Topologically Close-packed Research Articles

Role of Cobalt in the Microstructural Stability of Single Crystal Superalloys

The effects of Co additions on the evolution of γ' precipitates and topologically close-packed (TCP) phases during thermal exposure at 950 °C were investigated for two Ni-based single crystal superalloys with 7.9 wt.% Co and 9.1 wt.% Co. The results indicated that the γ′ morphology was not affected by Co content, whereas γ′ volume fraction decreased and precipitate size increased due to lower Co addition after standard heat treatment. The coarsening of γ′ precipitates in both alloys was controlled by diffusion. The γ′ coarsening rate increased, while the stability of γ′ volume fraction decreased due to lower level of Co addition during 950 °C thermal exposure because more Co addition suppressed the diffusion process. High Co addition promoted the formation of μ phase after thermal exposure at 950 °C for 1000 h due to higher γ′ volume fraction, more stable γ′ volume fraction and higher electron vacancy number. The experimental results of μ phase precipitation showed good agreement with thermodynamic calculation by JMatPro.

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.

Microstructure evolution of an EB-PVD NiAl coating and its underlying single crystal superalloy substrate

A NiAl coating was deposited onto a Ni-based single crystal superalloy with (001) crystal orientation by electron beam physical vapor deposition (EB-PVD). The as-deposited NiAl coating showed a columnar microstructure with (110) preferred orientation. The microstructure evolution behavior near interface between the NiAl coating and superalloy substrate at 1100 °C was investigated. Kirkendall voids were formed in the NiAl coating, indicating the different elements diffusion coefficients in the coating and substrate. Interdiffusion zone (IDZ) with rod-like and granular topological close-packed (TCP) phases and substrate diffusion zone (SDZ) with needle-like TCP phases were formed during diffusion annealing at elevated temperature. The equi-axed β-NiAl grains were developed in the IDZ after diffusion annealing at 1100 °C for 10 h, which showed different orientations from the coating and substrate. However, after 50 h diffusion annealing, the equi-axed β-NiAl phases in the IDZ were transformed into γ′-Ni3Al phases which had the same orientation as the substrate. Furthermore, oriented rafting of the substrate occurred during diffusion annealing and the rafts were parallel to the coating/substrate interface.

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.

铸造镍基高温合金中初生 M C碳化物的退化过程和机理

Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During longterm thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+g →M6C+g′, MC+g→M6C + M23C6+ g′ and MC+g→M6C + M23C6+h. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of g′ phase, coarsening of grain boundaries and precipitation of topologically closepacked (TCP) phase, and consequently the mechanical properties of alloys. Much research work has focused on the degeneration mechanism of primary MC carbide during long-term thermal exposure, however, it is not very clear so far. In this work, a cast Ni-based superalloy is fabricated and thermally exposed at 850 °C for 500~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primary MC carbide is observed by OM, SEM and TEM. High-angle annular dark field (HAADF) mode of TEM is used to clearly observe the degeneration of primary MC carbide and the element distribution in the degeneration areas. The results show that the primary MC degeneration is an inter-diffusion process which occurs between the primary carbide and the g matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the g matrix, while Ti, W and Mo come from both primary MC and g matrix. The precipitation of h phase is determined by *国家自然科学基金项目51001101和国家能源局项目NY20150102资助 收到初稿日期: 2015-07-17,收到修改稿日期: 2015-11-24 作者简介:孙文,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00399 第455-462页 pp.455-462

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.

Classification and microstructural stability of high generation single crystal Nickel-based superalloys

This paper presents a brief review on various types of single crystal Ni-based superalloys designed over the recent years. The design principles to achieve required mechanical properties and microstructural stability over the long time exposure to high temperatures are emphasized. The main attention is given to alloys with a high content of refractory elements. In particular the formation of brittle phases, like topologically close packed (TCP) phases, degrading mechanical properties of the Ni-based superalloys is emphasized.

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.

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.

Effect of Ru on interdiffusion dynamics of β-NiAl/DD6 system: A combined experimental and first-principles studies

As a diffusion barrier between thermal barrier coating (TBC) and advanced single crystal (SC) superalloy DD6, RuNiAl coating has attracted increasing attention recently. In this work, different diffusion couples including NiAl/DD6, RuNiAl/DD6 and RuAl/DD6, were prepared and their interdiffusion behavior was investigated at 1100°C, to understand the diffusion barrier mechanism of the RuNiAl coating. The addition of Ru to NiAl effectively reduced the interdiffusion coefficient of Al, thereby delaying the phase transformation from β to γ′ and suppressing the formation of topologically closed-packed (TCP) phases and primary interdiffusion zone (IDZ). It is verified by first-principles calculations that Ru restrained the formation of Ni and Al vacancies and Ni and Al antisite atoms. According to the calculations, the addition of Ru increases the defect formation energies, thus inhibiting the diffusion of Al in β-NiAl. The calculation results are consistent with the experimental data.

An overview of rhenium effect in single-crystal superalloys

Nickel-based single-crystal superalloys are the key materials for the manufacturing and development of advanced aeroengines. Rhenium is a crucial alloying element in the advanced nickel-based single-crystal superalloys for its special strengthening effects. The addition of Re could effectively enhance the creep properties of the single-crystal superalloys; thus, the content of Re is considered as one of the characteristics in different-generation single-crystal superalloys. Owing to the fundamental importance of rhenium to nickel-based single-crystal superalloys, much progress has been made on understanding of the effect of rhenium in the single-crystal superalloys. While the effect of Re doping on the nickel-based superalloys is well documented, the origins of the so-called rhenium effect are still under debate. In this paper, the effect of Re doping on the single-crystal superalloys and progress in understanding the rhenium effect are reviewed. The characteristics of the d-states occupancy in the electronic structure of Re make it the slowest diffusion elements in the single-crystal superalloys, which is undoubtedly responsible for the rhenium effect, while the postulates of Re cluster and the enrichment of Re at the γ/γ′ interface are still under debate, and the synergistic action of Re with other alloying elements should be further studied. Additionally, the interaction of Re with interfacial dislocations seems to be a promising explanation for the rhenium effect. Finally, the addition of Ru could help suppress topologically close-packed (TCP) phase formation and strengthen the Re doping single-crystal superalloys. Understanding the mechanism of rhenium effect will be beneficial for the effective utilization of Re and the design of low-cost single-crystal superalloys.

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.

Experimental determination of the Ni–Cr–Ru phase diagram and thermodynamic reassessments of the Cr–Ru and Ni–Cr–Ru systems

Ruthenium addition was found to be beneficial in inhibiting the formation of the detrimental topologically closed-packed (TCP) phases in Ni-based single crystal superalloys; thus, the Ni–Cr–Ru phase stability is very valuable for better understanding of the constitutional effects of Ru in multicomponent Ni-base superalloys. Four isothermal sections of the Ni–Cr–Ru system at 800 °C, 900 °C, 1000 °C and 1200 °C were constructed from results obtained from diffusion multiples using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The measured phase equilibria together with prior experimental results were used to optimize a CALPHAD thermodynamic description of the Ni–Cr–Ru ternary system using thermodynamic parameters of the Ni–Cr and Ni–Ru systems published in the literature and modified Cr–Ru parameters from the current study. The intermetallic compound sigma phase in the Cr–Ru system was described with a simplified two-sublattice model, (Cr,Ru)20(Cr,Ru)10, by taking into account future extension of the model into higher order systems. Comparison between the calculated results and the experimental measurements was made to demonstrate the reliability of the current thermodynamic parameters for the Ni–Cr–Ru system. For easy implementation of the thermodynamic parameters into some existing thermodynamic databases, a version of the thermodynamic description of the Ni–Cr–Ru system with an older sublattice model, (Cr,Ni,Ru)10(Cr)4(Cr,Ni,Ru)16, for the sigma phase was also developed.

A Criterion for Topological Close-Packed Phase Formation in High Entropy Alloys

The stability of topological close-packed (TCP) phases were found to be well related to the average value of the d-orbital energy level \( \overline{Md} \) for most reported high entropy alloys (HEAs). Excluding some HEAs that contain high levels of the elements aluminum and vanadium, the results of this study indicated that the TCP phases form at \( \overline{Md} \) > 1.09. This criterion, as a semi-empirical method, can play a key role in designing and preparing HEAs with high amounts of transitional elements.

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.

Effects of Cr content on microstructure and mechanical properties of single crystal superalloy

Two experimental single crystal superalloys with 2% Cr and 4% Cr (mass fraction) were cast in a directionally solidified furnace, while other alloying element contents were kept unchanged. The effects of Cr content on the microstructure, phase stability, tensile properties at 1100 °C and stress rupture properties at 1070 °C and 160 MPa of the single crystal superalloy were investigated. The results show that the size of γ' phase particles become small and uniform, and the cubic shape turns a little regular with the increase of Cr content. The γ' directional coarsening and rafting were observed in the 2% Cr and 4% Cr alloys after long term aging (LTA) at 1100 °C. The rafting rate of γ' phase increased with increasing Cr content. Needle-shaped topologically close packed (TCP) phases precipitated and grew along fixed direction in both alloys. The precipitating rate and volume fraction of TCP phases significantly increased with the increase of Cr content. The tensile property of the alloy increased and the stress rupture properties of the alloy decreased with the increase of Cr content at high temperature. The increase of Cr content increased the partition ratio of TCP forming elements, Re, W, and Mo, and the saturation degrees of these elements in γ phases increased. Therefore, the high temperature phase stability of the alloy decreased with the increase of Cr content.

Structural stability of Fe-based topologically close-packed phases

Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe–Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size.

Thermal stability of primary MC carbide and its influence on the performance of cast Ni-base superalloys

Investigations into the thermal stability of primary MC carbide (TSMC) during long-term thermal exposure and its effects on the performance of nine cast Ni-based superalloys with different Nb/Ti and (W + Mo)/Cr ratios have been undertaken. The results show that the chemical composition of primary MC can be precisely modified by the Nb/Ti ratio of alloys while the distribution and morphology of primary MC carbides is not affected. During long-term thermal exposure, the TSMC is dependent not only on the composition of primary MC, i.e. the Nb/Ti and (Nb + Ti)/(W + Mo) ratios of primary MC, but also on the (W + Mo)/Cr ratios of the alloys, and the latter has a more significant effect than the formers. With the decrease of these parameters, the TSMC decreases and more degeneration reactions of primary MC occur during thermal exposure. Moreover, with the (W + Mo)/Cr ratio of the alloys decreasing from 0.55 to 0.37 and finally to 0.22, the transformed carbides from primary MC change from M6C to coexist of M6C and M23C6 and to M23C6. With the decrease of TSMC, the stress-rupture life of alloy at 800 °C/294 MPa is improved, because that more secondary carbides precipitate in the matrix which can retard or even suppress the topologically close-packed (TCP) phases.

The influence of thermal exposure on the microstructure of a Ni3Al-based single crystal superalloy

The effect of thermal exposure on the microstructure of an Ni3Al-based single crystal superalloy was investigated. The Ni3Al-based single crystal superalloy used in this experiment contained a high content of molybdenum (Mo) and a low content of chromium (Cr), which had good high-temperature strength and oxidation resistance. The alloy was exposed at 1020–1200°C for different time periods. The microstructure after thermal exposure was examined by scanning electron microscope (SEM) and field emission microscope (FEM). The results indicated that precipitation of needle-like topologically close-packed (TCP) phases was only observed at certain intermediate temperature intervals (1020–1080°C). Moreover, once the TCP phases precipitated, they would not dissolve back even after thermal exposed for 1000 h. No precipitation phases were observed after high-temperature thermal exposure at 1100–1200°C for 25 h.