Topologically Close-packed Phases Research Articles

Determination of solubility limits of refractory elements in TCP phases of the Ni-Mo-Cr ternary system using diffusion multiples

The Ni-Mo-Cr system is critical for the design of nickel-based superalloys. This system stabilizes different topologically close packed (TCP) phases which precipitate in many of the commercially used superalloys. A diffusion multiple approach was applied to map the phase diagram of the ternary system in a highly efficient manner. Two isothermal sections of the Ni-Mo-Cr system at 1100°C and 1250°C were constructed from the results obtained from diffusion multiples using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). At both temperatures two TCP phases were observed, namely the tetragonal σ- and the orthorhombic P-phase. In the range of this study the σ-phase was found to be widely stable towards the Cr-poor side, whereas the P-phase was detected on the Ni-rich side. The phases were characterized mechanically by microhardness measurements. Determined values range from 8,5 to 9,2 kN/mm2 for σ- and P-phase. By analysing interdiffusion zones of quaternary systems the solubility limits of Re, Ru and W were determined and compared for both TCP phases in order to assess their stabilising impact. Maximum solubilities of up to 12,7 at.% were detected, whereas the P-phase exhibits higher solubility limits than the σ-phase. In both phases solubility limits decrease with increasing temperature. In contradiction to previous experience in nickel-based superalloys, the observed stabilising effect on TCPs in quaternary systems is stronger for Ru than for Re.

Effect of post-weld heat treatment on the microstructure and tensile properties of electron-beam-welded 21st century nickel-based super alloy 686

This work examines the influence of post-weld heat treatment (PWHT) on the fusion zone microstructure, and mechanical properties of electron beam (EB)-welded alloy 686. Comparative studies have been made on weld microstructure and tensile properties of the weldments both in as-welded and post-welded heat-treated conditions. PWHT consists of direct aging (DA/480°C for 3 h) and solution treatment (ST, 980°C for 1 h) followed by ageing and finally, homogenizing treatment (HT, 1200°C for 1 h) followed by DA and ST. The secondary topologically closed packed (TCP) phases formation, their distribution and microsegregation characteristics are studied with the aid of scanning electron microscope (SEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) is also performed to estimate the microsegregation of alloying elements in the dendritic core and interdendritic regions of the weldments. The result shows that there is no significant change in the microstructure of DA and solution-treated sample as compared to the as-welded sample. The microstructure of HT sample was entirely different from those of as-welded and other HT samples. The SEM/EDS analysis illustrates the presence of secondary TCP phases (σ, P and μ) in the as-weld, DA and ST condition, whereas HT weldments did not show the presence of TCP phases. Tensile test results show higher tensile strength in ST condition whereas homogenization samples show higher ductility compared with others.

Thermodynamic Study on Equilibrium Phases in Nickel-Base Single Crystal Superalloys

In order to improve the composition and microstructure of nickel-base single crystal superalloys, equilibrium phases of third generation single crystal superalloys RenéN6 and CMSX-10 have been researched by using thermodynamic calculation software JMatPro. The calculated results indicated that the two superalloys have the same equilibrium phases, such as liquid phase, γ phase, γ’ phase and TCP (topologically close-packed phases), however, there are differences in the quantity and temperature range. RenéN6 alloy has higher content of μ phase. And CMSX-10 alloy has higher γ’ phase precipitation temperature and more γ’ phase precipitates.

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.

Effect of chemical composition on particle morphology of topologically close-packed precipitates in a Ni-based single crystal superalloy

Topologically close-packed (TCP) phases in Ni-based single crystal superalloys are known to exhibit different morphologies and orientation relationships within the matrix. This study investigated the influence of chemical composition on lattice parameters, which in turn determine the interface lattice misfit between TCP phase and matrix and thus shape and orientation of the TCP phase. In this study, TCP phase particles of different morphologies were formed by ageing at different temperatures. The level of TCP-phase element enrichment was found to increase with increasing growth time and ageing temperature, and thus leads to a same evolution behavior of the atomic volume.

Fracture mechanisms induced by microporosity and precipitates in isothermal fatigue of polycrystalline nickel based superalloy

Isothermal fatigue (IF) of polycrystalline nickel based superalloy Inconel 718 was studied to reveal the effects of microporosity and TCP phase on the damage mechanisms and fatigue life. Two groups of as cast and heat isostatic pressed (HIP) superalloys were subjected to the isothermal fatigue at three temperatures (500 °C, 575 °C, and 650 °C), followed by microstructure characterization and fracture mode analysis via scanning electron microscope (SEM) and transmission electron microscope (TEM). Resulting data suggested that microporosity significantly shorten the fatigue life of the superalloy even temperature and applied stress load were controlled at low level. Fatigue cyclic life of the superalloy decreased with the temperature and applied stress load increasing. While as cast superalloy with low porosity exhibited a long fatigue life at 500 °C and 650 °C. Microporosity accelerated the crack growth rate in crack initiation zone and assisted the rapid propagation of secondary crack. Laves, as a topologically close packed (TCP) phase, cannot hinder enough dislocations before brittle rupture, while precipitation of strengthener γ′′ promoted formation of dislocation slip bands leading to high fatigue resistance. Abnormal strain localization at scripted carbide preferentially changed the shape of fatigue striation and introduced twin. As crack source, surface oxide scale can generate complicated crack networks.

Observation of the Dissolution of Topologically Close Packed Phases by an Additional Heat Treatment in Third Generation Nickel‐Based Single Crystal Superalloy Turbine Blades

Topologically close packed (TCP) phases degrade the superior creep and rupture properties of Ni‐based single crystal superalloys. Initially, small TCP phases are formed in the dendrite core during the early stages of the manufacturing process, such as solution heat treatment, and surrounded by a gamma prime (γ′) phase. Then, TCP phases continue to develop during full heat treatment. However, an additional heat treatment induces diffusion of refractory metals from the TCP phases into the γ′ phase and consequently, the TCP phases clearly disappear. After dissolution, the regions where the TCP phases existed are altered to a normal microstructure composed of γ channels and a normal cubic γ′ phase. Based on the observation result, the mechanism of the dissolution of TCP phases is discussed.

The effect of Ti addition on phase selection of CoCrCu0.5FeNi high-entropy alloys

ABSTRACTWith the increasing titanium, the volume fraction of face-centred cubic-structured dendrites decreased, and ordered B2 phase, σ, and Laves phase appeared in CoCrCu0.5FeNiTix alloy when x > 0.5. With the increase of Ti content, ΔSmix, ΔHmix, valence electron concentration (VEC), and Λ (=ΔSmix/δ2) of CoCrCu0.5FeNiTix alloys decreased, while atomic size difference (δ) and electronegativity difference (Δχ) increased. Topologically close-packed (TCP) structures are favoured when ΔSmix > 14.53 J K−1 mol−1. TCP phases can be found when δ > 5.05 and Δχ ≥ 0.124. Simple solid solution phase is favoured when VEC ≥ 8.26 but multi-phases including TCP phase coexist when VEC < 8.26. Single-disordered solid solution forms only when Λ > 0.637, solid solution and TCP phases coexist when 0.531 < Λ < 0.637, and TCP phases are favoured when Λ < 0.531.

Thermal cycling performance of La2Ce2O7/YSZ TBCs with Pt/Dy co-doped NiAl bond coat on single crystal superalloy

Thermal barrier coatings (TBCs) consisting of La2Ce2O7 (LCO) and Y2O3-stabilized-ZrO2 (YSZ) double-ceramic layer and Dy/Pt co-doped NiAl bond coat were produced by electron beam physical vapor deposition (EB-PVD). Thermal cyclic performance of the TBCs was evaluated by flame shock testing at 1300 °C. For comparison, the TBCs with a undoped NiAl bond coat were also studied. The microstructural evolution and failure mechanisms of the above TBCs during thermal cycling were investigated. Spallation failure of the TBCs with the undoped bond coat occurs after around 500 cycles by cracking at the interface between YSZ ceramic layer and thermally grown oxides (TGO) layer. The TBCs with Pt/Dy modified bond coat reveal improved interface bonding even after 1200 thermal cycles, whereas some delamination cracks are presented in the LCO layer. On the other hand, the Pt/Dy modified bond coat effectively suppresses the formation of the needle-like topologically closed packed phases (TCP) in the single crystal superalloy.

Minimum interface misfit criterion for the precipitation morphologies of TCP phases in a Ni-based single crystal superalloy

The precipitation and growth morphology of topologically close-packed (TCP) phases in a Re-containing Ni-based single crystal superalloy during thermal exposure were studied. The alloy was found to develop acicular-shaped coherent μ phase particles along <110>γ directions of the matrix after exposure at 950 °C. P phase precipitates were found to form with increased time and at higher temperatures, and in different morphologies, including acicular shaped particles along <112>γ and <110>γ directions, and plate-like particles in {110}γ planes of the matrix. The different P phase precipitates were found to have different lattice constants and different lattice correspondences with the matrix. The μ phase precipitate was also found to have different lattice correspondences with those reported in the literature. The multiplicity of lattice correspondences for the same TCP phase is attributed to the different lattice parameters, which render the precipitates to have different coherency relationships with the matrix.

Effect of dilution on the microstructure of AWS ERNiCrMo-14 alloy in overlay welding by the TIG process with cold wire feed

Dilution is an event that has a great influence on the microstructure and, consequently, in the properties of weld claddings. The present study evaluated the effect of dilution level on the microstructure of the fusion zone of alloy Inconel 686 weld claddings. The claddings were welded by the TIG cold wire feed process. The dilution was calculated from the geometric characteristics and based on chemical composition of the materials obtained by energy dispersive X-ray spectroscopy and optical emission spectroscopy. The microstructures of all claddings were composed by a γ-FCC matrix with some minor precipitates. These particles are called in the literature topologically close-packed phases (TCP). These precipitates assumed several morphological aspects, being the shapeless morphology observed in more frequency in all claddings evaluated, independent of dilution level. A phase with lamellar morphology, with growth establishing an orientation relationship with the γ-FCC matrix solidification direction, was observed only in claddings with a low dilution level. In addition, a shapeless phase with a fine precipitation aggregated was observed in claddings with a high dilution level.

The influence of high temperature annealing and creep on the microstructure and chemical element distribution in the γ, γ′ and TCP phases in single crystal Ni-base superalloy

The development of the microstructure in the CMSX-4 superalloy caused by high temperature annealing and creep and its relation with the chemical composition evolution of the γ, γ′ and topologically close-packed (TCP) phases was investigated at the micro- and nanoscales. The samples were creep deformed at a temperature of 900 °C and 1000 °C without previous treatment, and after pre-annealing at a temperature of 1100 °C. The changes in microstructure and elemental composition of phases were investigated using analytical electron microscopy, electron tomography and X-ray diffractometry methods. The combination of the different imaging techniques with the high precision of chemical composition microanalysis in nanoareas allowed to examine the compositional changes which accompany coalescence of the γ′ particles as well as precipitation and growth of the TCP phases in single-crystal nickel base superalloys. It was determined that in the temperature range of 900–1100 °C the P and μ phases with TCP structure are present in the CMSX-4 superalloy. The μ phase exhibits almost perfect crystal structure, while the P phase contains numerous planar defects. Precipitation of the TCP phases at the expense of the γ phase dissolution in their vicinity leads only to the localized depletion in refractory elements in the surrounding γ′ phase. The concentration of these elements in the γ phase remains on the similar level close to the TCP phases and away from them.

Topologically Close-packed Phase Formation in High Entropy Alloys: A Review of Calphad and Experimental Results

One of the major challenges in high entropy alloy (HEA) research is to obtain single-phase solid solutions by proper selection of components and processing techniques. Often one encounters situations where topologically close-packed (TCP) phases are present in the HEA microstructures. TCP phases are a class of intermetallic phases that are in general considered undesirable. The ability to predict these phases in HEAs using the Calphad (CALculation of PHAse Diagrams) method has been shown to accelerate the identification of promising compositions. In this review, an analysis of the reported Calphad studies and corresponding microstructural information on HEAs is done to evaluate the success of the Calphad method for TCP phases. A total of 52 alloys with 123 post-heat treatment microstructures reported so far have been compared. Challenges and issues in experiments and calculations are brought out with a possible way forward.

Investigation on the multi-pass gas tungsten arc welded Bi-metallic combination between nickel-based superalloy and Ti-stabilized austenitic stainless steel

Abstract

Composition design, phase transitions of a new polycrystalline Ni-Cr-Co-W base superalloy and its isothermal oxidation dynamics behaviors at 1300 °C

The composition, phase transitions and isothermal oxidation dynamics of a new polycrystalline Ni-Cr-Co-W base superalloy are investigated using CALculation of PHAse Diagrams (CALPHAD), Differential Scanning Calorimetry (DSC), ThermoGravimetric analysis (TG/dTG), Scanning Electron Microscope and Energy Dispersion Spectrum (SEM/EDS) methods. The microstructure of the designed alloy shows a uniform γ matrix with the precipitates of rich-Ti cellular η and rich-Nb acicular platelet δ phase, and no Topologically Close-packed Phases (TCPs) observed, and the oxide behavior obeys a parabolic law with a parabolic constant (kp) 2.945mg2/(cm4·h) at isothermal 1300°C for 30 hours. The oxide scales are composed of an outer loose oxide mixture containing Cr, Ni, Ti and Nb/Ta and generous pores presented for a volatilization of Cr2O3, and a dense aluminum-rich oxide layer.

Microstructural nature and stability of Co-rich TCP phases in Ru-containing single crystal superalloys

Destruction of microstructural stability caused by topologically close-packed (TCP) phase precipitation restricted the industrial application of advanced single crystal superalloys. However, systematic investigations on microstructural nature and evolution of Co-rich TCP phases in Ru-containing single crystal superalloys with high level of Co addition have been rarely reported. In this study, experimental results indicated that the main TCP phase was μ phase after thermal exposure at 950 °C for 1000 h, while R phase was the main phase after thermal exposure at 1100 °C for 1000 h in the alloys containing high level of Co and different levels of Cr and Mo. The experimental and thermodynamic calculation results showed that R phase was more stable at 1100 °C than that of μ phase in the experimental alloys. Meanwhile, μ phase started to transform to R phase at 1100 °C for 50 h, and the stress accelerated the transformation of μ phase and R phase during creep test at 1100 °C/140 MPa in an alloy containing high levels of Co, Cr and Mo additions. Both R phase and μ phase were enriched in Re, W, Mo, Cr and Co, while R phase contained more Cr and μ phase was more enriched in Mo. This study contributes to better understand the microstructural evolution of TCP phases at different temperatures and to optimize the alloy design and thermodynamic database of Ru-containing single crystal superalloys with high level of Co addition.

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.

On the crystallography and composition of topologically close-packed phases in ATI 718Plus®

ATI 718Plus® is a nickel-based superalloy developed to replace Inconel 718 in aero engines for static and rotating applications. Here, the long-term stability of the alloy was studied and it was found that topologically close-packed (TCP) phases can form at the γ-η interface or, less frequently, at grain boundaries. Conventional and scanning transmission electron microscopy techniques were applied to elucidate the crystal structure and composition of these TCP precipitates. The precipitates were found to be tetragonal sigma phase and hexagonal C14 Laves phase, both being enriched in Cr, Co, Fe and Mo though sigma has a higher Cr and lower Nb content. The precipitates were observed to be heavily faulted along multiple planes. In addition, the disorientations between the TCP phases and neighbouring η/γ were determined using scanning precession electron diffraction and evaluated in axis-angle space. This work therefore provides a series of compositional and crystallographic insights that may be used to guide future alloy design.

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.

Effect of Co on Discontinuous Precipitation Transformation with TCP Phase in Ni-based Alloy Containing Re

The effect of Co on discontinuous precipitation (DP) transformation involving the formation of topologically close-packed (TCP) phase was investigated in three Ni-Cr-Re model alloys containing different levels of Co. One typical TCP phase, σ, was generated within DP cellular colonies along the migrating grain boundaries in experimental alloys during aging treatment. As a result of the increased solubility of Re in the γ matrix and enlarged interlamellar spacing of σ precipitates inside of growing DP colonies, Co addition suppressed the formation of σ phase and associated DP colonies. This study suggests that Co could potentially serve as a microstructural stabilizer in Re-containing Ni-base superalloys, which provides an alternative method for the composition optimization of superalloys.