Formation Of Topologically Close-packed Phases Research Articles

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.

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.

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.

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.

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.

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.

Effect of TCP phase formation on the electrochemical corrosion behavior of the nickel-based superalloy UNS N26455

The effect of topologically close-packed (TCP) phase formation on the electrochemical corrosion behavior of the UNS N26455 (Ni–16Cr–16Mo) superalloy was evaluated using different electrochemical techniques. We verified that this superalloy is resistant to highly corrosive environments but its corrosion potential decreases and the anodic current density increases as the TCP phase concentration increases in the microstructure. Its anodic behavior indicates that there is a selective dissolution process associated with this secondary phase, which is completely dissolved before repassivation occurs. Although the repassivation potential remains nearly unchanged, the degraded points configure defects that may negatively affect the material’s performance during service.

Abnormal increase of TCP phase during heat treatment in a Ni-based single crystal superalloy

Generally, elevated solution heat treatment temperature decreases the residual dendritic segregation of refractory elements (i.e. Re and W) and suppresses the precipitation of deleterious topologically close-packed (TCP) phases in Ni-based superalloys. However, in the present study, elevated solution temperature resulted in an abnormal increase of TCP phases in a Ni-based single crystal superalloy after long-term thermal exposure at 900 °C. Microstructural analysis suggested that higher supersaturation in γ phase which promoted the formation of TCP phases was induced by the increased γ′ precipitation after heat treated at elevated solution temperature.

Abnormal Increase of TCP Phase During Heat Treatment in a Ni-Based Single Crystal Superalloy

Generally, elevated solution heat treatment temperature decreases the residual segregation of refractory elements (i.e. Re and W) and suppresses the precipitation of deleterious topologically close-packed (TCP) phases in Ni-based superalloys. However, in the present study, elevated solution temperature resulted in an abnormal increase of TCP phases in a Ni-based single crystal superalloy after long-term thermal exposure at 900 oC. Microstructural analysis suggested that higher supersaturation in γ phase which promoted the formation of TCP phases was induced by the increased γ' precipitation after heat treated at elevated solution temperature.

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.

Correlative characterization on microstructure evolution of Ni-based K403 alloy during thermal exposure

The microstructure evolution of the K403 Ni-based superalloy as a function of thermal exposure temperature and time was investigated using correlative characterization, including a combination of scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, high-angle angular dark-field scanning transmission electron microscopy imaging and electron energy loss spectroscopy. The as-cast microstructure of the K403 alloy shows a typical dendritic structure and consists of a γ solid solution, γ′ phase, (γ + γ′) eutectic phase and a metal carbide (MC) phase. The solutes of Ni and Ti are enriched in the γ solid solution, while, the solutes of Cr and Co are enriched in the γ′ phase. The morphology of the γ′ phase is nearly cubic. After thermal exposure at 800 °C or 950 °C for up to 200 h, the typical dendritic structure and a similar solute segregation behavior were still observed. However, the MC carbides were partially decomposed and further transformed to the M6C phase and M23C6 phase. Furthermore, the size of γ′ was increased from 302.88 ± 20.49 nm to 374.75 ± 29.76 nm (800 °C, 50 h) and 751.73 ± 123.14 nm (950 °C, 50 h), respectively. The morphology of γ′ was changed from cubic to triangular or round. Clearly, there is a significant coarsening of γ′ during thermal exposure. A topologically close-packed (TCP) σ phase was observed after thermal exposure at 800 °C for 100 h or 950 °C for 50 h. More interestingly, an in-situ phase transformation of the σ phase to other TCP phase (i.e. P phase) was also observed after thermal exposure at 950 °C for 50 h. The formation and transformation of carbide (i.e. M6C phase, M23C6 phase) and TCP phases (i.e. σ phase, P phase) is proposed to be a diffusion-controlled process and can be attributed to the solute diffusion during thermal exposure. The present investigation provides a better understanding on the high temperature performance of the K403 Ni-based superalloy, which is essential to predict the failure and thereby enhance the reliability and service life of the K403 Ni-based superalloy.

Microstructural Investigation of the Formation and Development of Topologically Close‐Packed Phases in a 3rd Generation Nickel‐Base Single Crystal Superalloy

Topologically close‐packed (TCP) phases generally form during long period aging or in‐service. In this study, however, the early stage formation and development of TCP phases, which has received relatively less attention, was extensively investigated from as‐cast samples through to fully heat‐treated ones. TCP phases were surrounded with a γ′ phase, and their compositions were slightly different due to the structural and compositional complexity. Most of all, extremely fine particles of about 100 nm containing large amounts of rhenium formed in the early processing. In addition, two different types of particles were detected at different formation stages: round‐shaped particles at the starting point, and needle‐like ones near the terminating point. Based on experimental observations, it was suggested that the needle‐like particle probably showed the growth of a TCP phase by the diffusion of rhenium after nucleation from a point containing high amounts of rhenium.

Phase evolution and crystallography of precipitates during decomposition of new “tungsten-free” Co(Ni)–Mo–Al–Nb γ–γ′ superalloys at elevated temperatures

This article reports the microstructural stability and consequent phase decomposition including the appearance of topologically close-packed (TCP) phases at high temperature of recently discovered tungsten-free gamma-gamma' alloys of base composition Co-10Al-5Mo-2Nb with or without the addition of Ni and Ti. On prolonged aging at 800 degrees C of the Co-10Al-5Mo-2Nb alloy, needle-shaped DO19-ordered precipitates with stoichiometry of Co-3(Mo, Nb) start appearing in the microstructure. In addition, growth of cellular domains from the grain boundaries featuring a three-phase composite lamellar structure could be observed. These phases are fcc gamma-Co with composition different from the original matrix, CoAl with B2 ordering and Co-3(Mo, Nb) with DO19 ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the gamma' phase. The Ni-containing alloy exhibits no phase decomposition until 100 h of aging at 800 degrees C without any significant effect on gamma' volume fraction (76 %). However, at 950 degrees C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr3Si-type cubic phase, a hexagonal Laves phase, rhombohedral mu phase, and solid solution of Co phase. The gamma-gamma' microstructure in the Co-10Al-5Mo-2Nb and Co-30Ni-10Al-5Mo-2Ta alloys is not stable at 800 and 950 degrees C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990 degrees C) are metastable solvus temperatures. We also report that the Ti-containing alloy exhibits superior stability with no evidence of either TCP phase formation or any other decomposition of gamma' precipitates, even after aging at 950 degrees C for 100 h.

Compositional effect on TCP phase formation in Ru-containing Ni-based single crystal superalloys

Microstructural instability involving the formation of topologically close-packed (TCP) phases is restricted during the alloy development of Ni-based single crystal superalloys. In this study, the effects of alloying elements including Co, Cr, Mo and Ru on the formation of different TCP phases were investigated in a series of single crystal superalloys. Experimental results showed that more additions of Cr and Mo promoted the TCP phase formation, while Co and Ru played a positive role in improving microstructural stability. It is indicated that σ, P and R phases existed with various morphology and compositions in different experimental alloys during thermal exposure at 1100 ∘ C. The content of Co, Cr and Mo in those alloys affected the types of TCP phases significantly, while Ru additions showed a negligible effect. R phase was prone to form in alloys containing high level of Co addition. Meanwhile, the ratio of Cr and Mo content had strong influence on the formation of σ and P phases in alloys containing low level of Co addition. The effects of alloying additions on the elemental partitioning ratios between γ and γ ′ phases contributed to their corresponding influence on TCP phase formation.

Effects of Ru on the microstructure and phase stability of a single crystal superalloy

Two experimental single crystal superalloys, the Ru-free alloy and the Ru-containing alloy with [001] orientation, were cast in a directionally solidified furnace, while other alloying element contents were kept unchanged. The effects of Ru on the microstructure and phase stability of the single crystal superalloy were investigated. γ′ directional coarsening and rafting were observed in the Ru-free alloy and Ru-containing alloy after long-term aging at 1070°C for 800 h. Needle-shaped σ topologically close packed (TCP) phases precipitated and grew along the fixed direction in both the alloys. The precipitating rate and volume fraction of TCP phases decreased significantly by adding Ru. The compositions of γ and γ′ phases measured using an energy-dispersive X-ray spectroscope (EDS) in transmission electron microscopy (TEM) analysis showed that the addition of Ru lessened the partition ratio of TCP forming elements, Re, W and Mo, and decreased the saturation degrees of these elements in γ phase, which can enable the Ru-containing alloy to be more resistant to the formation of TCP phases. It is indicated that the addition of Ru to the Ni-based single crystal superalloy with high content of the refractory alloying element can enhance phase stability.

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.

Secondary Reaction Zone Formations in coated Ni-base Single Crystal Superalloys

Ruthenium (Ru) has been added to the latest 4th Generation Ni-base superalloys to improve phase stability and modify creep life. Various coatings are routinely applied to these advanced alloys to protect the turbine blade at elevated temperature, however, this creates several problems such as the precipitation of brittle Topologically Close-Packed (TCP) phases and the formation of Secondary Reaction Zones (SRZ). The SRZ forms under the plat-aluminized coating of turbine blades and consists of γ, γ and TCP phases growing into substrate by the migration of high-angle grain boundaries. Surface residual stress and chemical super-saturation of alloying elements are associated to SRZ formation. In the thin sections of high-pressure turbine blades this is critical in determining blade performance and longevity. It is essential to know how Ru additions affect coating and SRZ morphologies during exposure. In this study, we focus on the effects of three variables on the SRZ formation: Ru concentration, alloy composition in Ru-containing alloys and surface finish. A series of Platinum-Aluminised superalloys containing 2–5wt% Ru and having various surface finishes was studied after isothermal exposure at 1100°C for up to 500h. The alloys were classified into two groups by their distinctive SRZ morphology. At the lowest Ru levels sporadic formation of SRZ was observed, whilst a continuous SRZ was formed in the higher Ru alloys. EBSD analysis revealed that the latter group have a higher nucleation rate of individual SRZ grains and also showed more rapid SRZ growth. The precipitation of TCPs in the substrate also inhibited the growth of the SRZ towards the end of the exposure further reducing the penetration of the SRZ into the substrate. It is concluded that Ru-additions to Ni-base superalloys are effective in impeding TCP phase formation in the substrate, but increase both the extent and the rate of SRZ formation beneath coating.

Damage accumulation during creep deformation of a single crystal superalloy at 1150 °C

Microstructural degradation in the CMSX-4 single crystal superalloy during creep deformation at 1150 °C and 100 MPa is studied. Tensile deformation in the 〈0 0 1〉 direction is considered due to its technological importance. Under these conditions, rafting of the γ′ structure is completed quickly and within the first 10 h. It is demonstrated that the creep rupture event is highly localised, the instability being associated with a critical and well-defined strain being reached with failure occurring within a further few tens of hours. It is shown that the high strain rates and shear stresses associated with the rupture process are sufficient to cause realignment of the rafted γ′ structure with respect to the γ matrix. Creep cavitation damage near to the rupture surface is prevalent, at microporosity inherited from the casting process but more significantly, at topologically close-packed (TCP) phases and associated pores and voids formed in their vicinity which have formed via vacancy condensation. Hot isostatic pressing (HIPing) prior to creep testing reduces the incidence of casting microporosity, but the creep rupture life is not improved significantly. It is suggested that it is the formation of TCP phases which limits creep rupture life.

Effects of Ru on the high-temperature phase stability of Ni-base single-crystal superalloys

Microstructural instabilities associated with the precipitation of refractory-rich topologically-close-packed (TCP) phases within the microstructure of advanced Ni-base single-crystal superalloys were quantified in two nominally identical alloys with and without additions of Ru. Differences in the microstructural kinetics associated with the formation of TCP precipitates in these experimental single-crystal superalloys enabled the influence of Ru to be assessed. Detailed microstructural investigations were carried out on specimens subjected to prolonged isothermal exposures at elevated temperature. Even after 1000 hours at temperatures in excess of 1100 °C, the microstructure of the Ru-bearing alloy was highly resistant to the formation of TCP phases. Transmission electron micro-analysis (TEM) coupled with X-ray diffraction (XRD) was used to identify the characteristic crystal structures of the TCP precipitates in both alloys as being primarily the orthorhombic P and tetragonal σ phase. The sluggish precipitation kinetics of TCP phases in the Ru-bearing single-crystal Ni-base superalloy prevents the breakdown of the parent γ-γ′ microstructure and greatly enhances the high-temperature creep characteristics.