Nickel-based Powder Metallurgy Research Articles

Role of inclusion clusters on fatigue crack initiation in powder metallurgy nickel-based FGH96 superalloy

Inclusion clusters with a size of several tens of micrometers were found to be inclined to early crack initiation, resulting in an abnormally reduced fatigue life. Fatigue tests were conducted to elucidate crack initiation behaviors of inclusion clusters in nickel-based powder metallurgy (P/M) FGH96 superalloy. The inclusion clusters with varying sizes and shapes located at grain boundaries are composed of Al2O3, MgO and ZrO2 particles. The inclusion cluster with large size within 25-35 μm, small average particle distance within 0-4 μm and large orientation relative to loading direction within 35-66° will be more likely to cause crack initiation. In particular, the inclusion clusters with large size and high aggregation degree experience extruding during processing and subsequent heat-treatments due to the difference in thermal expansion coefficient with the matrix behave multiple cracking before fatigue loading, thereby facilitating crack initiation in the surrounding matrix grains. Moreover, as the main cracking part of inclusion clusters, the Al2O3 particles exhibit self-cracking and interface debonding, owing to higher hardness and poorer deformation coordination degree with matrix compared to other component inclusion particles.

Thermal processing behaviour and microstructural evolution of high W and high γ’ content extruded nickel-based powder metallurgy superalloy FGH4109

ABSTRACT Nickel-based powder metallurgy superalloys with high W and high γ’ content exhibit excellent high-temperature properties, but their high deformation resistance poses significant challenges for hot processing. In this study, the hot compression behaviour of an extruded nickel-based powder superalloy FGH4109 with high W (6.1%) and high γ’ phase contents (60%) was systematically investigated at temperatures ranging from 1060 ℃ ~ 1140 ℃, strain rates from 0.001 s−1 ~1 s−1, and maximum true strains of 0.7. Combined with the hot working map and the microstructure evolution in different hot working zones, the optimal hot working window for the alloy was determined to be in the temperature range of 1060 ℃ ~ 1140 ℃ and strain rates of 0.001 s−1 ~0.012 s−1, where the power dissipation efficiency η was greater than 0.5, and the dominant deformation mechanism was discontinuous dynamic recrystallisation.

Study on surface chemical composition of argon atomized superalloy powder based on TOF-SIMS

Nickel-based powder metallurgy superalloys have been extensively employed in aerospace industry because of their superior performance. However, severe defects can be formed in primary particle boundaries (PPB) during manufacturing process of superalloys, significantly deteriorating their practical performance. This study comprehensively investigates the surface structure, elemental distribution, and chemical state of FGH96 superalloy powder via several advanced surface characterization techniques. The results indicate that the strong binding affinity between metal elements (e.g., Al, Cr, Ti) and O/H elements contributes to the formation of an oxide layer in both surface and interval areas, providing an essential chemical basis for the development of PPB. Furthermore, Ti and Al exhibit the highest binding strength with O, which may form stable oxides. Hopefully, these findings will facilitate the emergence of PPB-free superalloys with improved performance.

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.

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.

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.

Effect of Semi-Aging Heat Treatment on Microstructure and Mechanical Properties of an Inertia Friction Welded Joint of FGH96 Powder Metallurgy Superalloy

Inertia friction welded joints often present different microstructures than the base metal, and subsequent heat treatment processes are always needed to maintain superior performance. This study investigates the effect of semi-aging heat treatment after welding on the microstructure, residual stress, micro-hardness, and tensile properties of inertia friction welded FGH96 powder metallurgy superalloy using optical microscopy, scanning electron microscopy, X-ray diffraction, and hardness and tensile tests. The results show that the semi-aging heat treatment after welding does not affect the grain size or grain morphology of the base metal. However, the recrystallization process can be further promoted in the weld nugget zone and transition zone. Meanwhile, the grain size is refined and the residual stress is significantly reduced in the welded joint after the same heat treatment. Under the synergetic strengthening effect of the γ′ phase, semi-aging heat treatment increased the micro-hardness of the weld nugget zone from 470 HV to 530 HV and improved the average tensile strength at room temperature by 118 MPa. These findings provide a reference for the selection of the heat treatment process after inertia friction welding of nickel-based powder metallurgy superalloys.

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.

Effect of Boron and Carbon on the Hot Deformation Behavior of a Novel Third Generation Nickel-Based Powder Metallurgy Superalloy WZ-A3

Effect of Boron and Carbon on the Hot Deformation Behavior of a Novel Third Generation Nickel-Based Powder Metallurgy Superalloy WZ-A3

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

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

Gradient microstructure evolution under thermo-mechanical coupling effects for a nickel-based powder metallurgy superalloy－Dynamic recrystallization coexist with static recrystallization

In order to withstand special working conditions, it is of great significance to manufacture turbine disks with a gradient microstructure (disk core with fine and uniform grains and disk edge with coarse grains) along the radial direction. A new and effective forming way-gradient hot deformation is proposed, and a gradient structure from the edge to the center can be obtained. In the present work, the gradient deformation behavior, dynamic recrystallization kinetics, and microstructural evolution of a nickel-based powder metallurgy superalloy are studied by thermal gradient compression. Dynamic recrystallization kinetics of this superalloy is established. Recrystallization fraction increases from 72.6 %–93.6 % because of the gradient deformation degree. Microstructure changes significantly from the center to the edge along the radial direction, and average grain size ranges from 6.65 μm to 15.62 μm. There is a big difference between experimental value and calculated data of recrystallization fraction, which indicates that dynamic and static recrystallization take place in different areas simultaneously. Static recrystallization can coexist with dynamic recrystallization in the trapezoid sample by gradient hot deformation. The difference between experimental and calculated results is the static recrystallization, and the maximum values of static recrystallization fraction is around 30 %. A great number of dynamic recrystallization grains distribute along pre-existing grain boundary, second phases and twin crystal. The main means of static recrystallization nucleation is the bulge of original grain boundaries.

Influences of a Hot-Working Process on the Microstructural Evolution and Creep Performance of a Spray-Formed Nickel-Based Superalloy

A new third generation nickel-based powder metallurgy (PM) superalloy, designated as FGH100L, was prepared by spray forming. The effects of hot isostatic pressing (HIP) and isothermal forging (IF) processes on the creep performance, microstructure, fracture, and creep deformation mechanism of the alloy were studied. The microstructure and fracture were characterized by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The coupled HIP and IF process improved the creep performance of the alloy under the creep condition of 705 °C/897 MPa. As for both the HIPed and IFed alloys, the creep process was dominated by the accumulation of dislocations and stacking faults, cutting through γ′ precipitates. The microstructural evolution was the main factor affecting the creep performance, which mainly manifested as coarsening, splitting, and morphology change of γ′ precipitates. Both the creep fractures of the HIPed and IFed alloys indicated intergranular fracture characteristics. In the former, wedge-shaped cracks usually initiated at the trigeminal intersection of the grain boundaries, while in the latter, cavity cracks generate more easily around the serrated curved grain boundary and carbides.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Nickel-Based Powder Metallurgy Superalloy Processed by Selective Laser Melting

Effect of Heat Treatment on Microstructure and Mechanical Properties of Nickel-Based Powder Metallurgy Superalloy Processed by Selective Laser Melting