concentration to delay the dislocation shearing into the γ′ phase, which is beneficial to keep the boundary strength and enhance the creep resistance of the alloy.

Development of a Probabilistic Methodology for Predicting Hot Corrosion Fatigue Crack Growth Life of Gas Turbine Engine Disks

Advanced Ni-based gas turbine disks are expected to operate at higher service temperatures in aggressive environments for longer time durations. Exposures of Ni-based alloys to alkaline-metal salts and sulfur compounds at elevated temperatures can lead to hot corrosion fatigue crack growth in engine disks. Type II hot corrosion involves the formation and growth of corrosion pits in Ni-based alloys at a temperature range of 650 °C to 750 °C. Once formed, these corrosion pits can serve as stress concentration sites where fatigue cracks can initiate and propagate to failure under subsequent cyclic loading. In this paper, a probabilistic methodology is developed for predicting the corrosion fatigue crack growth life of gas turbine engine disks made from a powder-metallurgy Ni-based superalloy (ME3). The key features of the approach include: (1) a pit growth model that describes the depth and width of corrosion pits as a function of exposure time, (2) a cycle-dependent crack growth model for treating fatigue, and (3) a time-dependent crack growth model for treating corrosion. This set of deterministic models is implemented into a probabilistic life-prediction code called DARWIN. Application of this approach is demonstrated for predicting corrosion fatigue crack growth life in a gas turbine disk based on the ME3 properties from the literature. The results of this study are used to assess the conditions that control the transition of a corrosion pit to a fatigue crack and to identify the pertinent material parameters influencing corrosion fatigue life and disk reliability.