Powder Metallurgy Nickel-based Superalloy Research Articles

Effect of cooling rate on the Morphological changes in the secondary [formula omitted] precipitation in FGH97 nickel-based PM superalloy

The forming and evolving mechanism of γ′ precipitates during continuous cooling after super-solvus heat-treatment (SSHT) for a nickel-based powder metallurgy (PM) superalloy FGH97 was systematically investigated. A series of cooling rates ranging from 0.1 °C/s to 500 °C/s were carried out using the DIL 805A dynamic dilatation machine. The results show that the size of the secondary γ′ precipitates was linearly related to the logarithm of the cooling rate, and the serrated grain boundaries became straight with the increase of cooling rate. A bimodal size distribution of γ′ precipitates, including secondary γ′ precipitates and tertiary γ′ precipitates, was observed at a relatively lower cooling rate, in contrast to a monomodal size distribution of γ′ precipitates at higher cooling rates. The morphology of the secondary γ′ precipitates changed from spherical to cuboidal, butterfly-like and octet shapes as the cooling rate decreased, at the same time the γ/γ' lattice mismatch increased, and the microhardness decreased. The Al elements were found to be enriched in the secondary γ′ precipitates according to the TEM results, while Cr and Co elements were found to have a higher concentration in the γ matrix.

Thermo-mechanical fatigue crack growth in a nickel-based powder metallurgy superalloy

Fatigue crack growth is studied in a nickel-based powder metallurgy (PM) superalloy (FGH4099) subjected to in-phase (IP) and out-of-phase (OP) thermo-mechanical fatigue (TMF), as well as isothermal fatigue (IF) at peak temperature. The crack growth rate and path are evaluated for both coarse grain (CG) and fine grain (FG) FGH4099, especially the effects of phase angle and polycrystalline microstructure. The results show that the TMF crack propagation is mainly transgranular in OP condition; while in IP condition, fatigue crack propagates intergranularly at low ΔK and transforms to transgranular after passing the transition region. The crack propagation resistance for FG microstructure is lower than that for CG microstructure at elevated temperature, as a result of secondary cracks and grain boundary weakening effect. Crystallographic slip controls the crack propagation process, while the twin boundaries (TB) and special grain boundaries also exert a significant influence on crack deflection. The formation of secondary cracks is closely related to local misorientation and crystallographic deformation during TMF crack propagation. The crack deflection in a single grain reveals a competition mechanism between crystallographic slip and grain boundary effect, which explains the lower crack growth rate for OP when compared to IP or IF.

Effect of interrupted cooling on microstructural evolution and microhardness of FGH97 nickel-based PM superalloy

Interrupted cooling experiments of FGH97 nickel-based powder metallurgy (PM) superalloy with different cooling rates were carried out using a DIL805A quenching dilatometer. The microstructural evolution of the γ′ precipitates during cooling after solution heat-treatment was investigated by means of field emission scanning electron microscopy (FE-SEM), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Vickers microhardness tests were performed on the samples that were interrupted at different temperatures to establish a relationship between microstructure and mechanical properties. The results revealed that the secondary γ′ precipitates preferentially nucleated at a higher temperature, and gradually grew during the cooling stage. Meanwhile, the morphology of the secondary γ′ precipitates transitioned from spherical to cubic, to rounded cuboidal, to a butterfly-like shape, and ultimately splitting into octo-cubes, whose driving force was attributed to the mutual competition between surface energy and elastic strain energy. During the initial stages of the cooling process, the microhardness decreased due to the growth and coarsening of the secondary γ′ precipitates. As the temperature decreased, the supersaturation within the matrix became sufficient to overcome the energy barrier for a second burst of nucleation, the tertiary γ′ phases precipitated, resulting in an increase in microhardness.

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.

Diffusion bonding of nickel-based powder metallurgy superalloy FGH98 with pure nickel interlayer

FGH98 powder metallurgy superalloy were successfully bonded via diffusion bonding technique with pure Ni interlayer. It was indicated that adding the Ni interlayer can significantly reduce joint defects, promote elements diffusion, and improve the bonding rate comparing with direct diffusion bonded joint. The typical joint is clearly divided into the IZ with larger volume dendritic γ′ phase and the DZ with narrow primary γ′ phase, petal secondary γ′ phase and granular tertiary γ′ phase. It was observed that the thinner the interlayer, the easier it is for the joint to achieve alloying, the more uneven the transition of the joint structure. Properly increasing the temperature or holding time can help with elements diffusion, but too high bonding parameters can cause grain seriously coarsening and reduce joint performance. The optimized process parameters of 1130 °C −60 min-10 μm Ni were obtained with reaching the highest shear strength of 656 MPa, which is the 201% of the direct diffusion bonded joint.

Interfacial microstructure evolution and mechanical properties of FGH98 superalloy TLP joints with BNi-2 interlayer

FGH98 nickel-based powder metallurgy superalloy was successfully bonded via transient liquid phase (TLP) bonding technique with a BNi-2 interlayer. The typical joint consisted of three characteristic zones including: ASZ composed of multiple silicon borides, ISZ with uniform γ solid solution and dispersed small γ′ phases and DAZ containing multiple kinds of borides which exist within and at grain boundaries. With the increase of bonding temperature and holding time, ASZ was gradually replaced by the ISZ, and eutectic structure disappeared. But excessive temperature or time will cause grain coarsening, deteriorating the joint performance. Under the bonding parameters of 1130 °C −60 min, the joint with optimal shear strength was obtained, which is 882 MPa (94% of the base metal strength). The fracture occurs in the DAZ, which is a mixed fracture.

Dynamic Mechanical Properties and Modified Johnson-Cook Model Considering Recrystallization Softening for Nickel-Based Powder Metallurgy Superalloys.

The material undergoes high temperature and high strain rate deformation process during the cutting process, which may induce the dynamic recrystallization behavior and result in the evolution of dynamic mechanical properties of the material to be machined. In this paper, the modified Johnson-Cook (J-C) model for nickel-based powder metallurgy superalloy considering dynamic recrystallization behavior in high strain rate and temperature is proposed. The dynamic mechanical properties of the material under different strain rates and temperature conditions are obtained by quasi-static compression test and split Hopkinson pressure bar (SHPB) test. The coefficients of the modified J-C model are obtained by the linear regression method. The modified model is verified by comparison with experimental and model prediction results. The results show that the modified J-C model proposed in this paper can accurately describe the mechanical properties of nickel-based powder metallurgy superalloys at high temperatures and high strain rates. This provides help for studying the cutting mechanism and finite element simulation of nickel-based powder metallurgy superalloy.

Thermo-mechanical fatigue deformation and fracture mechanisms of nickel-based powder metallurgy superalloy

Thermo-mechanical fatigue (TMF) failure is one of the major concerns for aero-engine turbine discs under complex service loading conditions. By conducting both in-phase (IP) and out-of-phase (OP) TMF tests, the stress-strain hysteresis loops and the cyclic stress responses were analyzed for the third-generation nickel-based powder metallurgy (P/M) superalloy FGH4098. To reveal the failure mechanism, advanced microscopy analysis was carried out to characterize the fracture surface and microstructure of the alloy after failure. The phase angle between alternating mechanical and thermal loads plays an important role for the magnitude and evolution of mean stress. According to SEM fractography, the crack growth exhibits intergranular fracture under IP TMF and transgranular fracture under OP TMF. From the electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) characterization results, the geometrically necessary dislocation (GND) density is higher for IP TMF, indicating more severe plastic deformation. The material shows cyclic softening in the high-temperature half-cycle, but cyclic hardening in the low-temperature half-cycle. The cyclic deformation is mainly related to dislocation activities under IP loading conditions, for which fatigue damage dominates the failure process. Under OP loading conditions, the deformation behavior is mainly associated with the generation of stacking faults (SF), and its failure mechanism includes both fatigue and oxidation damage.

Hot deformation behavior and microstructure evolution of a novel nickel-based powder metallurgy superalloy

With the increase in service temperature of aero-engine hot components, the requirements for the high-temperature performance and processability of the powder metallurgy (PM) superalloys used for preparing turbine disks also increase. To overcome the bottlenecks that constrain the preparation and microstructure control of PM superalloys, this work investigates the impacts of hot compression parameters on the hot deformation behavior and microstructure evolution of a novel PM superalloy, FGH4113A. Hot compression tests were conducted on the FGH4113A alloy, which underwent hot isostatic pressing, within a temperature range of 1050 °C–1150 °C and at a consistent true strain rate ranging from 0.001s−1 to 1s−1. The results reveal that the flow stress decreases progressively with the reduction of strain rate and the rise in temperature. Optimal hot deformation conditions are identified at a temperature of 1110 °C with strain rates ranging from 0.1s−1 to 1s−1, and a temperature of 1130 °C with strain rates between 0.01s−1 and 0.1s−1. Within these optimal regimes, grains are uniform, and the average grain size is maintained within10μm. Grain evolution during hot compression involves original grain deformation, followed by recrystallization, subsequent grain coarsening, and then deformation again. Additionally, the γ′ area fraction diminishes with an increase in deformation temperature. Specifically, as the deformation temperature raises from 1050 °C to 1110 °C, and subsequently to 1150 °C, at a rate of 0.001s−1, the primary γ′ area fraction diminishes progressively from 6.2 % to 3.0 % and ultimately to zero. Above 1130 °C, the γ′ area fraction remained largely unaffected by variations in strain rate. These insights are pivotal, offering both experimental data and theoretical perspectives for optimal microstructure control in PM superalloys.

Microstructure and Mechanical Properties of a Novel Powder Metallurgy Nickel-Base Superalloy

Microstructure and Mechanical Properties of a Novel Powder Metallurgy Nickel-Base Superalloy

Interfacial reaction and interfacial strengthening mechanism of graphene nanosheets reinforced powder metallurgy nickel-based superalloy composite

Graphene nanosheet (GNS) reinforced nickel based superalloy (FGH96) composites with various interfacial reaction degrees were fabricated by powder metallurgy of Hot Isostatic Pressing (HIP) at sintering temperature range of 1120–1190 °C. The interfacial reaction products [Ti, Nb]C particles were formed in FGH96 metal matrix composite by the in-situ reaction between homogeneously-dispersed GNS and FGH96 matrix. In addition, the interfacial reaction products determined by Transmission Electron Microscopy, the interfacial structure was also determined and named clean interface, transition zone and in-situ generation of metal carbides interface. The interfacial [Ti, Nb]C particles on partially reacted with GNS, led to significant improvement of interfacial strength and consequent reinforced the mechanical transfer efficiency in FGH96 metal matrix consequently, compared to those FGH96 without GNS. The role of interfacial [Ti, Nb]C particles played in determining the interfacial bonding strength and interfacial bonding mode of GNS was discussed. This work emphasizes the importance of the interface state of GNS on the interface structure in metal matrix composites, and may provide new insights into the interfacial phenomena and load transfer mechanism in GNS reinforced metal matrix composites.

Microstructural Characterization and Prior Particle Boundary (PPB) of PM Nickel-Based Superalloys by Spark Plasma Sintering (SPS).

This research investigates the microstructure and defects of powder metallurgy (PM) nickel-based superalloys prepared by spark plasma sintering (SPS). The densification, microstructural evolution, and precipitate phase evolution processes of FGH96 superalloy after powder heat treatment (PHT) and sintering via SPS are specifically analyzed. Experimental results demonstrate that SPS technology, when applied to sinter at the sub-solidus temperature of the γ' phase, effectively mitigates the formation of a prior particle boundary (PPB). Based on experimental and computational findings, it has been determined that the presence of elemental segregation and Al2O3 oxides on the surface of pre-alloyed powders leads to the preferential precipitation of MC-type carbides and Al2O3 and ZrO2 oxides in the sintering necks during the hot consolidation process, resulting in the formation of PPB. This study contributes to the understanding of microstructural modifications achieved through SPS technology, providing crucial information for optimizing sintering conditions and reducing the widespread occurrence of PPB, ultimately enhancing the material performance of PM nickel-based superalloys.

The Effect of Forging and Heat Treatment Variables on Microstructure and Mechanical Properties of a Re-Bearing Powder-Metallurgy Nickel Base Superalloy

In our previous works, the effects of forging and heat treatment variables on microstructure evolution and mechanical properties have been studied for an ingot-metallurgy Re-bearing nickel base superalloy. To overcome the issues associated with the production of large-scale ingots and fine-grained workpieces, in the present work, the effect of hot forging and heat treatment variables was studied in a Re-bearing nickel base superalloy prepared via powder metallurgy. The purpose of the study was to reach the properly balanced mechanical properties for the potential use of the superalloy as a disc material. The initial as-HIPed workpieces were subjected to different hot forging and post-forging heat treatment or only to heat treatment (no forging). For the processed workpieces, the recrystallization behavior, size, morphology and volume fraction of γ′ precipitates were evaluated by scanning electron microscopy followed by a study of mechanical properties. The most properly balanced mechanical properties (strength, ductility, creep resistance and creep rupture lifetime) were reached for the γ grain size of dγ ≈ 13.6 µm. A finer and coarser γ grain size (down to dγ ≈ 2.6 and up to dγ ≈ 37.5 µm) even when superimposed with a higher volume fraction of dispersed secondary γ′ precipitates (in the case of dγ = 27–37.5 µm) was associated with worse mechanical properties.

Microsegregation of Alloying Elements at Deformation-Induced Stacking Faults in a Powder Metallurgy Nickel-Base Superalloy

Microsegregation of Alloying Elements at Deformation-Induced Stacking Faults in a Powder Metallurgy Nickel-Base Superalloy

A crack growth model for nickel-based powder metallurgy superalloy considering creep zone at crack tip

In this paper, the creep crack growth behavior of powder alloy materials for turbine disks was studied. The experimental results show that the creep crack growth rate at 700 ℃ is significantly higher than that at 600 ℃, and the main form of the crack growth is intergranular fracture. It is found that there is an obvious dispersion state in the early stage of creep crack growth, which cannot be described by creep fracture parameters based on the linear elastic assumption, such as C(t),Ct. To solve this problem, a rate model considering creep zone at crack tip was proposed, which can describe the crack growth rate curve uniformly. The prediction deviation of creep crack growth time is approximately 1.5X.

Correlation between secondary γ′ and high-temperature tensile behavior of a powder metallurgy nickel-based superalloy EP962NP

A powder metallurgy nickel-based superalloy EP962NP was subjected to solid solution treated and cooled in four media (water, oil, air and furnace) followed by aging treatment to investigate the correlation between secondary γ′ (γs′) and tensile properties of the experimental alloy at 750 °C. Results show that the size of γs′ increases sharply with the decreasing cooling rate, and morphologies of γs′ depend on the cooling regime and present various characteristics including spherical, rod-shaped, cubic, etc. By implementing the heat treatment process of solid solution treatment (1210 °C/2 h, air cooling/AC) and two-step aging treatment (i.e., 870 °C/8 h, AC + 760 °C/16 h, AC), superior comprehensive mechanical properties can be obtained with ultimate tensile strength, yield strength and elongation of 1251 MPa, 1079 MPa and 13.3%, respectively, due to the high density of stacking faults. Interaction between γ′ and dislocations indicates that the deformation mechanism of the alloy exhibits stacking faults on (1‾11‾) [121] and microtwins, which depends on the size of γs′. The increasing size of γs′ precipitates drives a transition in the dominant deformation mechanism from stacking faults shearing to microtwinning.

Comparative Study of Prior Particle Boundaries and Their Influence on Grain Growth during Solution Treatment in a Novel Nickel-Based Powder Metallurgy Superalloy with/without Hot Extrusion

The prior particle boundaries (PPBs), as one of the typical defects in the nickel-based powder metallurgy superalloy, largely affect the microstructure and thus properties/performance of alloys. However, the effect of PPBs on the microstructure evolution in nickel-based powder metallurgy superalloy during heat treatment is still unclear. In this paper, a comparative study of PPBs and their influence on grain growth during solution treatment in a novel nickel-based powder metallurgy superalloy FGH4113A (i.e., WZ-A3 from Shenzhen Wedge, China) with/without hot extrusion (HEX) was conducted. Firstly, through a combination of scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and transmission electron microscope (TEM) techniques, PPBs in FGH4113A alloys were characterized to be Al2O3, carbides (TiC, M6C, M23C6) and large-size γ′ particles. After HEX, the oxides broke, carbides deformed, and γ′ phase redistributed. After solution treatment at 950 °C, the TiC decomposed to M6C and M23C6, while no such decomposition occurred in FGH4113A alloys after solution treated at 1050 °C and 1150 °C. Secondly, the evolution of grain size in FGH4113A alloys was analyzed using the electron backscattered diffraction (EBSD) technique. At 950 °C, the decomposition of carbide TiC resulted in the increase of PPBs and the enhancement of their pinning effect on grain boundaries, thus inhibiting grain growth. At 1050 °C, the nucleation rate due to recrystallization is comparable to the grain growth rate, leading to the stable distribution of grain size. While at 1150 °C, the higher temperature can induce a higher content of PPBs. However, the driving force for grain growth surpassed the pinning force of PPBs, making the grains quickly coarsen. Finally, it was concluded that the HEX process is an effective method to modify the microstructure of powder metallurgy superalloy after HIP that can heavily refine the grains in the powder metallurgy superalloys. Furthermore, based on the present experiment and analysis, an appropriate solution treatment mechanism (i.e., 1050 °C for 2 h) was proposed for FGH4113A alloys.

Turning/Shot peening of Nickel-based powder metallurgy superalloy: Effect on surface integrity and high-temperature low-cycle fatigue properties

In this study, a nickel-based powder metallurgy alloy was processed via turning, ceramic bead peening, and shot peening by high-intensity cast-iron shots and low-intensity ceramic beads (denoted as compound shot peening). We investigated the surface morphology (surface stress concentration factor, Kst), the surface residual stress profile, microstructure, and the hardness gradient of three surface states and compared the high-temperature low-cycle fatigue lives. The results showed that the surface stress concentration coefficients after turning, ceramic bead peening, and compound shot peening were 2.39, 1.98, and 2.12, respectively. The average surface roughness, Sa, after turning was low, but the bottom of the tool mark was sharp, so the Kst value was higher than that after shot peening, which formed a round bottom by bead blasting. Compared with ceramic bead peening, a surface structure with a deeper compressive residual stress profile and a higher hardness gradient was formed after compound shot peening, and nanostructured features appeared on the outermost surface. The estimated low-cycle fatigue lives after the ceramic bead peening and compound shot peening were 0.99 and 3.13 times longer than after the turning process respectively with strain amplitude 0.5% at 538℃. Moreover the gain of compound shot peening on low cycle fatigue performance gradually decreases with the increase of the strain amplitude.

Understanding on processing temperature-metallographic microstructure-tensile property relationships of third-generation nickel-based superalloy WZ-A3 prepared by hot isostatic pressing

A third-generation powder metallurgy nickel-based superalloy (WZ-A3) was prepared by hot isostatic pressing (HIP) at 1100 °C, 1150 °C, 1200 °C, and 1250 °C, respectively. The characteristics of grains, precipitates, and elemental distribution revealed its microstructural development. The samples prepared at a sub-solvus temperature (1100 °C) consisted of fine recrystallized grains and dendritic structures. However, the microstructure became a completely recrystallized structure when the HIP temperature reached or exceeded 1150 °C. The average grain size increased from 4.0 µm (1100 °C) to 53.1 µm (1250 °C) due to the promoted recrystallization and atom diffusion rate. Prior particle boundaries (PPBs) with continuously distributed precipitates were identified as a mixture of (Nb, Ti)C carbides and Hf0.5Zr0.5O2 oxides. By increasing the HIP temperature from 1100 °C to 1200 °C, the PPBs were reduced but not eliminated. As a result, samples prepared at 1200 °C experienced interparticle debonding with relatively high strengths, and the ultimate tensile strength values were 1388.7 MPa and 1228.3 MPa at room temperature (RT) and 700 °C, respectively. When the HIP temperature was 1250 °C, the presence of coarse grains, large carbides, as well as eutectic structures formed at the junction of grain boundaries caused dramatic drops of tensile strengths and brittle fractures. These results emphasized the importance of parameter optimization for controlling the microstructure and mechanical properties during the HIPing process.

An innovative approach towards forming the serrated grain boundaries and refining the γ′ precipitates in nickel-based superalloys

The serrated grain boundaries have been proven to improve the creep performance and reduce crack growth rate of polycrystalline nickel-based superalloys. However, a slow cooling rate to form the serrated grain boundaries often results in the coarsening of γ′ precipitates, thereby deteriorating the high temperature strength of the nickel-based superalloys. Hereof, a new heat treatment processing, high temperature pre-precipitation treatment (HTPT), is developed to simultaneously achieve the serrated grain boundaries and fine γ′ precipitates. The effects of HTPT conditions on the microstructure of the powder metallurgy nickel-based superalloys are systematically characterized. Surprisingly, the HTPTed samples possess the serrated grain boundaries and obtain even finer γ′ precipitates than those in air cooled samples. With the increasing of holding time from 10 min to 30 min and 60 min at 1140 ℃ for pre-precipitation, the average size of tertiary γ′ precipitates decreases from 43.9 nm to 34.7 nm and 30.4 nm, and the amplitude of the serrated grain boundaries increases from 0.84 µm to 1.71 µm and 1.87 µm, respectively. On the other hand, the excessively long holding time causes the abnormal growth of the secondary γ′ precipitates and the suitable holding time is considered to be 30 min. The formation of the serrated grain boundary during HTPT is related to the growth of the pre-precipitated γ′ precipitates and the hindering effect on the migration of grain boundaries. Our work demonstrates a new way to control the microstructure of nickel-based superalloys.