Topologically Close-packed Phases Research Articles

The origin of the straight propagation of the σ phase in a Ni-based single crystal superalloy at elevated temperature

Topologically close-packed (TCP) phases, although inherently hard, are commonly perceived as detrimental in various alloys. In Ni-based single crystal superalloys (Ni-SXs), the primary adverse impact of TCP phases on material mechanical properties stems from their direct propagation, disrupting the original γ/γ' dual-phase structure and leading to cleavage along the TCP phases under applied stress. Understanding the specific reasons for the straight propagation of TCP phases through the complex phase structure is essential for mitigating or potentially reversing their detrimental effects on material performance, a challenge that remains elusive. This study addressed this challenge by employing ultra-high spatial and energy resolution transmission electron microscopy (TEM) characterization, coupled with in situ scanning transmission electron microscopy (STEM) heating experiments in a fourth-generation Ni-based single crystal superalloy. Our research unveiled a significant phase-separation process during the incubation stage preceding σ phase formation at elevated temperatures, driven by the diffusion of atoms. Notably, domains of the γ' phase, characterized by a scale of tens of nanometers, emerged within preexisting γ phases, and vice versa. This phenomenon reconstructed the original γ/γ' dual-phase structure, leading to a finely refined dual-phase structure. The refined structure effectively reduced the diffusion distance of alloy elements necessary for forming σ phases. Consequently, the σ phases nucleated randomly and propagated directly through the newly refined γ/γ' dual-phase structure. Throughout the thickening process, the notable difference in the diffusivity of alloying elements eventually resulted in the growth of a highly defective σ phase structure.

Structural pathway for nucleation and growth of topologically close-packed phase from parent hexagonal crystal

The solid diffusive phase transformation involving the nucleation and growth of new phase particles is universal and frequently employed but has not yet been fully understood at the atomic level. Here, our first-principles calculations reveal a structural formation pathway of a series of topologically close-packed (TCP) phases within the hexagonally close-packed (hcp) matrix. The results show that the nucleation follows a nonclassical nucleation process, and apart from atomic diffusion, the entire hcp→TCP transformation only involves shuffle-based displacements, with a specific 3-layer unstable hcp-ordering serving as the basic structural transformation unit (BSTU). The thickening of plate-like TCP phases relies on the formation of these unstable hcp-orderings at their coherent TCP/matrix interface to nucleate a ledge, but the ledge lacks the dislocation characteristics which is commonly considered in the conventional view. Additionally, the atomic structure of the critical nucleus for the Mg2Ca and MgZn2 Laves phases was predicted in terms of Classical Nucleation Theory (CNT). The formation of polytypes and off-stoichiometry in TCP precipitates is found to be related to the nonclassical nucleation behavior. These insights contribute to a deeper understanding of solid diffusive phase transformations at the atomic level and provide a foundation for investigating other technologically significant transformations.

Nucleation and transition sequences of TCP phases during heat-exposure in a Re-containing Ni-based single crystal superalloy

The nucleation and transition sequences of topologically close-packed (TCP) phases in a Re-containing Ni-based single crystal superalloy were systematically investigated using in-situ transmission electron microscopy (TEM) and three-dimensional atom probe technology (3D-APT). During the initial stage of heat-exposure at 1100 °C, the TCP phase forming elements (Re, Co, Cr, etc.) segregated at the γ/γ′ interface near the γ matrix side to provide the concentration undulations for the nucleation sites of TCP phases, following which the σ and P phase coherently nucleated along the (111‾)γ and (022)γ planes from the γ/γ′ interface near the γ matrix side, respectively. With prolonged heat-exposure time, transitions from σ phase to P phase, σ phase to μ phase, and P phase to μ phase occurred. Besides, the orientation relationships of TCP phase intergrowth structures indicated that the P phase grew along the (1¯01)σ plane of the σ phase by co-lattice precipitation, meanwhile, the µ phase grew with smaller lattice misfits along the (04¯0)σ plane of the σ phase and the (400)P plane of the P phase. Additionally, the result by first-principles calculation evidenced that the μ phase had the lowest system energy to make the transition of σ phase and P phase to μ phases inevitable, therefore, the TCP phase ultimately existed as the most stable μ phase. Finally, the transition sequences of TCP phase during heat-exposure could be summarized into three types: γ matrix→σ→μ, γ matrix→P→μ, and γ matrix→σ→P→μ.

Effect of Ru on the precipitation of the TCP phase from the γ-γʹ microstructural degradation analysis

This work investigates the effect of Ru on the precipitation of the topologically close-packed (TCP) phase from the γ-γʹ microstructural degradation analysis. The addition of Ru is found to delay the precipitation time of the TCP phase from 100 h to 1000 h after thermal exposure at 950 °C. The effect of Ru on the γ-γʹ microstructural degradation is also investigated. Adding Ru slightly decreases the coarsening rate of the γʹ size and promotes the reduction of the volume fraction of γʹ phase. The reduction of the volume fraction of γʹ phase causes the "inverse partitioning" effect and increases the solution limit of the TCP forming element in the matrix. These help inhibit the precipitation of the TCP phases in Ni-based single crystal superalloys during thermal exposure.

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.

Microstructures Evolution of a Second Generation Single Crystal Superalloy after Long Term Aging at 980°C

The microstructure evolution of the second-generation single crystal superalloy DD412 aged at 980°C for 1000 h were investigated. The results show that needle-shaped topologically close packed (TCP) phases rich in Re and W were firstly found in the dendritic core region after aging at 980°C for 400h, while the size and amount of TCP phases increased gradually with aging time, which is due to the fact that high melt point elements Re and W segregated to the dendritic core region. With the increment of aging time, the size of γ′ phase increased and volume fraction of γ′ phase decreased. The γ′ phase in dendrite core and inter-dendrite region remained cuboidal basically, and the γ′ phase in second dendritic arm showed directional coarsening after aging for 800 h. The MC carbides in inter-dendritic region decomposed gradually and transformed to the M6C carbides partially during long term aging, while few fine blocky M6C carbides precipitated in dendritic core after aging for 400h. The different microstructure evolution in dendritic region is due to the dendrite segregation of alloying elements and elements diffusion during aging.

On phase stability and phase relations of topologically close packed phases in binary Cr-Os system over the temperature range of 900–1300 ℃

Osmium (Os) addition has been found to be beneficial in enhancing the creep resistance and microstructure stability of nickel-based single-crystal superalloys. The phase stability and phase relationship of topologically closely packed (TCP) phases in a binary Cr-Os system is thus essential for developing a better understanding of the constitutional effects of Os in multicomponent nickel-based single-crystal superalloys, but are still not fully understood in the literature. In this paper, a series of Cr-Os alloys were prepared, and the phase constituents and the microstructure of both as-cast and annealed alloys over a temperature range of 900–1300 ℃ were analyzed using the X-ray diffraction (XRD) and electron probe microanalysis (EPMA) techniques. The experimental results show that (Cr) solution, (Os) solution and Cr2Os phases are formed during solidification. After long-term annealing at 900–1300 ℃, the phase constituents and the microstructure of the alloys display obvious changes. Based on the measured data, the phase stability and phase equilibria of TCP between 900 and 1300 ℃ in the binary Cr-Os system are accordingly updated. The stability results for the two TCP phases in the binary Cr-Os system demonstrate that the Cr3Os phase exists stably, while the Cr2Os phase decomposes into Cr3Os and (Os) at 900–1300 ℃.

Atomic-scale study of linear reciprocating friction of TCP/γ phase in nickel-based single crystal alloy

In order to systematically investigate the role of TCP (topologically close-packed) phases in the fretting wear process of nickel-based single crystal alloys (NBSC), this study employed molecular dynamics to conduct comparative analyses of mechanical properties, atomic displacements, wear depth, defects, dislocation density, and the influence of temperature under constant load on the friction process in material wear. The research revealed that during the repetitive friction process, the friction force exhibited a peak at the extreme positions of reciprocating friction on the workpieces, and this peak increased with the number of friction cycles. The dislocation density in the worn area increased, resulting in hardening, and the removal rate of material decreased. At the initial stages of friction, the presence of interfaces notably hindered the transfer of temperature, defects, and atomic displacements in the workpiece, and this inhibitory effect weakened with an increasing number of friction cycles. The TCP phases experienced stratification due to the overall deformation they underwent. Furthermore, as the relaxation temperature increased, the workpiece exhibited enhanced plastic deformation capacity, an increase in dislocation density, and adhesion between abrasive particles and the grinding ball occurred.

Insights into the effects of Re and Ta on TCP phase evolution in nickel-based SX superalloys: Experimental and simulation analysis

This study explores the effects of Re and Ta interactions on TCP (topologically close-packed) phase precipitation in experimental alloys under long-term thermal exposure. TCP phase is thoroughly examined and analyzed in terms of its location, morphology evolution, structural identification, and phase transition, employing thermodynamic calculations and first-principles analysis. The main findings indicate that the standard heat treatment used in the experiment does not fully eliminate microstructure segregation. Particularly, W and Re exhibit significant segregation in the dendrite region, resulting in preferential TCP phase formation in this region. Re induces destabilization of the γ-matrix and greatly enhances TCP phase precipitation, whereas Ta enhances the supersaturation of the γ-matrix. The interaction between Ta and Re further enhances the precipitation of the TCP phase. Thermodynamic calculations and first-principles analysis indicate that Re significantly improves the driving force of TCP phase precipitation from the γ-matrix, whereas the effect of Ta is found to be slight. Under thermal exposure at 900 °C, the σ-phase is firstly observed in the 8Ta2Re alloy, exhibiting a long needle-like shape, followed by subsequent evolutions resulting in the transition to the P-phase. The phase transition from the σ-phase to the P-phase is governed by the diffusion of elements, with Ni playing a crucial role in continuous diffusion.

Microstructure Evolution and Dislocation Mechanism of a Third-Generation Single-Crystal Ni-Based Superalloy during Creep at 1170 °C.

The present study investigates the creep behavior and deformation mechanism of a third-generation single-crystal Ni-based superalloy at 1170 °C under a range of stress levels. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) were employed to observe the formation of a rafted γ' phase, which exhibits a topologically close-packed (TCP) structure. The orientation relationship and elemental composition of the TCP phase and matrix were analyzed to discern their impact on the creep properties of the alloy. The primary deformation mechanism of the examined alloy was identified as dislocation slipping within the γ matrix, accompanied by the climbing of dislocations over the rafted γ' phase during the initial stage of creep. In the later stages of creep, super-dislocations with Burgers vectors of a<010> and a/2<110> were observed to shear into the γ' phase, originating from interfacial dislocation networks. Up to the fracture, the sequential activation of dislocation shearing in the primary and secondary slipping systems of the γ' phase occurs. As a consequence of this alternating dislocation shearing, a twist deformation of the rafted γ' phase ensued, ultimately contributing to the fracture mechanism observed in the alloy during creep.

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 Topologically Closed Packed Phases during the Heat-Treatment of Rhenium Containing Single Crystal Ni-Based Superalloys

Continual development of nickel-based superalloys for single-crystal turbine applications has pushed their operating temperatures higher and higher, most notably through the addition of rhenium. However, this has left them susceptible to the precipitation of topologically closed packed phases (TCPs), which are widely considered detrimental. Whilst these have long been reported as an end-of-life phenomenon in in-service components, they have more recently been observed during the manufacture of turbine blades. Several rhenium-containing alloys (CMSX-4, CMSX-10K, and CMSX-10N) were cast into single-crystal test bars and studied at different times along their solution heat-treatment process to discern if, when, and where these TCPs precipitated. It was seen that all alloys were susceptible to TCPs at some point along the process, with the higher rhenium-containing alloy CMSX-10N being the most prone. They occurred at the earliest stages of the solution process; this was attributed to aluminium diffusion from the segregated interdendritic regions into the dendrite core, causing the concentration of rhenium into the ɣ-matrixes until sufficient potential was achieved for TCP precipitation. As the samples became more homogeneous, fewer TCPs were observed; however, in the case of CMSX-10N, this took longer than the typical 24-h solution time used in industry, leading to components entering service with TCPs still present.

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.

The element occupation preference of μ phase in ternary Ni-Cr-Mo model superalloy

Topologically close-packed (TCP) phases seriously endanger the properties of Superalloy. Investigating the microstructure of the TCP phases will provide a crystallographic reference for the design of superalloys with better properties. The μ phase is a common TCP phase in superalloys. It belongs to the space group R-3m, and has a hexagonal structure with five Wyckoff sites of 3a, 18h, 6c1, 6c2 and 6c3. In this paper, occupation of the three kinds of atoms at five Wyckoff sites of the μ phase in Ni-Cr-Mo model superalloy was determined using aberration-corrected High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), QSTEM software simulation and first-principles calculations. It was found that Mo atoms with larger size and smaller electronegativity tend to occupy 6c3, 6c1 and 6c2 sites in order of priority. Ni atoms mainly occupy the sites with coordination number (CN) of 12 at 3a and 18 h, and 3a sites contain more Cr atoms than 18 h sites.

Effect of Re-base diffusion barrier on the oxidation performance of a gradient NiCoCrAlY coating

A gradient NiCoCrAlY coating with a Re-base diffusion barrier (RG-NiCoCrAlY) was prepared by sequential treatments of NiRe coelectrodeposition, arc ion plating and chemical vapor aluminization. For comparison, normal NiCoCrAlY and simply aluminized NiCoCrAlY coatings (G-NiCoCrAlY) were also prepared to assess their cyclic oxidation behavior at 1050 °C. The results indicated that the oxide scale formed on the normal NiCoCrAlY coating peeled off after 400 cycles, while the G-NiCoCrAlY and RG-NiCoCrAlY coatings developed and maintained continuously dense alumina scale after oxidation for 600 cycles, confirming the benefits of increasing the Al content in NiCoCrAlY. In contrast to G-NiCoCrAlY, the presence of Re-base diffusion barrier mitigated element interdiffusion, which not only reserved Al in the coating but also reduced the outward diffusion of refractory elements from the substrate, thus was capable of minimizing precipitation of topologically close-packed phase (TCP) in single-crystal superalloy. Instead of outward diffusion, refractory elements were dissolved in the diffusion barrier and became fixed. Undoubtedly, the best anti-cycling performance was achieved by the RG-NiCoCrAlY coating. The mechanisms of the effect of the Re-base diffusion barrier on oxide scale growth and elemental interdiffusion, which influence the overall oxidation behavior of the NiCoCrAlY coating, are discussed.

Microstructural evolution and element interdiffusion of NiAlHf and NiAlHfY coatings deposited on a Ni-based superalloy

The NiAlHf and NiAlHfY coatings were deposited on a nickel-based single crystal superalloy using arc ion plating technology. The microstructural evolution and element interdiffusion behavior of the NiAlHf and NiAlHfY coatings were investigated under a 1200 °C thermal exposure process. The results showed that the NiAlHfY coating had a higher residual Al content and fewer topologically close-packed (TCP) phases, which implied that Y effectively retarded the diffusion rate of related elements. Besides, the inward diffusion of Al and outward diffusion of Ni lead to a γ → γ´ phase change and the formation of different TCP phases, resulting from the main precipitating elements (Cr, Mo, Re, W, and Ta) having a lower solubility in γ´. Moreover, it is found that the σ and Laves phase precipitate in the interdiffusion zone, which is described by an intergrowth relationship, where the σ phase is a metastable phase and plays a key role in forming the Laves phase. Finally, the interdiffusion mechanism of the elements is proposed.

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 &lt;1 1 0&gt; 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.