Topologically Close-packed Phases Research Articles

Grain Boundary Precipitation Control of GCP Phase Using TCP or A2 Phase in Ni-Based Alloys

To cover the grain boundary (GB) of the Ni phase with precipitates, the GB precipitation behavior of both topologically close-packed (TCP) or A2 and geometrically close-packed (GCP) phases was investigated in two Ni–Nb–(Co, Cr) ternary systems. The Ni/TCP or A2/GCP three-phase region existed in both systems. In the Ni-Nb-Co ternary system, Nb was approximately equally partitioned into both Co7Nb2 (mC18 structure, TCP) and (Ni, Co)3Nb (D019 structure, GCP) phases. In the Ni–Nb-Cr ternary system, Nb and Cr were mainly partitioned into the Ni3Nb (D0a structure, GCP) and Cr (A2 structure) phases, respectively. In the Ni–Nb–Co ternary system, the Co7Nb2 phase grew along the GB, whereas the (Ni, Co)3Nb phase grew toward the grain interior (GI). However, the growth of the Ni3Nb phase toward the GI was suppressed in the Ni–Nb–Cr ternary system. The suppression of growth of the GCP phase and covering the GB using both the TCP or A2 and GCP phases might be possible in a system where the precipitation of the GCP phase nucleating on the GB prior to the TCP or A2 phase increases supersaturation for precipitation of the TCP or A2 phase.

Effect of Mo on the Microstructures and Mechanical Properties of the Polycrystalline Superalloy with High W Content.

The effect of the Mo contents of 1.0 wt.%, 1.5 wt.%, 2.0 wt.%, and 3.0 wt.% on the microstructures and mechanical properties of the polycrystalline superalloy with a high W content was studied. The typical dendrite morphology was observed in the high-W superalloy with different Mo contents, containing γ matrix, γ' phase, eutectic, and MC carbide. After the heat treatment, the primary MC carbides were decomposed into M6C carbides, while a needle-like topologically close-packed (TCP) phase was formed in the alloy with high Mo content, in contrast to the other three alloys with low Mo content. The Mo addition increased the lattice parameter of the γ and γ' phases and also changed the lattice misfits of the γ and γ' phase lattice misfits towards a larger negative. The addition of Mo improved the yield strength at room temperature due to the solid solution strengthening and coherency strengthening. The improvement of the stress rupture lives at 975 °C/225 MPa was due to the combination of the suppressed propagation of the microcracks by the carbides and a more negative misfit. When the Mo content reached 3.0 wt.%, the TCP phases formed and decreased the ultimate tensile strength and the stress rupture lives as a result.

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.

Topologically Closed Packed Phase and Its Interaction with Dislocation Movement in Ni–Based Superalloy during High–Temperature Creep

In superalloys, topologically close–packed (TCP) phases, which contain refractory elements, usually significantly influence the mechanical properties. The current work investigates the structure and composition of the TCP phase in an Al–Mo–rich Ni–based single crystal superalloy. It is shown that after 40 h of thermal exposure, a large number of strip–like TCP phases are formed, which are enriched in Mo and Re. The structure of the TCP phase is identified as the tetragonal σ phase with the lattice parameter a being 0.93 nm and c being 0.50 nm. During the creep process, the single crystal tilts obviously and leads to orientation variation from <1 1 0> direction. Two groups of dislocations are observed in the deformed sample. One group contains straight dislocation lines and another group contains dislocation networks. The interaction between TCP phase and dislocation in the single crystal superalloy is studied to reveal the effect of the TCP phase on the deformation behavior. During creep, the σ phase hinders the dislocation movement, which may contribute to the propagation of the cracks and the final fracture.

Promotion of topologically close-packed phases in a Ru-containing Ni-based superalloy

We report on the effects of Ru addition on the formation of topologically close-packed (TCP) phases in a Ni-based superalloy during high-temperature exposure. The Ru addition reduces the lattice misfit between the γ and TCP phase, unexpectedly promoting nucleation of the TCP phase. Furthermore, the γ matrix is destabilized by Ru, while the stability of TCP phase is less affected according to the prediction of an electronic structure map. The results of this work indicate that γ destabilization is another important factor for the role of Ru in TCP phase formation.

The Effect of Cooling Rate from Solution Treatment on γ′ Reprecipitates and Creep Behaviors of a Ni-Based Superalloy Single-Crystal Casting

Slowing down the coarsening of the γ′ phase and suppressing the precipitation of the topologically close-packed (TCP) phase is crucial for optimizing the creep properties of Ni-based single crystal superalloys, which are affected by the solution treatment history. In the present study, the effect of cooling rate on the morphology, size and lattice misfit of γ′ reprecipitates after solution treatment, as well as the microstructural evolution (e.g., the coarsening of γ′ reprecipitate and precipitation of the TCP phase) and creep properties of samples under different cooling rates at 1100 °C were investigated. The findings suggested that as the cooling rate increasing, the size of γ′ reprecipitates decreased, while the morphology transformed from polygonal to cubic. Additionally, faster cooling rate, on the one hand, the lower the degree of lattice misfit of γ/γ′ phases, which is beneficial to slow down the coarsening of the γ′ phase; on the other hand, the supersaturation of the γ-phase was reduced, i.e., the Mo and Re contents in the γ matrix were lower compared to the slow-cooling sample, which led to a lower TCP phase area fraction during thermal exposure at 1100 °C. It is crucial that the creep life of the alloy significantly improved by increasing the cooling rate after solution treatment; this is facilitated by the formation of rafts from finer cubic γ′ phase and reduction in the TCP phase as a source of microcracks. In conclusion, the findings of this study provide new insights into suppressing the precipitation of the TCP phase and optimizing alloy heat treatment processes to improve creep properties.

Wear resistance performance of high entropy alloy–ceramic coating composites synthesized via a novel combined process

Titanium nitride (TiN), characterized by its high hardness and strength, was widely used as ceramic coating to improve the wear resistance of matrix materials. In this work, AlCrFeNiTix high-entropy alloy (HEA) powders were synthesized by direct electrochemical reduction in molten salt from the mixed metal oxides. Then, TiN ceramic coating on the AlCrFeNiTix bulk HEA containing the topologically close-packed (TCP) phase (σ phase, Laves phase, and Ti3Al phase) was prepared by vacuum hot pressing sintering, where nitride element come from boron nitride parting agent sprayed on the graphite mold. The effect of titanium content on the crystal structure, microstructure, hardness, and wear resistance of the products were investigated by X-ray diffraction, field emission scanning electron microscope, field emission electron-probe microanalysis, Vickers hardness tester, and friction–abrasion testing machine. The bulk HEAs exhibit excellent hardness and its hardness increases significantly with the increase of titanium content. The wear mechanism changes from both of predominantly delamination and accompanied oxidative wear to single delamination wear, which is due to ultra-high melting point and high hot hardness of TiN, that can effectively prevent the oxidation and deformation of the worn surface. Formation of the ceramic coatings containing the TiN second phase and TCP phase are the key factor to AlCrFeNiTix alloy with the excellent hardness and wear properties.

Interplay of chemistry and deformation-induced defects on facilitating topologically-close-packed phase precipitation in nickel-base superalloys

Topologically close packed (TCP) phases are often formed in nickel-base superalloys with high refractory elements during service, and they are detrimental for the high temperature performance of superalloys. The precipitation process of TCP phases is under scrutiny in particular for deformations that integrate strain and temperature but replicate the working conditions of superalloys. In this work, TCP phase precipitation is studied in nickel-base single crystal superalloys with or without Ru addition under thermomechanical fatigue deformation. Deformation twins on different {111} planes are observed intersecting with each other and forming large number of high angle boundaries. The structure of these high angle boundaries has high similarity to topologically close packed σ phase, and the boundaries are enriched in Re, Ru, Co and Cr, thus it provides both structural origins and constituent elements for the formation of σ phase. Ru is revealed intensely segregating to semi-coherent and incoherent interfaces between TCP phase and the matrix, this reduces the interface energies and leads to a dramatic change of the morphology of TCP phase precipitates. These results provide insight to effects of lattice imperfections and coevolution chemistry on TCP phase formation in superalloys, and shed light on inhomogeneous precipitation in alloys in general.

Atomistic mechanism of phase transformation between topologically close-packed complex intermetallics

Understanding how topologically close-packed phases (TCPs) transform between one another is one of the challenging puzzles in solid-state transformations. Here we use atomic-resolved tools to dissect the transition among TCPs, specifically the μ and P (or σ) phases in nickel-based superalloys. We discover that the P phase originates from intrinsic (110) faulted twin boundaries (FTB), which according to first-principles calculations is of extraordinarily low energy. The FTB sets up a pathway for the diffusional in-flux of the smaller 3d transition metal species, creating a Frank interstitial dislocation loop. The climb of this dislocation, with an unusual Burgers vector that displaces neighboring atoms into the lattice positions of the product phase, accomplishes the structural transformation. Our findings reveal an intrinsic link among these seemingly unrelated TCP configurations, explain the role of internal lattice defects in facilitating the phase transition, and offer useful insight for alloy design that involves different complex phases.

Competitive deformation induced by TCP precipitation and creep inconsistency on dendritic structures in a nickel-based single crystal superalloy crept at high temperatures

Creep properties of a 4th generation nickel-based single crystal superalloy (Ni-SX) are investigated at 1150 °C and 1100 °C. Fracture mechanism is revealed as competitive microcrack initiation induced by topologically close-packed (TCP) phase formation and creep inconsistency on dendritic structures showing the strong temperature-and-time dependent relations. At 1150 °C, alloy displays the high nucleation rate of TCP phases. Dense TCP particles dissolve the surrounding γ/γ' microstructures forming large depleted zone where is easily stress-concentrated. Dislocations are found to emit from the interface between TCP and depleted zone that promotes dislocation shearing and microcrack initiation in the large depleted zone, as well as the fracture of alloy. On contrary, alloy crept at 1100 °C suffers to relatively restricted TCP nucleation. But the longer creep time gives to larger size of TCP precipitates. Compared to alloy crept at 1150 °C, the depleted zone is much smaller that is not prone for stress concentration. For this creep inconsistency induced by dendritic segregation plays the main role in determining the creep properties. The larger local strain rate forms microcracks primarily in interdendritic region causing the final fracture.

Effects of the orientation relationships between TCP phases and matrix on the morphologies of TCP phases in Ni-based single crystal superalloys

Observation regarding the SEM and TEM images of the precipitation and growth morphology of topologically close-packed (TCP) phase indicates that the orientation relationships between the TCP phase with different morphologies and the matrix are not that similar. Concerning with this phenomenon, via DFT calculations, the possible interface structures between the σ phase and the matrix γ, as well as the μ phase and the matrix γ have been established and discussed under different orientation relationships. The experimental characterizations and simulated calculations reveal that the orientation relationships between TCP phase and the matrix determine interplanar spacing mismatches and the interface energy of the low index dense arrangement interfaces of the two phases, while the two parameters exert influence on the growth and morphology of TCP phase. The interface energy of the σ/γ interface with (111) γ //(001) σ orientation is small while its interplanar spacing mismatch is large, so the TCP phase under this orientation relationship tend to take planar expansion along interface instead of growing layer by layer parallel to the plane, giving rise to plate-like morphology. However, those of the σ/γ with (110) γ //(1–10) σ and the μ/γ with (110) γ //(111) μ are just the opposite, so the TCP phases under these orientations tend to grow layer by layer instead of expand along plane, forming needle-like or rod-like morphology eventually. • Building TCP/γ interfaces under different orientation relationships (ORs) via DFT. • Different ORs between TCP phases and γ correspond to different morphologies of TCP. • Interface energy γ i of (001) σ /(111) γ is small, and spacing mismatches δ d of it is big. • The γ i of (1–10) σ /(110) γ and (111) μ /(110) γ are big, and the δ d of them are small. • The two parameters δ d and γ i combined to influence the growth and morphology of TCP.

Influence of Mo/Cr ratio on the lamellar microstructure and mechanical properties of as-cast Al0.75CoCrFeNi compositionally complex alloys

The Al0.75CoCrFeNi alloy (Al16Co21Cr21Fe21Ni21 in at.%) presents a lamellar microstructure in the as-cast state consisting of a spinodally-decomposed B2/BCC matrix and Widmanstätten-type FCC plates. In this study, to retain the lamellar microstructure and improve tensile strength, Al16Co21Cr21-xFe21Ni21Mox alloys with x ≤ 10 at.% were investigated. For x = 2 at.%, the Widmanstätten microstructure changed into a vermicular one due to the stabilization of the BCC phase. With increasing the Mo/Cr ratio, the BCC phase transformed into topologically close-packed (TCP) phases, i.e., σ phase for x = 4 at.% and R phase for x ≥ 6 at.%, whose volume fractions increases with x. The as-cast alloys with x = 10 and 4 at.% presented the largest microhardness of ~600 HV0.5. The former had the highest volume fraction in TCP phases, which are hard and brittle while the latter presented the finest microstructure (enhanced phase boundary strengthening). While the alloys with x > 4 at.% were too brittle to machine tensile specimens, the others were tested between 20 and 700 °C. The ultimate tensile strength increased with increasing x up to ~1460 MPa for x = 4 at.% at 400 °C. At 700 °C, the strength of all alloys significantly decreased due to the softening of the B2 phase. Most of them had limited ductility and showed intergranular fracture except for x = 4 at.% presenting pronounced necking with ~38% ductility. The latter effect was attributed to the occurrence of interfacial sliding resulting in cavitation at grain boundaries and interphase boundaries.

Formation mechanism and thermal stability of C15-Laves phase in a Hf-containing Co-based superalloy

Topologically close-packed (TCP) phases such as μ and Laves phases formed during solidification has an important influence on the mechanical properties of superalloys. Understanding the formation and elimination mechanism of TCP phases is the key to promote the development of superalloys. In this study, C15-Laves phase was first found in a Hf-containing Co-based superalloy, meanwhile, the formation mechanism and thermal stability of the C15-Laves phase were clarified. The results showed that the formation of C15-Laves phase was associated with the segregation behavior of alloying elements during solidification: Co, W and Cr segregated to the dendrite core, and Hf, Ta, Ti, Mo, Ni, Al, Si segregated to the interdendritic region. In the final stage of solidification, the C15-Laves phase enriched in Hf, Ta and Ti formed, and it had similar lattice parameters to the C15-Co2Hf phase. The incipient melting point of C15-Laves phase was between 1170 ℃ and 1180 ℃, and it gradually decomposed to form the Hf-rich MC carbide during solution treatment. The large formation energy differences between the MC carbide and C15-Laves phase promoted a rapid elimination of the C15-Laves phase, which reduced solution treatment time of the alloy. The results can provide a theoretical basis and experimental data support for the composition design and heat treatment process optimization of multicomponent novel Co-based superalloys.

On the interdiffusion and precipitation of topologically close-packed phases in (Ni,Pt)Al coating/superalloy system: Effects of Pt and Ni contents in coating

In combination with the electroplating and electron beam physical vapor deposition (EB-PVD), four types of (Ni,Pt)Al coatings, namely Ni45Al, (Ni,Pt)45Al, Ni50Al, and (Ni,Pt)50Al were prepared. In order to study the interdiffusion and topologically close-packed (TCP) phases precipitation behavior, annealing experiments were carried out at 1100 °C for 20 h in vacuum. The addition of Pt layer has little effect on the thickness of interdiffusion zone (IDZ) because the inhibition effect of Pt on the Al diffusion weakens with the continuous diffusion of Pt. Meanwhile, the increase of Pt content in substrate promotes the Al diffusion. Besides, the effect of Pt on the TCP phases precipitation depends strongly on the Ni content in coating-For Ni45Al coating, adding Pt makes the volume fraction of TCP phases reduces from ~16.35 % to ~3.15 %, whereas this value is comparable in both Ni50Al and (Ni,Pt)50Al coatings (18.3 % and 17.9 %). This phenomenon is attributed to the competition mechanism between the acceleration effect of Pt on Al diffusion and the inhibition effect of Pt on TCP phases growth. The results provide a better understanding about the microstructure, especially the IDZ and TCP characteristics of (Ni,Pt)Al coating, which is significant for controlling the interdiffusion and TCP precipitation behavior in (Ni,Pt)Al coating.

Microstructural degradation after thermal exposure of a Re-containing single crystal superalloy

Microstructural degradation after thermal exposure of a 5% Re -containing single crystal (SX) superalloys has been studied. The alloy was subjected to thermal exposure at temperature of 1100 °C for periods of 100, 500, and 1000 h. The microstructure of the alloy was degraded after long-term thermal exposure, mainly in γ' coarsening, the precipitation topologically close packed (TCP) phases. Moreover, the stress-rupture properties of the samples before and after long-term thermal exposures were characterized. The results showed that the γ' coarsening and TCP formation after exposed for 500 h, which led to a damaged of the stress-rupture life time. Besides, the growth of pores during exposure also influences the stress-rupture life. • γ' coarsening and TCP phase precipitation are characterized after long-term exposure. • The TCP phase does not embrittle the alloy. • The severe γ' coarsening is the main factor in the degradation of the life time.

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.

Influence of long-term aging on microstructural stability and performance of DD6 superalloy

In order to investigate the deterioration of microstructure in service, the intrinsic microstructural stability and performance of DD6 during long-term aging without an external load are investigated. During aging at 980°C, γ′ rafting in 〈100〉 and 〈010〉 directions show no anisotropy due to small driving forces. γ′ coarsening and rafting lead to a monotonic decrease in hardness between 200 and 1000 h. However, precipitation of topologically close-packed (TCP) phases occurs significantly during aging at 1050°C. The amount of TCP phases increases initially, and then almost saturates, which results in a decrease in hardness. However, owing to the reduction in the growth of the TCP phases and coarsening of γ′, the hardness saturates for an aging time greater than 500 h.

Effect of post-treatment on the microstructure and tensile properties of Ni–Co-based superalloy manufactured by selective laser melting

ABSTRACT A newly designed Ni–Co-based superalloy was manufactured by selective laser melting (SLM) and the effect of hot isostatic pressing (HIP) and heat treatment on the microstructure and tensile properties was investigated. The as-built sample showed epitaxial grains including sub-structures, whereas recrystallisation occurred in the HIP-treated samples. Furthermore, carbide particles appeared at the grain boundaries, and the γ’ phase formed extensively. The sample that was subjected to HIP-solution-treatment and aging (STA) contained a larger fraction of carbide and γ’ phase compared to the HIP sample, with a needle-shaped topologically close-packed (TCP) phase also generated inside the γ matrix. The yield and tensile-strengths increased by approximately 60% after HIP, and the elongation increased by 0.8%. However, the HIP-STA sample underwent a significant decrease in elongation by TCP phase. The correlation between the microstructure and mechanical properties of the SLM-built superalloy is also discussed on the basis of the fractography results.

Thermodynamic assessment of the Co–Cr–Ni, Co–Cr–W and Co–Ni–W

The thermodynamic description of the Co–Cr–Ni, Co–Cr–W and Co–Ni–W systems is the basis of material design for Co-WC cutting tools and superalloys. A common feature of the three ternary systems is that the detrimental topologically close packed (TCP) phases (μ and σ phases) have large ternary extensions, which makes it crucial to provide a reliable description of the TCP phase boundaries. In order to obtain an accurate description of the TCP phase boundary, the effect of magnetism on the Gibbs energy of the TCP phases containing ferromagnetic elements must be considered. In the present work, the effect of magnetism on the Gibbs energy for the Ni-related TCP phases has been discussed. The magnetic related parameters TcNiσ, βNiσ, GoNidis_σ, TcNiμ, βNiμ, GoNidis_μ were evaluated. CALPHAD (CALculation of PHAse Diagrams) assessment for the ternary systems has been performed. The present CALPHAD description can well describe the experimental data.

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%.