Precipitation Of Topologically Close-packed Phase Research Articles

Crystallographic texture and multiscale boundaries mediated creep anisotropy in additively manufactured Ni-based Hastelloy C276 superalloy

The microstructure and high-temperature creep mechanisms of Ni-based Hastelloy C276 superalloy fabricated using wire and arc-based directed energy deposition were investigated systematically and innovatively. The microstructural investigation revealed that the as-fabricated samples comprise γ-Ni matrix and topologically close-packed (TCP) P phase precipitates. The γ matrix subgrains and grains are spread over multiple highly textured columnar dendrites, with a majority of γ <001> crystallographic orientations closely aligned along the deposition direction. Moreover, the interdendritic regions exhibit severe Mo segregation, P phase particles, and dislocation bands. Creep tests were conducted on miniature samples under various temperature and stress conditions, loaded either in the deposition direction (DD) or travel direction (TD). DD samples exhibit lower minimum strain rates, greater strains-to-failure, and longer creep rupture lifetimes than TD samples, indicating significant creep anisotropy. Dislocation creep was identified as the primary creep mechanism for both DD and TD conditions. During creep, dynamic precipitation of TCP phases occurred in the interdendritic regions, resulting in varying creep resistance between interdendritic and dendritic core regions. Isostress and isostrain models, considering both crystallographic texture and precipitation strengthening, reasonably predicted the observed creep anisotropy during the secondary creep stage. Additionally, variations in the Schmid factor led to significant deformation incompatibility among dendrites in TD samples. Dislocation accumulation in TD sample interdendritic regions promoted new grain nucleation, triggering dynamic recrystallisation, facilitating grain boundary sliding, and accelerating tertiary creep. Furthermore, TCP phase particles in the interdendritic regions contributed to microcrack development, further accelerating creep fracture, especially in the TD condition.

A fabrication of low diffused PtHf co-doped Ni-Al coating on Ni-based superalloy

Two PtHf co-doped Ni-Al coatings were fabricated by the following steps: first, NiHf layer and Pt layer were electroplated on the substrate, then vacuum annealed at 800 ℃ for 5 h; Second, Al layer was deposited via arc ion plating (AIP); Finally, vacuum annealing was proceeded at 1050 ℃ for 2 h. The Al contents in PtHf co-doped γʹ coating and PtHf co-doped γʹ+β coating were about 24 at.% and 42 at.%, respectively. In addition, no topologically close-packed (TCP) phases were precipitated in the diffusion affected zone (DAZ) in the PtHf co-doped γʹ coating, while many TCP phases were precipitated in the DAZ in the PtHf co-doped γʹ+β coating. The interdiffusion coefficient of Al gradually decreased with the reduction of Al content and the increase of Pt content in coating, consequently suppressing the precipitation of TCP phases.

Precipitation of TCP phases with R/P intergrowth structure during directional solidification in a Ru-containing nickel-based single crystal superalloy

The topologically close-packed (TCP) phases with R/P intergrowth structure formed at the final stage of directional solidification in a Ru-containing nickel-based single crystal (Ni-SX) superalloy were investigated. The nodular TCP phases were distributed in the interdendritic regions of the as-cast microstructure. Precipitation of the β-NiAl phases and the subsequent incomplete peritectic reaction (Liquid + β-NiAl → γ′ + β-NiAlresidual) resulted in the accumulation of the TCP-forming elements (Cr, Co, Mo, W, Ta, and Re) in the residual liquid around the β-NiAl phase, which provided a favorable nucleation condition, leading to the precipitation of TCP phase in the vicinity of the β-NiAl phases. In addition, the TCP phases were found to be the R/P intergrowth structure with an orientation relationship of [2,7,10]R//[001]P and (322¯)R//(200)P, (44¯2)R ∼3.2° from (11¯0)P. Further, the formation mechanism of these TCP phases with R/P intergrowth structure was systematically discussed in terms of crystal structure, interface characteristics, and chemical composition. The result can provide theoretical guidance and experimental data for the composition design and heat treatment process optimization of the novel Ru-containing high-generation Ni-SX superalloys.

Effect of Mo on microstructural stability of a 4th generation Ni-based single crystal superalloy

The microstructural stability during thermal exposure at 900 °C of Ni-based single crystal superalloys with various Mo content was investigated. The coarsening of γ′ precipitates and formation of TCP (topologically close-packed) phases were discussed in detail. The LSW theory and TIDC theory were applied to study the coarsening behavior of γ′ particles, and both models fitted well with the experimental results. Increasing Mo content enhanced the partitioning behavior of alloying elements, thus increasing the lattice misfit and interfacial stress of γ′/γ phases. Additionally, Mo addition also decreased γ′ coarsening rate by retarding the diffusion process of elements. However, Mo addition significantly promoted the precipitation of TCP phases, consuming massive solution strengthener elements, such as Re, W and Mo. TCP precipitation may significantly limit the creep life at 900 °C.

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa. Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared. For specimen with higher inhomogeneity, stronger segregation of refractory elements reinforces the local strength in dendrite core, but damages the strength in interdendritic region. Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing in γ′ phase, faster degradation of dislocation networks and facilitated topological inversion of rated structures. Although the segregation of refractory elements produces a high density of topologically close-packed (TCP) phase in dendrite core, faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen. In another specimen, increased solid solution time gives rise to overall reduced inhomogeneity. Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region. The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity. Importantly, the balance of local strength between dendrite core and interdendritic region results in over 40% increase of creep rupture life of the second specimen.

Creep prediction model for nickel-based single-crystal superalloys considering precipitation of TCP phase

The microstructure of nickel-based single-crystal (SC) superalloys has a pivotal influence on their creep properties. The addition of the Re element not only enhances the long-term creep properties of nickel-based SC superalloys, but also results in the formation of a topologically close-packed (TCP) phase which is a harmful and brittle hard phase. Here, high-temperature creep interruption tests of a nickel-based SC superalloy that contains 4.8 wt% Re were performed under various temperatures and stress conditions, and the evolution of microstructure during creep was observed by scanning electron microscopy (SEM). The volume fraction of the TCP phase was also extracted to explore the mechanism that controls the impacts of the TCP phase on the creep properties. According to the microstructure evolution mechanism, the influence of the TCP phase was attributed to the initial damage and critical shear stress of the material. A creep performance prediction model for nickel-based SC superalloys considering the precipitation of the TCP phase that is based on the crystal plasticity theory and a modified creep damage model was established. The simulation curves fit well with the experimental results and the errors between prediction creep life with test results are within 5%.

Effect of Ta on microstructural evolution of NiCrAlYSi coated Ni-base single crystal superalloys

The effect of Ta on microstructural evolution of NiCrAlYSi coated Ni-base single crystal superalloys at 1000 °C and 1100 °C has been investigated. Interdiffusion occurs between the NiCrAlYSi coating and the substrate, leading to the formation of inter-diffusion zone (IDZ) and secondary reaction zone (SRZ). After exposure at 1000 °C for 100 h, the initial γ/γ′ transforms to γ′ phases in the IDZ, and SRZ with σ and μ topologically close-packed (TCP) phases is formed primarily due to the inward diffusion of Al. Different behavior of SRZ formation in the alloys with various Ta content can be observed after exposure at 1000 °C, which highlights the potency of Ta addition in promoting SRZ formation. This may be ascribed to the fact that Ta promotes the precipitation of TCP phases. After exposure at 1100 °C for 100 h, the initial γ/γ′ transforms to γ layer and coarse γ′ phases in the IDZ, and no SRZ can be found. The formation of IDZ is significantly suppressed by Ta addition, which may be related to the retardation of interdiffusion between the coating and the substrate due to the increased γ′ volume fraction. Furthermore, γ′ rafting parallel to and/or perpendicular to the coating/substrate interface in the substrate beneath IDZ are found in the alloys after exposure at 1000 °C and 1100 °C. Microstructural observation shows that Ta has a potent influence on the development of perpendicular γ′ rafting. The mechanism of parallel γ′ rafting and perpendicular γ′ rafting has been discussed, respectively.

First-principles theoretical and experimental studies of effects of ruthenium on precipitation behavior of μ phase and μ/matrix interface stability in Ni-based single crystal superalloys

Precipitation of deleterious topologically close-packed (TCP) phases worsens the mechanical behavior of Ni-based single crystal superalloys. It is acknowledged that Ru will inhibit precipitation of TCP phase. In this paper, precipitation behaviors of TCP in alloy with and without Ru have been investigated through experimental characterization means, combined with the first principles calculations. For the first time, the interface structure between precipitated μ phase and matrix γ phase has been established, and stable interfacial termination types in different orientation relationships were obtained. Effects of solute atoms like Ru on interface stability and site substitution behaviors of different solute elements in two phases were further calculated, and compared with precipitation behavior and composition information of TCP phases in two alloys. It is verified that Ru is different with other elements, which can remarkably increase the interfacial energy of μ/γ interface, decrease interface stability, and rise the difficulty of nucleation of μ phase; conversely, W will lower the interfacial energy of μ/γ, making it easier for μ phase to nucleate; the effects of Co and Cr are not that obvious; the effects of Re and Mo on interfacial energy and nucleation of μ phase vary with orientation relationships. Also, Re, Cr, and Ru have larger trends to diffuse from matrix γ to μ phase during growing process of μ phase. The above reasons give rise to blocking effect of Ru on the nucleation of TCP phase, but Ru has trifling influence on inhibition of precipitation growth.

Stress-induced formation of TCP phases during high temperature low cycle fatigue loading of the single-crystal Ni-base superalloy ERBO/1

The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states.

Stress-Induced Formation of TCP Phases During High Temperature Low Cycle Fatigue Loading of the Single-Crystal Ni-Base Superalloy ERBO/1

The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950°C (load-ratio: 0.6, tensile stress range: 420 to 740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 2000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only µ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary µ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states.

σ and η Phase formation in advanced polycrystalline Ni-base superalloys

In polycrystalline Ni-base superalloys, grain boundary precipitation of secondary phases can be significant due to the effects they pose on the mechanical properties. As new alloying concepts for polycrystalline Ni-base superalloys are being developed to extend their temperature capability, the effect of increasing levels of Nb alloying additions on long term phase stability and the formation of topologically close packed (TCP) phases needs to be studied. Elevated levels of Nb can result in increased matrix supersaturation and promote the precipitation of secondary phases. Long term thermal exposures on two experimental powder processed Ni-base superalloys containing various levels of Nb were completed to assess the stability and precipitation of TCP phases. It was found that additions of Nb promoted the precipitation of η-Ni6AlNb along the grain boundaries in powder processed, polycrystalline Ni-base superalloys, while reduced Nb levels favored the precipitation of blocky Cr and Mo – rich σ phase precipitates along the grain boundary. Evaluation of the thermodynamic stability of these two phases in both alloys using Thermo-calc showed that while σ phase predictions are fairly accurate, predictions of the η phase are limited.

Influence of W, Re, Cr, and Mo on microstructural stability of the third generation Ni-based single crystal superalloys

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Effect of Co on microstructural stability of the third generation Ni-based single crystal superalloys

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Effects of platinum group metals addition on the precipitation of topologically close-packed phase in Ni-base single crystal superalloys

Precipitation of topologically close-packed (TCP) phase in Ni-base single crystal superalloys with additions of platinum group metals (PGMs) was investigated by in situ transmission electron microscopy (TEM). Pt, Ir and Ru additions are all helpful to suppress the precipitation of TCP phase, among which Ru has the largest efficiency while Pt has the least one. Comparing with the TCP precipitation in non-PGMs alloys, addition of Ru mainly retards the nucleation rate of TCP phase, but Pt and Ir mainly reduces the growth rate of TCP precipitates. Elemental partitioning investigation reveals that Pt and Ir prefer partitioning to the surrounding γ matrix rather than in the precipitated μ phase, and Ru distributes homogenously in both γ matrix and μ phase. Cr distribution in precipitated μ phase is higher in Pt/Ir-containing alloys than in Ru-containing alloy, which is supposed to be related to the different metallic bonding energy of Cr–Pt, Cr–Ir and Cr–Ru.

Influences of ruthenium and crystallographic orientation on creep behavior of aluminized nickel-base single crystal superalloys

The influences of ruthenium and surface orientation on creep behavior of aluminized Ni-base single crystal superalloys were investigated by comparing two different types of NKH superalloys. The aluminized coated specimens were then subjected to creep rupture tests at a temperature of 900°C and a stress of 392MPa. The coating treatment resulted in a significant decrease in creep rupture lives for both superalloys. The diffusion zones between the coating and substrate led to changes in microstructure, which diminished the creep behavior of the aluminized superalloys. Because of the interdiffusion of Ru, Al and Ni, the solubility of some of the refractory elements, such as W, Re. Mo, Co and Cr decreased in the diffusion zone; the precipitation of topologically close-packed (TCP) phases was thus inevitable. In the present study, the addition of Ru increased the degree of Re and Cr supersaturation in the γ matrix. Consequently, the addition of Ru indirectly promoted the precipitation of TCP phases in aluminized Ni-base single crystal superalloys. Furthermore, the growth of TCP precipitates was greatly influenced by the specific surface orientations of the Ni-base single crystal superalloys. In conclusion, the {110} specimens showed shorter creep rupture life than the {100} specimens, this was due to the difference in the crystallographic geometry of {111}〈101〉 slip system and TCP precipitates between the two side-surface orientations of the specimens.

Effect of long-term aging on the fatigue crack growth rate of a nickel-based superalloy

Fatigue crack growth rate test was conducted for a nickel-based superalloy at room temperature to investigate the effect of long-term aging at 700 °C and 750 °C. It is found that the precipitation of topologically close packed (TCP) phases as well as the coarsening of γ′ phase and carbides at grain boundaries is induced by long-term aging. Fatigue crack growth rate of the superalloy is increased by long-term aging except that the alloy under aging at 700 °C for 500 h exhibits better crack growth resistance in the near-threshold region than the alloy under standard heat treatment. The difference in fatigue crack growth rate at the transition point from Paris regime to near-threshold regime can be rationalized by considering crack closure effect, crack tip stress shielding effect and crack tip effective stress intensity factor range. The precipitation of TCP phases and coarsening of γ′ phase cause changes in the three aspects. The crack closure effect and crack tip stress shielding effect are mainly induced by roughness and crack branching, respectively.

Topologically close-packed phase promotion in a Ru-containing single crystal superalloy

It is widely accepted that Ru suppresses the formation of topologically close-packed (TCP) phases in advanced Ni-based single crystal superalloys. However, in the current study, the addition of a large amount of Ru promoted the precipitation of TCP phases in a Ni-based single crystal superalloy containing a high level of Cr during thermal exposure at both 1000 and 1100 °C. It is suggested that the addition of Ru increased the degree of Re and Cr supersaturation in the γ phase and resulted in the TCP phase formation.

Topologically close-packed phase precipitation and thermal stability in alloy 22

The thermal stability of Alloy 22 affected by the precipitation of topologically close-packed (TCP) phases was investigated by means of experimental measurements and thermodynamic calculations. Both mill-annealed and welded specimens were exposed to various thermal aging and solution-annealing treatments. Microstructural analyses showed progressive precipitation of TCP phases in both the mill-annealed and welded specimens upon aging at 870 °C. Solution annealing of the welded material results in homogenization of the fusion zone; however, a high annealing temperature of 1300 °C leads to undesirable grain growth. All aging and solutionizing treatments of the welded material enhance precipitation of the secondary phases. Thermodynamic calculations predicted a solvus temperature of 1271 °C for the TCP phase in the interdendritic region as a result of Mo segregation during the solidification of the welded metal. This model prediction is consistent with experimental results showing that precipitates are observed in the welded material after various solution-annealing treatments. Results obtained from the present study suggest that solution-annealing treatments for Alloy 22 disposal containers should be carefully evaluated to assure that a homogeneous single-phase structure will be obtained.

第４世代Ｎｉ基超合金のＴＣＰ相生成に関するＴＴＴ線図

To improve the high temperature strength, solid solution strengthening elements with amounts well above the solubility limit imposed by the solvus, have been added to high-strength Ni-base superalloys. Under long-term exposure at elevated temperatures, the formation of Tertiary phases causes a serious degradation in terms of creep strength. The Tertiary phases, also known as Topologically Close Packed (TCP) phases, and can be classified into μ, P, R, and σ types. Recently, it has been found that Platinum Group Metals (PGMs) elements are effective in controlling precipitation of TCP phases, although the exact mechanism of this phenomenon is not well understood.This study was carried out based on a 3rd generation Ni-base superalloy, TMS-121, which contains 5 mass pct rhenium and is known to precipitate TCP phases; a series of alloys with different amount of ruthenium additions to TMS-121 were cast and heat treated in a variety of time and temperature conditions. Based on the microstructural examinations, Time Temperature Transformation (TTT) diagrams were constructed by heating the alloys at different aging time and temperatures. These diagrams indicated the critical amount of TCP phases that were known to deteriorate the high temperature strength. It was concluded that Ru addition is effective in stabilizing the microstructure by increasing the solubility of solid solution strengthening elements in Ni-base superalloys at high temperatures. It was also found that delaying the precipitation of TCP phases at all temperature ranges.

Development of an iridium–tantalum modified aluminide coating as a diffusion barrier on nickel-base single crystal superalloy TMS-75

An iridium–tantalum modified aluminized (Ir–Ta–Al) coating obtained by a two-step process is investigated. Ir–71 at.% Ta alloy was first coated on a nickel-base single-crystal superalloy TMS-75 by an electron beam physical vapor deposition (EB-PVD), and the resultant Ir–Ta alloy coating on the TMS-75 was treated by a conventional low-activity pack-cementation aluminizing process. It is found that a triplex layer structure, Ir–Ta alloy layer/β-NiAl/Al-diffusion zone, is formed in the order from the surface. In order to assess the oxidation resistance of the coated material, cyclic oxidation tests were carried out at 1373 K in air. It is found that Ir–Ta–Al coated TMS-75 had better thermal cyclic oxidation resistance than simply aluminized TMS-75. The microstructural evolution caused by the interdiffusion of solute elements shows remarkable difference on the precipitation of detrimental phases such as topologically close-packed (TCP) phase. The precipitation of TCP phase in the Ir–Ta–Al-coated TMS-75 was limited only within 180 μm in depth from the surface, whereas in the simply aluminized TMS-75, such detrimental phases were precipitated to the depth of more than 300 μm from the surface, indicating that the Ir–Ta–Al coating functions as an effective diffusion barrier for the alloying elements.