Powder Metallurgy Ni-based Superalloy Research Articles

Microstructure and microtexture evolution characteristics of a powder metallurgy Ni-based superalloy during static recrystallization

Microstructure and microtexture evolution characteristics of a powder metallurgy Ni-based superalloy during static recrystallization

Dissolution behavior and kinetics of the γ′ precipitates within a novel powder metallurgy Ni-based superalloy

Isothermal heat treatment experiments were used to studied the dissolution behavior and kinetics of the γ′ precipitates within the hot-extruded novel powder metallurgy Ni-based superalloy A1. The characteristics of the γ′ precipitates were accurately and rapidly extracted by deep learning methods. The JMAK models were established to predict the change of the area fraction of γ′ precipitates during heat treatment. The determination coefficient R2 of model is 0.99. The results indicate that the dissolution sequence, morphological evolution, and dissolution mechanisms of the three types of γ′ precipitates are different. The small-sized tertiary γ′ precipitates (γ′t) dissolve first, while the larger-sized primary γ′ precipitates (γ′p) dissolve last. After 15 min at 1120 °C, there were only a small amount of γ′ precipitates with 0.19 ± 0.03 % area fraction. And there was not significant changed over time. After 1 min at 1150 °C, the γ′ precipitates completely dissolved. During the process of low temperature subsolvus heat treatment (1060 °C, 1090 °C), the large-sized γ′s grown at the expense of consuming the small-sized γ′ precipitates, and the γ′s tend towards directional arrangement due to elastic interaction energy. During the subsolvus heat treatment process, the γ′p spilt due to elastic strain energy and the uneven diffusion of alloying elements, while neck-connected coalescence coarsening occurs due to the overlapping diffusion fields.

Study of Fatigue Crack Initiation and the Propagation Mechanism Induced by Pores in a Powder Metallurgy Nickel-Based FGH96 Superalloy.

Thermally induced pores (TIPs) are generally the source of fatigue crack initiation in the powder metallurgy (PM) Ni-based FGH96 superalloy. The effect of TIPs on fatigue crack initiation on the surface of the FGH96 superalloy was detected using scanning electron microscopy (SEM). The cause of fatigue crack deflection was studied using electron backscatter diffraction (EBSD) analysis. The results indicated that there are two states of TIPs including isolated TIPs and clustered TIPs located at the grain boundary. The investigation of crack initiation and propagation around TIPs was conducted in detail through the comprehensive integration of experimental findings and computational results. For cracks initiated by isolated TIPs, the maximum equivalent size and the ratio of the vertical-parallel axis to the loading direction of the TIPs reveal a linear relationship, and both of them determine crack initiation. Regarding clustered TIPs, the constituent pores of the clustered TIPs will compete to initiate cracks based on the experimental results, and the largest pore will be more likely to initiate cracking. Moreover, the results showed that fatigue crack propagation can be hindered by hard-orientation grains and twins with a low Schmid factor (SF). Large-angle crack deflection due to twins with a low SF can significantly increase crack length and resistance to crack propagation.

Microstructure evolution characteristics of a powder metallurgy Ni-based superalloy during hot deformation

Microstructure evolution characteristics of a powder metallurgy (PM) Ni-based superalloy during hot deformation were investigated by isothermal compression tests at 1050 ˚C–1200 ˚C for strain rates of 0.001 s−1–1 s−1 in this work. And the relationships between the dynamic recrystallization (DRX) behavior, DRX mechanisms, twins, and γ´ were discussed. The results showed that discontinuous dynamic recrystallization (DDRX) which was nucleated by grain boundary bulging, occurred in all deformation parameters in this superalloy. For γ/γ´ poly-phase region deformation (1050 ˚C-1140 ˚C), the volume fraction of dynamically recrystallized (DRXed) grains increased with the increase in strain rates. The finding suggested that γ´ particles promoted the nucleation of subgrains in deformed grains, causing the continuous dynamic recrystallization (CDRX) mechanism, characterized by subgrain continuous rotation, to be another dominant DRX mechanism except for DDRX. Moreover, the formation of twin boundaries occurred under the comprehensive control of DRX and grain boundary migration. DRX promoted the formation of twin boundaries, but at high temperatures, the reduction of the free energy from grain growth was higher than twin formation, causing the possibility of twin formation to be reduced. For γ/γ´ poly-phase deformation, the area fraction and size of γ´ increased with decreasing compression temperatures and increasing strain rates, and high compression temperatures and low strain rates promoted the further dissolution of γ´. The optimum hot deformation processing conditions for PM Ni-based superalloy is 1110–1140 ˚C/0.01–0.1 s−1.

Tensile and creep properties of novel powder metallurgy Ni-based superalloy

The development of novel powder superalloys for aero-engine turbine disks holds significant implications. In this paper, the microstructure and mechanical properties of a novel powder nickel-based superalloy are evaluated, followed by a brief discussion of the relationship between tensile and creep properties. The results revealed that the new alloy exhibits good microstructural stability after long-term aging and excellent tensile strength at different temperatures. In particular, the creep properties are significantly superior compared to other alloys. In addition, it is verified that the Monkman-Grant relationship modified by tensile parameters has better validity.

STUDY ON SURFACE INTEGRITY AND ITS INFLUENCE ON HIGH TEMPERATURE FATIGUE BEHAVIOR WHEN TURNING POWDER METALLURGY NI-BASED SUPERALLOY

As a member of the latest generation of powder metallurgy Ni-based superalloy, FGH96 has already been widely applied in the manufacturing of aero-engines because of its distinguished mechanical performances. The surface integrity plays an essential role in the final fatigue life of the machined parts. However, surface modification induced by the machining process is inevitable, which profoundly affects the high temperature fatigue behavior of the manufactured part during the service lifetime. To cover the gap, the present work was carried out to specially study the surface integrity and its influence on high temperature fatigue behavior of powder metallurgy Ni-based superalloy when subjected to the mechanical turning process. The surface integrity, including surface morphology, micro-hardness, and depth-dependent residual stress, was initially characterized under different input parameters. High temperature fatigue tests were then conducted on the processed specimens to correlate the surface integrity and its fatigue behavior. Results showed that input parameters greatly influence the surface condition and thus the high temperature fatigue behavior. Poor integrity on machined surface leads to poor fatigue performance with typical fatigue fracture morphology, including crack initiation, propagation, and final fracture. While fine integrity on machined surface contributes to better fatigue performance. More interestingly, the fracture morphology in this situation shows similarity to the static tensile fracture, which presents shear failure along the direction of maximum shear stress.

Prediction of Grain Size in a High Cobalt Nickel-Based Superalloy.

With the advancement in computational approaches and experimental, simulation, and modeling tools in recent decades, a trial-and-validation method is attracting more attention in the materials community. The development of powder metallurgy Ni-based superalloys is a vivid example that relies on simulation and experiments to produce desired microstructure and properties in a tightly controlled manner. In this research, we show an integrated approach to predicting the grain size of industrial forgings starting from lab-scale cylindrical compression by employing modeling and experimental validation. (a) Cylindrical compression tests to obtain accurate flow stress data and the hot working processing window; (b) double-cone tests of laboratory scale validation; (c) sub-scale forgings for further validation under production conditions; and (d) application and validation on full-scale industrial forgings. The procedure uses modeling and simulation to predict metal flow, strain, strain rate, temperature, and the resulting grain size as a function of thermo-mechanical processing conditions. The models are calibrated with experimental data until the accuracy of the modeling predictions is at an acceptable level, which is defined as the accuracy at which the results can be used to design and evaluate industrial forgings.

Effect of humid atmosphere on the microstructure and mechanical properties of a PM Ni-based superalloy: From powders to bulk alloys

Understanding the effects of powder storage condition is helpful to improve the mechanical properties of powder metallurgy (PM) and additive manufacturing (AM) superalloys. In this work, nickel-based FGH96 superalloy powders were stored under ambient air (relatively dry) and humid (25 °C/95%RH and 90 °C/95%RH) atmospheres and then consolidated by hot isostatic pressing (HIP). The powders and corresponding bulk alloys were characterized by XPS, OM, FESEM, EPMA, EBSD, and TEM. The results indicate that the humid atmosphere mainly affects the gas content, chemical state, and oxide/hydroxide layer thickness of powders, leading to highest values of oxygen content (~450 ppm), hydrogen content (~25 ppm), and NiO/Ni(OH)2 thickness (~13.9 nm) for 90 °C/95%RH powders, and further forms much severe prior particle boundaries (PPBs) defects in HIPed alloy. The detrimental PPBs are composed of large-sized γ', MC carbides-(Ti, Nb)C, Al2O3 and ZrO2 oxides. In comparison with ambient air powders compact, the strength of 90 °C/95%RH powders compact is unexpectedly increased, which is due to the internal grains pinned inside the original particle boundaries by higher severity degree of PPBs, and leads to the increase of grain boundary strength. Meanwhile, the ductility is significantly decreased, because the fracture mode transited from ductile transgranular fracture to brittle inter-particle debonding. The study reveals the relationship between humid atmosphere and properties of both powders and bulk alloys, and intends to obtain good combination of strength and ductility for PM Ni-based superalloys.

Segregation of alloying elements at planar faults during creep in the PM Ni-based superalloy FGH4096

In this work, the segregation of alloying elements at planar faults during creep in the powder metallurgy (PM) Ni-based superalloy FGH4096 was characterized. It was found that Co, Cr, Mo, Ti, and Nb segregated to the superlattice extrinsic stacking fault and microtwin boundaries in the γ′ phase during creep, leading to the disordered local phase transformation in the γ′ phase. This element segregation behavior of FGH4096 was different from other superalloys which had been reported. This study contributed to understanding the interaction between alloying elements and planar faults in FGH4096 during creep.

Experiment and Modelling of the Pre-Strain Effect on the Creep Behaviour of P/M Ni-Based Superalloy FGH96.

FGH96 is a powder metallurgy Ni-based superalloy used for turbine disks of aero-engines. In the present study, room-temperature pre-tension experiments with various plastic strain were conducted for the P/M FGH96 alloy, and subsequent creep tests were conducted under the test conditions of 700 °C and 690 MPa. The microstructures of the pre-strained specimens after room-temperature pre-strain and after 70 h creep were investigated. A steady-state creep rate model was proposed, considering the micro-twinning mechanism and pre-strain effects. Progressive increases in steady-state creep rate and creep stain within 70 h were found with increasing amounts of pre-strain. Room-temperature pre-tension within 6.04% plastic strain had no obvious influence on the morphology and distribution of γ' precipitates, although the dislocation density continuously increased with the increase in pre-strains. The increase in the density of mobile dislocations introduced by pre-strain was the main reason for the increase in creep rate. The predicted steady-state creep rates showed good agreement with the experiment data; the creep model proposed in this study could capture the pre-strain effect.

Investigation on the Microstructure and Mechanical Properties of Ni-Based Superalloy with Scandium

In this work, a method concerning thermal consolidation is proposed to simulate the traditional powder metallurgy process and accomplish the composition screening of powder metallurgy Ni-based superalloys U720Li and RR1000 with rare metal scandium, and superalloys with zero scandium addition, medium scandium addition and high scandium addition are selected. Then effects of scandium on the microstructure and mechanical properties of superalloys are further investigated through fast hot pressed sintering. The results indicate that scandium doping can effectively refine the grain through modifying the size and volume fraction of primary γ’ precipitates at the grain boundary. Meanwhile, scandium can promote the growth and precipitation of secondary γ’ precipitates to some extent. Due to the comprehensive effects of γ’ precipitate modification and grain boundary strengthening, as-sintered U720Li with 0.043 wt.% scandium presents an excellent combination of tensile strength and ductility at ambient and elevated temperature while as-sintered RR1000 with 0.064 wt.% scandium has a good performance at elevated temperature.

Cellular automata simulation of grain growth of powder metallurgy Ni-based superalloy

Cellular automata simulation of grain growth of powder metallurgy Ni-based superalloy

Influence of shot peening on the microstructure and high-temperature tensile properties of a powder metallurgy Ni-based superalloy

Influence of shot peening on the microstructure and high-temperature tensile properties of a powder metallurgy Ni-based superalloy

Dendrite evolution and quantitative characterization of γ′ precipitates in a powder metallurgy Ni-based superalloy by different cooling rates

The effect of cooling rate after super-solvus heat treatment on the morphological evolution of γ′ precipitates in an advanced powder metallurgy Ni-based superalloy, FGH4113A, was systematically investigated. The quantitative relationship between the cooling rate and γ′ precipitates was elucidated in detail. The results showed that at cooling rates of 200 °C/min and 600 °C/min, the primary branch of dendritic γ′ grew along the< 111 > direction, whereas the secondary branch grew along the< 100 > and {111} directions. In addition to the interfacial and elastic strain energies, the width of the γ′ channel significantly impacted the evolution of the secondary dendritic orientation of γ′. As the cooling rate decreased, the size and volume fraction of the γ′ precipitates increased, whereas the nucleation density of the γ′ precipitates decreased. A strong power-law relationship between the characterization of the γ′ precipitates (such as volume fraction, nucleation density, particle size) and the cooling rate was revealed analytically. Meanwhile, the γ′ morphologies evolved from spheres to cuboids, concave cuboids, octets, and dendrites as the cooling rate decreased. The evolution mechanism and stability of γ′ precipitates were proposed to control their morphology and optimize the properties of superalloys. • The width of γ channel affected the dendrite evolution direction of γ′ secondary branches were demonstrated. • The growth mechanisms of different secondary branches of dendrite γ′ were illustrated. • The different morphology evolution paths of γ′ during cooling rates were revealed. • The power exponential relationship between d s γ ′ , d t γ ′ , f , ρ d grain boundary width and cooling rates were revealed.

Failure behaviors and processing maps with failure domains for hot compression of a powder metallurgy Ni-based superalloy

Processing maps are key to guiding the thermo-mechanical processing (TMP) of superalloys. However, traditional processing maps are incapable of delimiting failure, which is an essential factor to be concerned about during the TMP of superalloys. Employing isothermal hot compression experiments and finite element analysis (FEA), the present study examined the failure behaviors of a powder metallurgy (P/M) Ni-based superalloy and constructed processing maps with failure domains based on the predicted failure threshold. The micromechanical Gurson-Tvergaard-Needleman (GTN) damage model was employed in the FEA to model the cavity-driven intergranular fracture of the superalloy. Deformation temperature and strain rate were considered in the range of 1050–1150 °C and 0.001–1 s−1, respectively. The FEA results reveal that the maximum tensile stress locates at the outer budging surfaces of the samples, which causes failure initiation and subsequent propagation into longitudinal cracks, being consistent with the experiments. It is further demonstrated that the failure is strain-controlled and the critical failure strain remains nearly insensitive to the range of strain rates considered while increasing with the increase of temperature in a third-order polynomial. Finally, an optimized processing window for hot deformation of the superalloy is formulated to warrant good hot workability while avoiding flow instability and failure. The present study offers direct insights into the failure behaviors of P/M Ni-based superalloys and details a modeling strategy to delineate optimized parametric spaces for the TMP of superalloys.

Characterisation of strain localisation under cyclic loading at 450 °C by SEM-DIC in a PM Ni-based superalloy

In this study, the strain localisation under cyclic loading in an advanced powder metallurgy Ni-based superalloy at 450 °C was characterised by SEM-DIC, and the related deformation substructures were characterised by electron channelling contrast imaging under controlled diffraction conditions (cECCI). The results indicate strain mainly accumulates in slip/strain bands due to dislocation slip on the {111} plane, and the strain saturates at 1000 cycles with no evident increase in strain value within strain bands or strain band density from 1000 cycles to 10,000 cycles. Analysis of the strain components in the local coordinates associated with each strain band shows the strain band mainly consists of shear strain ϵxy along the strain band and transverse strain ϵyy normal to the strain band. The strain bands can transmit through, be deflected, or be blocked at grain boundaries depending on the grain boundary characteristics and the neighbouring grain orientation. Comparing the strain bands obtained by SEM-DIC and deformation substructures obtained by cECCI shows that the strain bands originate from underlying continuous slip bands which show the sheared γ′ within the slip bands and dislocation pairs looping around the γ′ precipitates, but the deformation localised in the discontinuous slip bands is not well characterised by SEM-DIC at this lateral resolution of strain.

Effect of tantalum on the microstructure stability of PM Ni-base superalloys

The effect of Ta on the microstructure stability of powder metallurgy (PM) Ni-base superalloy was examined at standard heat treatment and after long-term thermal exposure. The results reveal that the previous particle boundary structure is eliminated with the addition of Ta. As Ta content increases, the γ' morphology changes from spheres to cuboids and begins to split, which is caused by the increase in the γ/γ' mismatch. Reversal coarsening of γ' particles in Ta-containing alloys are observed during exposure at 750 °C. The main reasons are related to γ/γ' lattice mismatch, elements partitioning and soft impingement. Besides, the increase of Ta content promotes the precipitation of σ phase at grain boundaries and in grains. • The PM Ni-based superalloys were fabricated by plasma rotation electrode process and hot isostatic pressing. • Systematical study on microstructure evolution of the PM superalloy with increase of Ta content. • The addition of Ta effectively inhibits the previous particle boundary structure in PM superalloy. • The addition of Ta changes the partitioning of alloying elements in γ and γ'. • Reversal coarsening of γ' particles occurred in the Ta-containing alloys during the aging at 750 °C.

Low cycle fatigue and stress relaxation behaviours of powder metallurgy Ni-based superalloy FGH4098

In this study, the effects of microstructure, temperature and dwell time in a loading cycle on low cycle fatigue (LCF) and stress relaxation behaviours of a powder metallurgy Ni-based superalloy (i.e. FGH4098) for aeroengine turbine disc application were studied along with detailed microscopic characterisation of fracture features and deformation substructures. The results indicate that the LCF failure mode of FGH4098 transits from transgranular to intergranular with the increase of temperature and dwell time, and the propensity of intergranular fatigue cracking is higher in fine-grained FGH4098. FGH4098 predominantly shows cyclic hardening behaviour which is mainly related to dislocation interactions. With the increase of temperature and/or dwell time, the cyclic hardening behaviour diminishes to some extent due to the increasing shearing of secondary γ′ precipitates by stacking fault and partial dislocation pairs. Similarly, stress relaxation in FGH4098 is also associated with the shearing of γ′ precipitates and becomes more prominent at high temperature with long dwell period. Micro-twinning may also contribute to stress relaxation at 750 °C.

Fatigue performance improvement of laser shock peened hole on powder metallurgy Ni-based superalloy labyrinth disc

It is well understood that the components with holes used in aviation and aerospace industries are generally vulnerable to alternate cycles of working stress during their service, which are weakest parts and highly prone to failure and fracture due to the stress concentration at the edge of hole. To improve the resistance towards fatigue failure of the hole, laser shock peening (LSP) technology is successfully applied to the hole on powder metallurgy (P/M) Ni-based superalloy labyrinth disc in present work and significant 18% increase in high cycle fatigue limit is achieved (from 224 ± 4 MPa to 265 ± 6 MPa). The notable effects of LSP on the residual stress, microhardness and microstructure of the hole with and without LSP is investigated based on experimental and simulation method. It is found the large improvement of fatigue limit is attributed to the comprehensive action of high amplitude compressive residual stress and microhardness, grain refinement and the removement of inclusions caused by LSP. The finding here in promoting the fatigue resistance of hole by LSP are general and flexible, thereby exhibiting a potential application to a wide spectrum of engineering components with hole structures.

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