Microstructure Of Ni-based Superalloy Research Articles

Multi-scale Modeling and Experimental Study on Microstructure of Ni-Based Superalloys in Additive Manufacturing

Multi-scale Modeling and Experimental Study on Microstructure of Ni-Based Superalloys in Additive Manufacturing

A novel coherent particles-reinforced FCC-based high-entropy superalloy with superior high-temperature compressive properties

A novel κʹ-strengthened FCC-based Al0.5CoFeNiC0.1 high entropy superalloy (HESA) with minor B2 phase was prepared by arc melting. The FCC/κʹ dual-phase microstructure with thermal stability is similar to γ/γʹ microstructure of Ni-based superalloys. Surprisedly, the large volume fraction (nearly 50%), high density and nano-scale coherent κʹ particles with uniform distribution are easily accessible only by direct solidification, which gives rise to superior compressive yield strengths at both room and elevated temperatures, reaching 1130 MPa at 25 °C and 830 MPa at 700 °C, respectively. The sustained high-temperature yield strength is contributed to both the antiphase boundaries (APBs) and stacking faults (SFs) shearing κʹ particles. Further, optics and electron microscopy analyses revealed the changes (transition point about at 0.6 Tm, where Tm is melting temperature) of the dominant deformation and failure mechanisms, dependent on the temperature. The dominant deformation mode is planar slip and SFs at relatively low temperatures, while the cross slip and dislocation dissociation above 800 °C (0.6 Tm), coupled with the diffusion-controlled recovery mechanisms. The failure mechanism shows failure mode from shearing fracture to thermal softening. Our κʹ-strengthened FCC-based superalloy provides a route for elevated-temperature applications.

The Hardness Evolution of Cast and the High-Cycle Fatigue Life Change of Wrought Ni-Base Superalloys after Additional Heat Treatment.

Concerning the use of modern technologies and manufacturing systems in the production of high-stress components from Ni-base superalloys and the optimization of the production process, knowledge of the microstructure–mechanical properties relationship is very important. The microstructure of Ni-base superalloys is very closely related to the chemical composition. With the high number of alloying elements, various phases are presented in the structure of Ni-base superalloys, which have a predominantly positive effect on the mechanical properties, but also phases that reduce, in particular, the heat resistance of these materials. The aim of the presented paper is the quantification of structural parameters of two types of cast alloys, ZhS6K and IN738, where the effect of dwell at 10 and 15 h at 800 °C on the change in morphology and volume fraction of the γ′-phase precipitate was studied. The detected changes were verified by the Vickers hardness test. The IN718 superalloy was chosen as a representative of the wrought superalloy. This alloy was also annealed for 72 h at a temperature of 800 °C, and the quantification of structural parameters was performed by EDS mapping and TEM analysis. Another partial goal was to assess the effect of changes in the volume fraction of the γ′-phase and δ-phase on the change in the high-cycle fatigue life of superalloy IN 718. This superalloy was tested by dynamic cyclic loading with cycle asymmetry parameter R = −1 at an ambient temperature of 22 ± 5 °C and at a temperature of 700 ± 5 °C and with cycle asymmetry parameter R < 1 (three-point bending load) after annealing at 700 °C/72 h. The results of the quantitative analyses and fatigue tests will be further used in optimizing the design of Ni-base superalloy components by modern technologies such as additive technologies for the production of turbine blades and implemented within the philosophy of Industry 4.0.

Effects of Precipitates and Oxide Dispersion on the High-temperature Mechanical Properties of ODS Ni-Based Superalloys

In this study, we investigated the effects of precipitates and oxide dispersoids on the high-temperature mechanical properties of oxide dispersion-strengthened (ODS) Ni-based super alloys. Two ODS Ni-based super alloy rods with different chemical compositions were fabricated by high-energy milling and hot extrusion process at 1150℃ to investigate the effects of precipitates on high-temperature mechanical properties. Further, the MA6000N alloy is an improvement over the commercial MA6000 alloy, and the KS6000 alloy has the same chemical composition as the MA6000 alloy. The phase and microstructure of Ni-based super alloys were investigated by X-ray diffraction and scanning electron microscopy. It was found that MC carbide precipitates and oxide dispersoids in the ODS Ni-based super alloys developed in this study may effectively improve high-temperature hardness and creep resistance.

Thermodynamic Analysis of the Effects of Alloying Elements on the Stacking Fault Energy in Ruthenium-Bearing Nickel Alloys

The effects of Al, Co, Re, and Ru on the stacking fault energy in Ni alloys were analyzed using computational thermodynamics. The effects of adding up to 5 at% Re or Ru to a Ni-15at%Co system were found to be weak at 300 °C, 700 °C, and 900 °C. However, Al addition decreased the stacking fault energy in a Ni-15at%Co-Xat%Ru system, where X = 0, 3, 5. In addition, this decrease in the stacking fault energy due to Al addition became more significant as the amount of Ru increased. Furthermore, in Ni–Co–Al–Ru alloys containing 9at%Al, the addition of 5at%Ru decreased the stacking fault energy as much as the addition of 12.5at%Co at 900 °C. The effects of Co and Ru addition on the γ/γ’ microstructure of Ni-based superalloys were also discussed.

A Review on Inertia and Linear Friction Welding of Ni-Based Superalloys

Inertia and linear friction welding are being increasingly used for near-net-shape manufacturing of high-value materials in aerospace and power generation gas turbines because of providing a better quality joint and offering many advantages over conventional fusion welding and mechanical joining techniques. In this paper, the published works up-to-date on inertia and linear friction welding of Ni-based superalloys are reviewed with the objective to make clarifications on discrepancies and uncertainties reported in literature regarding issues related to these two friction welding processes as well as microstructure, texture, and mechanical properties of the Ni-based superalloy weldments. Initially, the chemical composition and microstructure of Ni-based superalloys that contribute to the quality of the joint are reviewed briefly. Then, problems related to fusion welding of these alloys are addressed with due consideration of inertia and linear friction welding as alternative techniques. The fundamentals of inertia and linear friction welding processes are analyzed next with emphasis on the bonding mechanisms and evolution of temperature and strain rate across the weld interface. Microstructural features, texture development, residual stresses, and mechanical properties of similar and dissimilar polycrystalline and single crystal Ni-based superalloy weldments are discussed next. Then, application of inertia and linear friction welding for joining Ni-based superalloys and related advantages over fusion welding, mechanical joining, and machining are explained briefly. Finally, present scientific and technological challenges facing inertia and linear friction welding of Ni-based superalloys including those related to modeling of these processes are addressed.

Effect of a high magnetic field on microstructures of Ni-based superalloy during directional solidification

The effect of a high magnetic field on the dendrite morphology of superalloy DZ417G during directional solidification at a low solidification velocity has been investigated experimentally. It was found that the magnetic field induces columnar to equiaxed transition (CET) and makes the primary dendrite arm spacing decrease. In addition, the magnetic field causes deformation of the solid–liquid interface shape and the macrosegregation in the mushy zone. Based on these results, it was found that both of the thermoelectric magnetic convection (TEMC) and the thermoelectric magnetic force (TEMF) cause CET, the change of solid–liquid interface shape and the formation of macrosegregation. This is in good agreement with predicted values of the TEMC and TEMF, respectively. The primary dendrite arm spacing was changed by the interdendritic TEMC in the magnetic field.

Dislocations in a Ni-based superalloy during low temperature creep

The nature and variety of the dislocations passing through the two-phase γγ′ microstructure of Ni-based superalloys is key to the properties of these materials. The chemistry, size and arrangement of the precipitates greatly affects the nature of these dislocations. We present High Angle Annular Dark Field (HAADF) TEM observations of the structure of dislocations entering, passing through the γ′ precipitates in the single-crystal superalloy CMSX-4®. The creep deformation of the sample was interrupted after 8 hours at 750 °C and 750 MPa, a critical stage just as secondary creep was being established, and shows a range of defects in both phases, not always those predicted by the Schmid factor for the deformation geometry. We show that dislocations lodged in the γγ′ interfaces have a significant effect on the structure of the interface and that they combine to produce stacking faults which cut through the γ′. The implications of these observations for secondary creep deformation are discussed.

Effect of cooling rates from partial solution temperature and aging on γ′ precipitation in IN792 superalloy

One of the important factors affecting the microstructure of Ni base superalloys is the cooling rate from partial solution temperature; therefore, the prime purpose of the present research is to investigate the effects of this factor on some microstructural features of superalloy IN792. The results showed that by increasing the cooling rate from 5 to 160°C min−1 after partial solution treatment at 1120°C for 2 h, the size and volume fraction of primary γ′ decreased, but during the subsequent aging the volume fraction of secondary γ′ increased initially exponentially and later linearly. When high cooling rates >90°C min−1 were applied, the formation of secondary γ′ was totally suppressed. However, after aging at 845°C for a period of 24 h, the amount of secondary γ′ increased with a decelerating rate up to 60°C min−1 and then with a linear rate up to 160°C min−1. The later observation has not been reported elsewhere.

Influence of casting conditions on the solution treated microstructure of Ni-base superalloy

Abstract Ni-base superalloy specimens were cast under different superheat levels, low and high, and various cooling rates. Solution treatment was carried out for 2 h at 1120 °C or 1180 °C then air cooled. The solution treatment at 1120 °C represents the standard solution treatment (SST) while that at 1180 °C represents the modified solution treatment (MST). DSC measurements, SEM and EDS emphasized that the α nodular and interdendritic γ/γ' phases found in as cast structure were entirely removed after solution treatment either at 1120 °C or 1180 °C. The volume fraction of η phase in microstructure of all specimens decreased with MST in comparison with SST. After SST and MST, needle like η phase had disappeared except in alloy H1 that was produced under lower cooling rate and high superheat, with SST. In addition to needle like shape, platelet η phase was found in the microstructure of H1 alloy after SST. The microstructure of other alloys contains η phase only with platelet shape. Generated metastable nano-scale γ' particles precipitated in alternating layers with η phase at the expense of transformed η phase. At the platelet η phase circumference, new γ phase with high Cr content was formed at the expense of the dissolved η phase during solution treatment.

Microstructure of creep-exposed single crystal nickel base superalloy CSMX4

Main changes which appear in microstructure of Ni base superalloys during creep exposure is the directional coarsening (rafting) of originally cubic γ' precipitates embedded in γ matrix. The progress in γ' degradation can be assessed with help of the analysis of geometrical parameters of the γ' microstructure. Determination of the morphological changes of γ' in creep-exposed single-crystal Ni-base superalloy CMSX4 using SANS was carried out. A new sample design (conic specimen) was tested in order to get continually changing stress levels in one sample and corresponding variety of micro structural change. Evolution of rafts with stress magnitude is clearly observable from the parameters obtained using SANS (the mean thickness of the γ channel in different crystallographic directions of the single-crystal, the evolution of the γ' precipitate size and distance, the evolution of the specific interface). The use of conic specimen and spatial scan facilitate determination of microstructure evolution in dependence on the exposure parameters.

Influence of Ru and Cr on the Heat-Treated Microstructure of Ni-Based Superalloys

The effects of ruthenium and chromium contents have been investigated on phase transformation temperatures and the morphology of γ’ precipitates as well as microstructural stability in high refractory Ni-base superalloys. The solidus and liquidus temperatures were determined by differential scanning calorimetry (DSC), suggesting that the addition of Cr resulted in a decrease in solidus/liquidus temperatures while the Ru addition (3.5 at.%) had the neutral effect. The morphology of γ’ precipitates in the heat-treated microstructure was changed under the influence of the Cr and Ru additions, suggesting that Ru and Cr contents affected the γ−γ’ lattice misfit through changes in the associated partitioning to the constituent phases. The microstructural instability has been investigated at 1000 oC. High levels of Cr addition (8 at.%) strongly promoted the formation of TCP phases, while Ru improved the microstructural stability to some extent.

ＣＭＳＸ４のクリープ強度におよぼす時効熱処理の影響

In single crystals of Ni-based superalloys, the γ' precipitate size has been shown to affect both the tensile strength and the creep behavior at intermediate temperatures. The microstructure of Ni-based superalloys contains primary cuboidal γ' particles that precipitate during aging treatment, and cooling γ' particles that precipitate during cooling from the aging temperature. In the Ni-based superalloy containing a high volume fraction of γ' precipitates, the change in the morphology of the secondary γ' precipitates in a matrix channel has been found to affect the creep strength. In this study, three kinds of aged specimens were prepared to investigate the influence of the morphology of γ' precipitates on the strengths of CMSX4 alloy. Slow furnace cooling from the aging temperature was employed to obtain a clean matrix channel completely devoid of secondary γ' precipitates. In order to clarify the effect of secondary γ' precipitates, the strengths of the furnace-cooled specimens were compared with those of the forced-airquenched specimens containing numerous superfine γ' precipitates in the matrix channel, and those of the standard two-step-aged specimens. The purpose of the present investigation is to gain a better understanding of the effect of precipitation heat treatment on the material strengths at an intermediate temperature, where the secondary γ' precipitates remain undissolved during creep deformation.

Different Behavior between Edge and Screw Dislocations at the .GAMMA./.GAMMA.' Interface of Ni-Based Superalloy: A Molecular Dynamics Study

Different behaviors between the edge and screw dislocations in the γ/γ'microstructure of Ni-based superalloy are investigated by using molecular dynamics simulations. The simulations are implemented with a slab cell of Ni matrix involving an apex of a cuboidal Ni3A1 precipitate, that mimics a part of the idealized γ/γ' microstructure with about 3 million atoms. Mode II and III type displacements are applied on the cell to nucleate the edge and screw dislocations, respectively, and proceed them toward the precipitate. The edge dislocation decreases its velocity at the γ' precipitate, showing dislocation pinning there, then penetrates it under the force from the follow-on dislocation.The screw dislocation runs through the precipitate without slowdown by shrinking the with between its Shockley partials. Detail investigation of the stress distribution suggests that the constriction is due to interactions between the stress field around the precipitate and the partials : the stress causes repulsive Peach-Koehler force on the leading partial and attractive on the trailing since their edge components have opposite Burgers vectors. In addition to the fact that the strain energy of a screw dislocation is smaller than that of an edge, the constricted screw dislocation makes smaller atomic step on the γ' precipitate than the extended edge dislocation. Thus the screw dislocation easily penetrates the precipitate while the edge is pinned.

γ/γ'の格子ミスフィットに起因した転位挙動 : 離散転位動力学法による検討(J08-3 転位・変形,J08 原子系の計算力学シミュレーション)

In the present study, a DDD-FEM analysis is conducted on the dislocation behavior in the γ/γ' microstructure of Ni-based superalloys. Tensile/compressive stress is applied on the γ' precipitates in the FEM, in order to clarify the effect of internal stress due to the lattice misfit. The dislocation approaching the γ' precipitate is decelerated by the negative stress of γ', while that is accelerated by the positive stress. Detail observation on the distribution of shear stress suggests that the stress becomes negative near the γ/γ' interface, resulting in the repalsive force on the approaching dislocation.

Small-Angle Neutron Scattering: A Tool for Microstructural Investigation of High-Temperature Materials

The suitability of SANS for microstructural investigation of materials for high- temperature applications is highlighted. The presented examples of SANS application were oriented namely to the characterization of microstructure of Ni-base superalloys. The bulk parameters of single-crystal alloy SCA433 were determined from the measured 2D data at two sample orientations. The in-situ SANS study of polycrystalline alloy Re13 led to the determination of the suitable solution treatment procedure.

Molecular Dynamics Simulation of Edge Dislocation Piled at Cuboidal Precipitate in Ni-Based Superalloy.

In order to clarify the fundamental mechanism of dislocations in the γ/γ' microstructure of Ni-based superalloy, three molecular dynamics simulations are conducted on the behavior of edge dislocations nucleated from a free surface and proceeding in the pure Ni matrix (γ) toward cuboidal Ni3Al precipitates (γ') under shear force. One involves dislocations near the apices of two precipitates adjoining each other with the distance of 0.04 μm, as large as the width of the γ channel in real superalloys. Others simulate dislocations piled at the precipitates as well, however, the scale of the microstructure is smaller than that in real superalloys by one order of magnitude, and one of them have precipitates with atomistically sharp edge. Dislocations are pinned at precipitates and bowed-out in the γ channel, then they begin to penetrate into the precipitate at the edge in both the real-scale and smaller microstructures when the precipitates have blunt edges. On the other hand, an edge dislocation splits into a superpartial in the γ' precipitate and a misfit screw dislocation bridging between two adjacent precipitates at the atomistically sharp edge of γ' precipitates. It is also observed that two superpartials glide in the precipitate as a superdislocation with anti-phase boundary (APB), of which the width is evaluated to be about 4 nm.

X-ray Diffraction Investigations on Individual Grains in the Polycrystalline Ni-base Superalloy IN939 During Cyclice Loading I: X-ray Rocking Curve Broadening

X-ray diffraction investigations were carried out to prove that X-ray methods can be applied for the assessment of fatigue-induced changes in the microstructure of Ni-base superalloys. Since the investigations were performed on single grains of a coarse grained alloy (IN 939), analysis of X-ray rocking curves can be applied. It was shown that the width of a rocking curve is sensitive to even small plastic deformations and that for the given experimental conditions the broadening can be taken as a measure for the accumulated plastic strain resulting from cyclic loading. The observed anisotropic behaviour of rocking curve broadening could be explained by the concept involving activation of selected slip systems