Topologically Close-packed Phases Research Articles

Characterization of the μ and P phase precipitates in the CMSX-4 single crystal superalloy.

A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning-transmission electron microscopy (STEM) using high-angle annular-dark-field (HAADF) imaging, focussed ion beam- scanning electron microscopy (FIB-SEM) tomography, selected area electron diffraction with beam precession (PED), as well as spatially resolved energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), was used to investigate topologically close-packed (TCP) phases, occurring in the CMSX-4 superalloy subjected to high temperature annealing and creep deformation. Structural and chemical analyses were performed to identify the TCP phases and provide information concerning the compositional partitioning of elements between them. The results of SEM and FIB-SEM tomography revealed the presence of merged TCP particles, which were identified by TEM and PED analysis as coprecipitates of the μ and P phases. Inside the TCP particles that were several micrometres in size, platelets of alternating μ and P phases of nanometric width were found. The combination of STEM-HAADF imaging with spatially resolved EDS and EELS microanalysis allowed determination of the significant partitioning of the constituent elements between the μ and P phases.

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

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

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

Abstract

Experimental and thermodynamic analysis of differences in phase transformation of Pt-modified nickel aluminide coating during isothermal and cyclic oxidation

Phase transformation of Pt-modified nickel aluminide coating β-(Ni,Pt)Al into γ′ phase was studied during isothermal and cyclic oxidation at 1100 °C. The transformation occurred with homogeneous β and γ′ phases with chemical compositions depending on oxidation conditions. Cyclic oxidation led to a higher volume fraction of topologically close-packed (TCP) phases formed in the interdiffusion zone and the chemical composition of TCP phases was found to vary with oxidation conditions. This difference in TCP precipitation in the interdiffusion zone for isothermal and cyclic oxidation was suggested to be responsible for different β and γ′ compositions. The experimental observations were discussed using thermodynamic calculations by Thermo-Calc software, and some explanations were given.

Ab-initio study of C and N point defects in the C14-Fe2Nb phase

Nb-alloying of steels can lead to the formation of topologically close-packed (TCP) phases, particularly Fe2Nb Laves and Fe7Nb6μ phases. The stability of these TCP phases is strongly affected by the presence of light elements like C and N. We calculate the solution energy of C and N in C14-Fe2Nb using density functional theory. N shows a strong preference to dissolve in larger interstitial voids while C shows a strong tendency to bind with a neighbouring Nb atom. The computed solution energies suggest N incorporation into Fe2Nb Laves phases while C is hardly soluble. The N-N interaction in Fe2Nb is strongly attractive and twice as strong as that of C-C. A comparison to C interstitials in the μ-Fe7Nb6 phase shows similar dependence of the solution energy on the atomic environment. In order to aid future work, we additionally provide the coordinates of interstitial sites in all TCP phases (A15, Zr4Al3, C14, C15, C36, χ, μ, σ, M, P, δ and R.) in the supplementary material.

Efeito da Diluição Sobre a Microestrutura da Liga AWS ER NiCrMo-14 na Soldagem de Revestimentos pelo Processo TIG com Alimentação de Arame Frio

Resumo: A diluição é um evento de forte influência na microestrutura e, consequentemente, nas propriedades de revestimentos soldados. O presente estudo avaliou o efeito do nível de diluição na microestrutura da zona fundida de revestimentos da liga Inconel 686. Os revestimentos foram depositados por processo TIG com alimentação de arame frio. A diluição foi calculada a partir das características geométricas dos revestimentos e por medidas de composição química de EDS e de espectroscopia de emissão ótica. As microestruturas de todos os revestimentos foram compostas por uma matriz γ-CFC com alguns precipitados secundários. Essas partículas são chamadas de fases topologicamente compactas. Esses precipitados assumiram diversos aspectos morfológicos, nos quais os precipitados com morfologia disforme foram observados com maior frequência em todos os revestimentos avaliados, independentemente do nível de diluição. Uma fase com morfologia lamelar, que estabelece uma relação de orientação com a matriz γ-CFC na direção de solidificação, foi observada nos revestimentos de menor diluição. Além disso, uma fase com morfologia disforme com uma fina precipitação agregada foi observada nos revestimentos mais diluídos.

Interaction between Re and W on the microstructural stability of Ni-based single-crystal superalloys

The influences of Re and W as well as their interaction on γ′ and topologically close-packed (TCP) phases have been investigated in seven kinds of Ni-based single-crystal superalloys. The results show that after full heat treatment, the γ′ size is reduced with increasing Re, but does not change with increasing W. After thermal exposure at 1000°C for 1000 h, the TCP phase is dramatically increased with increasing Re, but increased slightly with increasing W. The TCP phase volume fraction in higher Re alloys is much more than that in lower Re alloys which have the same total content of Re and W. This indicates that W instead of Re could effectively improve the microstructural stability of Ni-based single-crystal superalloys.

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.

Microstructure and cyclic oxidation behaviour of low-Pt/Dy co-doped β-NiAl coatings on single crystal (SC) superalloy

Abstract The low-Pt/0.1 at.% Dy co-doped β-NiAlCrSi coating was produced on single crystal (SC) superalloy IC21 by electro-plating of a ~ 3 μm thick Pt film followed by electron beam physical vapour deposition (EB-PVD) of a β-NiAlDyCrSi over layer. For comparison, the single low-Pt doped and single Dy-doped β-NiAlCrSi coatings were also prepared, respectively. The cyclic oxidation behaviour of the above coatings at 1200 °C was investigated. Minor Dy-doping led to refinement of grains of the as-deposited coating. The addition of Pt effectively suppressed precipitation of topologically close-packed phase (TCP) in the interdiffusion zone (IDZ) during heat-treatment. The coupled effects of Pt/Dy co-doping led to lower oxidation rate, less scale rumpling and improved scale adherence than the single-doping cases and the related mechanisms were discussed.

拓扑密堆 μ 相对含Hf的镍基粉末高温合金组织和性能的影响

It is widely acknowledged that topologically close packed (TCP) phases are detrimental to comprehensive properties of superalloys, as TCP phases deplete strengthening elements from matrix and easily become crack initiations. In this work, the precipitation kinetics and morphology of topologically close packed m phase in FGH4097 powder metallurgy (PM) superalloy with (0~0.89%)Hf and the effect of m phase on the mechanical prop*国家国际科技合作专项资助项目2014DFR50330 收到初稿日期: 2015-07-16,收到修改稿日期: 2015-12-23 作者简介:张义文,男, 1964年生,教授级高级工程师 DOI: 10.11900/0412.1961.2015.00389 第445-454页 pp.445-454

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

Abstract

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

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

Crystal-Structure Analysis with Moments of the Density-of-States: Application to Intermetallic Topologically Close-Packed Phases

The moments of the electronic density-of-states provide a robust and transparent means for the characterization of crystal structures. Using d-valent canonical tight-binding, we compute the moments of the crystal structures of topologically close-packed (TCP) phases as obtained from density-functional theory (DFT) calculations. We apply the moments to establish a measure for the difference between two crystal structures and to characterize volume changes and internal relaxations. The second moment provides access to volume variations of the unit cell and of the atomic coordination polyhedra. Higher moments reveal changes in the longer-ranged coordination shells due to internal relaxations. Normalization of the higher moments leads to constant (A15,C15) or very similar (χ, C14, C36, μ, and σ) higher moments of the DFT-relaxed TCP phases across the 4d and 5d transition-metal series. The identification and analysis of internal relaxations is demonstrated for atomic-size differences in the V-Ta system and for different magnetic orderings in the C14-Fe 2 Nb Laves phase.

GCP and TCP Phases Presented in Nickel-base Superalloys

The paper gives a short description of intermetallic phases presented in cast and wrought nickel based superalloys. Generally, the intermetallic phases presented in nickel based superalloys can be divided into three main groups: i - primary and secondary carbides; ii - GCP (Geometrically Close-Packed); and iii - TCP (Topologically Close-Packed). Of course, mentioned intermetallic phases have various influences on mechanical properties. Some of them have increasing strength or creep rupture life as well as fatigue properties. On the other hand, some act negatively to mechanical properties. Its influence strongly depends from morphology (shape or orientation in matrix), and quantity (e.g. small amount of delta phase Ni3Nb increases tensile strength and fatigue properties when situated on grain boundary of wrought IN 718 alloy) as well. Most of phases are presented after applied heat treatment, which commonly consist of solution annealing followed by artificial ageing.

The thermal stability of topologically close-packed phases in the single-crystal Ni-base superalloy ERBO/1

In Ni-base superalloys, the addition of refractory elements such as Cr, Mo, Co, W, and Re is necessary to increase the creep resistance. Nevertheless, these elements induce the formation of different kinds of intermetallic phases, namely, the topologically close-packed (TCP) phases. This work focuses on intermetallic phases present in the second-generation single-crystal (SX) Ni-base superalloy ERBO/1. In the as-cast condition, the typical γ/γ′ structure is accompanied by undesirable intermetallic phases located in the interdendritic regions. The nature of these precipitates as well as their thermal stability between 800 and 1200 °C has been investigated by isothermal heat treatments. The investigation techniques include DSC, SEM, EDX, and TEM. The experimental information is complemented by (1) comparison with a structure map to link the local chemical composition with phase stability, as well as (2) thermodynamic calculations based on the CALPHAD method to determine the occurrence and composition of phases during solidification and in equilibrium conditions. The TCP phases Laves, µ and σ were identified in various temperature/time ranges.

Thermal cyclic oxidation and interdiffusion of NiCoCrAlYHf coating on a Ni-based single crystal superalloy

A NiCoCrAlYHf coating was deposited onto a third generation single crystal superalloy DD10 by arc ion plating (AIP). In the present study, thermal cyclic oxidation and interdiffusion of NiCoCrAlYHf coating at 1100 °Cwere investigated to 1000 h. The coating surface, interface morphology and phase structure of the organization were characterized by scanning electron microscopy (SEM), energy dispersion X ray spectroscopy (EDS) and X ray diffraction (XRD). An interdiffusion zone (IDZ) and a secondary reaction zone (SRZ) were formed after 300 h. Severe inward diffusion of Al, Co and Cr from NiCoCrAlYHf coating and outward diffusion of Ni and refractory elements such as Re and W from the superalloy occur at 1100 °C in air. The SRZ consisting of γ, γ′ and topologically closed-packed phase (TCP) was formed 60 μm distance from the surface of coating in the superalloy after 300 h oxidation. Additionally, the development of IDZ and growth of the SRZ was also observed during thermal cyclic oxidation. Subsequently, interdiffusion coefficients of Al were presented by using a modified form of the Boltzmann–Matano analysis.

Development of advanced metallic alloys for solid oxide fuel cell interconnector application

Topologically closed packed (TCP) phases bearing Ni-based alloys have been developed as candidate materials for interconnector application in solid oxide fuel cell (SOFC). The high contents of W and Mo have promoted the formation of TCP phases and decreased the coefficient of thermal expansion (CTE) of these alloys. Furthermore, newly developed alloys can form continuous chromia for protection against oxidation, and exhibit low area specific resistance. They also show strong resistances against softening and Cr poisoning at 800 °C. The potentials of these low CTE Ni-based alloys for SOFC interconnector application are discussed in present article.

The promotion of Ru on topologically close-packed phase precipitation in the high Cr-containing (∼9wt.%) nickel-base single crystal superalloy

The partitioning behaviors of alloying elements Cr, Co, Al, Ta, etc. on the γ and γ′ phases of high Cr-containing (∼9wt.%) nickel-base single crystal superalloys have been investigated with and without the addition of 3wt.% Ru. The “reverse partitioning effect” occurs, which means that the addition of Ru causes the partition of Cr and Co into γ′ phase, while Al an Ta concentrate into γ matrix. However, Ru promotes the precipitation of topologically close-packed (TCP) phases, since the diffusion coefficient between the TCP phase and the matrix interface increases with the diffusion behaviors of refractory elements. For the coarsening and slightly rafting of γ′ phases, the increase of elemental diffusion plays much more important role in its kinetic process than the decrease of lattice misfit.

Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation

The precipitation of topologically close packed (TCP) phases is detrimental for the high temperature strength of high refractory Ni-based superalloys. The beneficial influence of Ru with respect to this so called instability is nowadays well accepted. In the present paper the precipitation of topologically close packed (TCP) phases is studied quantitatively in two experimental alloys (one Ru-free and one with addition of Ru) to clarify the mechanism of the Ru effect. It is confirmed that the TCP phase precipitates undergo sequential phase transformation with the tetragonal σ-phase precipitating first. Ru retards the phase transformation and leads to decreased equilibrium volume fraction of TCP phases. The results clearly indicate that Ru decreases the driving force for TCP phase precipitation. Investigations of crystallography and chemistry of the TCP/matrix interface point to an additional effect by increase of misfit strain energy.

Nanoindentation studies of the mechanical properties of the μ phase in a creep deformed Re containing nickel-based superalloy

Addition of Re in nickel-based superalloys results in an increase of the creep life. However, Re is also known to segregate to the dendrite core and to promote the formation of topologically closed packed (TCP) phases. In the present work, the local segregation of Re was studied in the heat treated and creep deformed state of a nickel-based superalloy. Tensile creep deformation at 1050°C resulted in the formation of TCP phases in the dendritic regions. Characterization using TEM confirmed the presence of μ phase that grows on {111} planes. Measurements with a nanoindenting AFM show that the μ phase is harder and shows less work hardening than both the γ and the γ′ phases. Furthermore, in the creep deformed state the hardness of the matrix phase is very similar in the dendrite core and in interdendritic areas, although Re is still enriched in the dendrite core. It is shown that Re is consumed in the dendrite core by the TCP phases.