Topologically Close-packed Research Articles

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.

HVOF 용사공정에 의한 단결정 초내열합금 코팅계면 이차 반응층 생성기구 고찰

The formation of the secondary reaction zone (SRZ) at the interface between the CoNiCrAlY coating and experimental Ni-based single crystal superalloy substrate was investigated. Transmission elec- tron microscope (TEM) observation revealed that the SRZ was composed of recrystallized γ΄ grains with a high density of fine topologically close-packed (TCP) precipitates. The TCP precipitate was identified as the faulted and twinned rhombohedral μ phase. TCP precipitation appears to be analogous to the discontinuous precipitation. The γ΄ grains nucleated epitaxially with grains in the coated layer. Preferential formation of the SRZ was observed along the direction. This anisotropy of SRZ formation may be essentially related to the preferential dislocation gliding direction where recrystallization would be most active due to high strain energy and the supply of fast interdiffusion pathways. Based on the suggested SRZ formation mechanism, the intermediate annealing is proposed to suppress SRZ formation in terms of mitigation of strain energy and interdiffusion. †(Received September 26, 2013)

In situ investigation on the precipitation of topologically close-packed phase in Ni-base single crystal superalloy

In situ transmission electron microscopy was first applied to study the precipitation behavior of topologically close-packed (TCP) phase at elevated temperature in Ru-free and Ru-containing Ni-base single crystal superalloys. Ru addition can decrease both the nucleation and growth rate, hence to suppress TCP precipitation effectively. Higher exposure temperature reduces the nucleation rate but increases the growth rate, finally accelerates the precipitation. The TCP phases nucleate homogeneously in γ matrix without preferential sites, but tend to grow toward primary γ/γ′ interface for element supply. An orientation relationship is kept during nucleation and growth of μ phase in γ matrix, which is [110]μ // [001]γ and (001)μ // (−110)γ.

The effects of Ta on the stress rupture properties and microstructural stability of a novel Ni-base superalloy for land-based high temperature applications

A novel polycrystalline Ni-base superalloy was developed for land-based high temperature applications, such as isothermal forging dies and industrial gas turbines. The alloy possessed surprisingly high stress rupture life of 52h at 1100°C/118MPa which is comparable to the first generation single crystal (SC) superalloy and exhibited good microstructural stability. The effects of Ta addition on the phase change, stress rupture properties and microstructural stability of the alloy were investigated. The results indicated that Ta is a γ′-former and promotes the formation of eutectic γ′. The alloys with ∼7vol.% eutectic γ′ possess higher stress rupture life at 1100°C/118MPa than the alloys with higher ∼20vol.% eutectic. However, ∼20vol.% excessive eutectic phases will enhance the stress rupture life at intermediate temperature of 760°C for 686MPa but weaken high temperature stress rupture properties. The (Al+Ta) content over 14.4at.% led to the formation of large amounts of eutectic γ′ and exceeded the solubility of W and Mo in the residue liquid pool, which then promoted the precipitation of primary α-(W,Mo) and M6C phases. Tantalum was also found as a strong MC carbides forming element. The order of ability to form monocarbide decreased from NbC to TaC to TiC. 6Al–0Ta (wt.%) alloys possessed good microstructural stability with no harmful topologically close-packed (TCP) phases being observed after thermal exposure at 850°C/3000h, 900°C/1000h. Only trace amounts of secondary plate-like M6C carbides appeared in Ta-free and 5Al–4Ta (wt.%) alloys at 1100°C/100–500h. However, excessive Ta addition will destabilize the alloy and large amounts of secondary plate-like M6C carbides precipitated after thermal exposure at 1100°C. The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) results showed the existence of the plate-like M6C carbides.

Effect of Long-Term Aging on the Microstructures and Stress Rupture Properties of a Ni-Based Single Crystal Superalloy at 980°C

The effect of aging time at 980°C on the microstructures and mechanic properties for a single crystal Ni-based superalloy was investigated in this paper. The results showed that γ’ size distribution portrayed a low growth rate and maintained well cubic during aging for 0~1000h at 980°C. However, γ’ phase was gradually coarsened directionally during the aging for 1000~2500h at 980°C, indicating that the γ’ phase coarsening was controlled by diffusion during aging for 0~1000h at 980°C. In addition, the γ’ phase coarsening was also susceptible to the interface reaction during aging for 1000~2500h at 980°C. Furthermore the precipitated phase was analyzed, and the results showed that only slightly Topologically Close-Packed (TCP) phase was precipitated after aging for 1500h at 980°C which indicates that the microstructures of alloy were stable during aging for up to 1500 hours. At the same time, stress rupture life decreased rapidly for 0~250h, but basically kept constant value when aged for 250~1500h, then decreased rapidly again aging for 1500~2000h.

Effects of electro-negativity on the stability of topologically close-packed phase in high entropy alloys

Topologically close-packed (TCP) phases with complex structures are often observed in high entropy alloys (HEAs). Currently, these TCP phases are garnering significant interest from both theoretical and experimental perspectives due to the ductility deterioration of the high strength HEAs. Alternatively, there are instances when TCP phases can actually benefit the mechanical performances of alloys, such as the wear resistance. Therefore, the stability of TCP phases should be taken into consideration in the alloy design. In this paper, the relationship between the TCP phase stability and the physicochemical/thermodynamic properties of alloying components in HEAs was systematically studied. The stability of TCP phases was found to correlate well with the electro-negativity difference (ΔX) for most of the reported HEAs. The stability of TCP phases is well delineated by the electro-negativity difference (ΔX): i.e., TCP phases are stable at ΔX > 0.133 except for some HEAs that contain a significant amount of aluminum.

Effect of Ru on microstructural evolution during heat treatment in single crystal superalloys

The effect of Ru addition on the as cast microstructure, structural evolution during heat treatment and long term aging was employed in single crystal superalloys with different contents of Ru addition (0, 1·5 and 3·0 wt-%). The results show that NiAl based β phase can occur in the interdendritic region in 3Ru alloy. With increasing Ru content in the alloys, the incipient melting temperature of alloy decreases gradually, while the liquidus and solidus changed slightly. After solution treatment and the same aging treatment, the γ′ phase size decreased with increasing Ru addition. An remarkable inverse partition of alloying elements, i.e. more matrix forming elements Re and Mo distribute to the γ′ phase and more Ni element distribute to the matrix, was observed in the alloys with more Ru content after aging treatment. Ru addition can accelerate the rafting of γ′ phase and suppress the precipitation of topologically close packed (TCP) phases effectively.

Effects of Long-Time Aging on Fatigue Crack Propagation Behaviors of a Hardworking Ni-Based Superalloy

Fatigue crack propagation behaviors coupled with the microstructure evolution of a hardworking Ni-based superalloy after long-time aging at 1023K were studied, to provide fundamental data for the fatigue life prediction of the superalloy. The results show that the microstructures remarkably change and the fatigue crack propagation resistance decreases with the aging time prolongs. It is found that the precipitation and the growth of topologically close packed (TCP) phases as well as the coarsening of γ' phase and carbides on grain boundaries can significantly affect the fatigue crack growth rate. On one hand, coarsened γ' phase and carbides at grain boundaries block dislocation movements near the crack tip, thus the fatigue crack propagation is hindered in near-threshold region and Paris region. On the other hand, the stress concentration accumulates continually with carbides precipitation increases, so that the grain boundaries become the main fatigue crack propagation rout. As well as, the effect of the TCP phases on the fatigue crack propagation behavior ascribes to the size and the distribution of TCP phases. Very small quantity of TCP phases contribute to pinning dislocation and enlarging fatigue crack propagation absorption energy, but high quantity of TCP phases with short rod shape changed to the needle, which gradually precipitate uniformly within the grain after 1000h besides on grain boundaries in the earlier aging, leads to much higher stress concentration degree. Those discussed above are the most important reasons why the fatigue crack growth rate increases after long-time aging.

The effect of Ru on precipitation of topologically close packed phases in Re – containing Ni base superalloys: Quantitative FIB-SEM investigation and 3D image modeling

A new approach to clarify the ruthenium effect on the precipitation of topologically close packed (TCP) phases is described in the paper. It is based on focused ion beam – scanning electron microscopy (FIB – SEM) dual beam methodology as well as three-dimensional imaging. The high-temperature capabilities of nickel base superalloys can be improved by alloying with refractory elements. With excessive refractory element content or excessive exposure to high temperature, brittle TCP phases precipitate resulting in a drop of strength. The undesirable phase transformation can be suppressed by addition of ruthenium. Although the effect is well known, its real mechanism remains open. In the present paper, the volume fraction and particle density, as well as the exact three-dimensional morphology of TCP phases as measured by FIB-SEM will be presented. The effect of ruthenium content and time of exposure is studied quantitatively. The results show that increased Ru additions slow down all stages of phase transformation and also reduce the equilibrium TCP volume fraction. The Ru effect might be due to either reduced driving force for precipitation or reduced interfacial energy.

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.

Influences of ruthenium and crystallographic orientation on creep behavior of aluminized nickel-base single crystal superalloys

The influences of ruthenium and surface orientation on creep behavior of aluminized Ni-base single crystal superalloys were investigated by comparing two different types of NKH superalloys. The aluminized coated specimens were then subjected to creep rupture tests at a temperature of 900°C and a stress of 392MPa. The coating treatment resulted in a significant decrease in creep rupture lives for both superalloys. The diffusion zones between the coating and substrate led to changes in microstructure, which diminished the creep behavior of the aluminized superalloys. Because of the interdiffusion of Ru, Al and Ni, the solubility of some of the refractory elements, such as W, Re. Mo, Co and Cr decreased in the diffusion zone; the precipitation of topologically close-packed (TCP) phases was thus inevitable. In the present study, the addition of Ru increased the degree of Re and Cr supersaturation in the γ matrix. Consequently, the addition of Ru indirectly promoted the precipitation of TCP phases in aluminized Ni-base single crystal superalloys. Furthermore, the growth of TCP precipitates was greatly influenced by the specific surface orientations of the Ni-base single crystal superalloys. In conclusion, the {110} specimens showed shorter creep rupture life than the {100} specimens, this was due to the difference in the crystallographic geometry of {111}〈101〉 slip system and TCP precipitates between the two side-surface orientations of the specimens.

Topologically close-packed phases in binary transition-metal compounds: matching high-throughput ab initio calculations to an empirical structure map

In steels and single-crystal superalloys the control of the formation of topologically close-packed (TCP) phases is critical for the performance of the material. The structural stability of TCP phases in multi-component transition-metal alloys may be rationalized in terms of the average valence-electron count and the composition-dependent relative volume-difference . We elucidate the interplay of these factors by comparing density-functional theory calculations to an empirical structure map based on experimental data. In particular, we calculate the heat of formation for the TCP phases A15, C14, C15, C36, χ, μ and σ for all possible binary occupations of the Wyckoff positions. We discuss the isovalent systems V/Nb–Ta to highlight the role of atomic-size difference and observe the expected stabilization of C14/C15/C36/μ by at ΔN = 0 in V–Ta. In the systems V/Nb–Re, we focus on the well-known trend of A15 → σ → χ stability with increasing and show that the influence of is too weak to stabilize C14/C15/C36/μ in Nb–Re. As an example for a significant influence of both and , we also consider the systems Cr/Mo–Co. Here the sequence A15 → σ → χ is observed in both systems but in Mo–Co the large size-mismatch stabilizes C14/C15/C36/μ. We also include V/Nb–Co that cover the entire valence range of TCP stability and also show the stabilization of C14/C15/C36/μ. Moreover, the combination of a large volume-difference with a large mismatch in valence-electron count reduces the stability of the A15/σ/χ phases in Nb–Co as compared to V–Co. By comparison to non-magnetic calculations we also find that magnetism is of minor importance for the structural stability of TCP phases in Cr/Mo–Co and in V/Nb–Co.

Effects of precipitated phases on the crack propagation behaviour of a Ni-based superalloy

Fatigue crack growth rate tests were carried out on a nickel-based superalloy GH4586 (in Chinese series) with various aging treatments at room temperature, to investigate the effect of precipitated phases on the crack propagation behaviour. The results show that the topologically close packed (TCP) phases precipitate and the γ′ phase and carbides at grain boundaries are coarsened after long time aging. The size of TCP phases becomes larger and the volume fraction of these phases increases with the increasing of aging time. The fatigue crack growth rate of the alloy at 700°C for 500h can be reduced by the small amount of TCP phases and coarsened γ′ phase with low coarsening degree, but it increases if these secondary phases are with large size and volume fraction. Very small quantity of TCP phases and small size γ′ phase in the alloy at 700°C for 500h can be cut through by dislocations, contributing to the yielding strength, thus the crack growth rate is reduced. Furthermore, cut-able TCP phases and γ′ phase leads to the formation of serrated and tortuous fracture surface, resulting in remarkable closure effect. On the other hand, the increasing of TCP phases and coarsened γ′ phase leads to the weakening of the crack closure effect and crack tip stress shielding effect.

Size versus electronic factors in transition metal carbide and TCP phase stability

The contributions of atomic size and electronic factors to the structural stability of transition metal carbides and topologically close-packed (TCP) phases are investigated. The hard-sphere model that has been used by Cottrell to rationalize the occurrence of the octahedral and trigonal local coordination polyhedra within the transition metal carbides is shown to have limitations in TiC since density functional theory (DFT) predicts that the second most metastable phase closest to the B1 (NaCl) ground state takes the B (BN) structure type with 5-atom local coordination polyhedra with very short Ti–C bond lengths. The importance of electronic factors in the TCP phases is demonstrated by DFT predictions that the A15, and phases are stabilized between groups VI and VII of the elemental transition metals, whereas the and Laves phases are destabilized. The origin of this difference is related to the bimodal shape parameter of the electronic density of states by using the bond-order potential expansion of the structural energy within a canonical tight-binding model. The importance of the size factor in the TCP phases is illustrated by the DFT heats of formation for the binary systems Mo–Re, Mo–Ru, Nb–Re and Nb–Ru which show that the and Laves phases become more and more stable compared to A15, and as the size factor increases from Mo–Re through to Nb–Ru.

Microstructural Stability of a Single Crystal Superalloy DD10 under Stressed and Un-Stressed Thermal Exposure at 980 °C

The effects of applied tensile stress on the microstructural stability of a third generation single crystal superalloy DD10 have been investigated under stressed and un-stressed thermal exposure at 980 °C. The results indicated that μ phase precipitated in the dendrite core after both stressed and un-stressed thermal exposure at 980 °C for 450 h. The μ phase formation, γ′ coarsening and rafting processes were promoted by the applied tensile stress. However, the precipitation of μ phase was not sensitive to the magnitude of applied tensile stress. Meanwhile, the applied tensile stress did not affect the type and morphology of the topologically close packed (TCP) phase. It is suggested that the interaction of the applied tensile stress and the misfit stress decreased the lattice misfit between μ phase and γ matrix and promoted the formation of μ phase.

실험 계획법 및 열역학 계산법을 이용한 초고온가스로용 니켈계 초합금 설계 방법론

This work is concerning a methodology of Ni-base superalloy development for a very high temperature gas-cooled reactor(VHTR) using design of experiments(DOE) and thermodynamic calculations. Total 32 sets of the Ni-base superalloys with various chemical compositions were formulated based on a fractional factorial design of DOE, and the thermodynamic stability of topologically close-packed(TCP) phases of those alloys was calculated by using the THERMO-CALC software. From the statistical evaluation of the effect of the chemical composition on the formation of TCP phase up to a temperature of 950 oC, which should be suppressed for prolonged service life when it used as the structural components of VHTR, 16 sets were selected for further calculation of the mechanical properties. Considering the yield and ultimate tensile strengths of the selected alloys estimated by using the JMATPRO software, the optimized chemical composition of the alloys for VHTR application, especially intermediate heat exchanger, was proposed for a succeeding experimental study.

Oxidation behavior characteristics of an aluminized Ni-based single crystal superalloy CM186LC between 900 °C and 1100 °C in air

Oxidation behavior of an aluminized Ni-based single crystal superalloy CM186LC was performed between 900°C and 1100°C in air. The oxidation kinetics approximately followed a parabolic oxidation law at 900°C and 1100°C. The mass gains were significantly increased owing to the formation of θ-Al2O3 during initial oxidation stage. After 100h oxidation, the mass gain rates were then decreased due to the transformation from θ-Al2O3 to α-Al2O3. The microstructures after 500h oxidation at all temperatures generally consisted of scale, coating layer, interdiffusion zone (IDZ), substrate diffusion zone (SDZ) accompanied with the topologically close-packed (TCP) and substrate.

The Effects of Ni-Plating and Prolonged High Temperature Oxidation at 1423 K (1150 °C) on a CMSX-10 Single-Crystal Ni-Based Super-Alloy and Coating System

A hypothesis was investigated, to assess if Ni-plating a 3rd-generation single-crystal alloy could favorably alter the diffusion profile of critical elements such that brittle, deleterious topologically close-packed (TCP) phases did not form or at least the extent to which they did was reduced. In conjunction with delaying the onset of these phases, it was hoped that more favorable alternatives could be promoted, such as martensite (β′). This study showed that Ni-plating did have some positive effects on the super-alloy/coating system. While the coating produced on the Ni-plated alloy was thinner, it retained a higher Al content than its unplated counterpart when subjected to oxidation. The retention of Al within the coating delayed the phase evolution of the coating from a β or β + β′ to a γ′-dominant coating as the Ni-plated system had a greater driving force for Ni and Al diffusion, which helped to establish the Ni-rich diffusion barrier that entrapped Al in the coating. Unfortunately, Ni-plating does not sufficiently alter the diffusion profiles within the alloy to prevent precipitation of the TCP phases. Four pairs of the CMSX-10 alloy were used for this study. While they were all aluminized, only half of them, one in each pair, were Ni plated prior to aluminizing. Three of the four pairs were then oxidized at 1423 K (1150 °C), while the first pair was kept as a standard. X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy were used to characterize each alloy-coating system in an effort to better understand their performance under high temperature oxidation.

Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys

Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-base single crystal superalloys have been investigated in the present study. The addition of Re seemed to be effective for the creep strength improvement of Ni-base single crystal superalloys. The creep strength was significantly decreased in aluminized specimens due to the change in microstructure under coating layer by the formation of the interdiffusion zone (IDZ) and substrate diffusion zone (SDZ). The specimens with {100} side-surface showed longer creep rupture lives than the specimens with {110} side-surface which means that the anisotropic creep behavior took place. The anisotropic creep behavior was more evident in aluminized CM186LC than in aluminized PWA 1480. This anisotropic creep behavior was mainly induced by the discrepancy of fracture modes due to the different arrangement of slip systems, the geometry as well as the depth penetration of topologically close-packed (TCP) phases between the two side-surface orientations.

SOLIDIFICATION BEHAVIOR OF A Pt-CONTAINING Ni-BASED SINGLE CRYSTAL SUPERALLOY

The Ni-based single crystal superalloy is the primary material for the manufacture of blades of the advanced areoengine and gas turbine.As the alloying design theory improved,the concentration of refractory elements increased and some new elements were introduced.Among these refractory elements,Ru became a fresh and sign element which was introduced into the fourth generation Ni-based single crystal superalloy.A large amount of research had indicated that Ru,as a member of Platinum group metals(PGMs),had played a significant role on the enhancement of phase stability and rupture life for Ni-based single crystal superalloy.Inspired by these works,other PGMs like Pt have been suggested to be the major alloying elements of the next generation advanced Nibased single crystal superalloy.But the research for the effects of Pt addition on solidification behavior or creep property of the single crystal superalloy is rare.To explore the possibility of Pt using as a major alloying element,the present work investigated the influence of Pt on the solidification behavior of the Ni-based single crystal superalloy.Directional solidification method was used to grow the single crystal.DTA and EPMA were used to determine the effect of Pt on the phase transition temperatures and composition variety,respectively.In addition,OM and SEM were used to show the phases morphology.Some results are described as follow.Firstly,it has been found that the addition of Pt changes the eutectic morphology and increases eutectic fraction.Grid-like eutectic increases with the addition of Pt.Secondly,Pt promotes not only the segregation of refractory elements but also the eutectic-forming element Al.In addition,Pt prefers to segregate to the interdendritic region and is able to form an ordered PtsAl phase with Al which may be a reason for the increase of eutectic fraction.But Pt reduces the segregation of Mo element whose content is a sensitive factor for the formation of the topological close-packed(TCP) phase.Thirdly,Pt decreases initial melting temperature and enhances γ' phase precipitation temperature;thereby reduces the solution heat treatment window of the alloys.The solution heat treatment of the alloys therefore becomes more difficult.Since the element segregation is hard to be eliminated by heat treatment in the Pt-containing alloys,Pt addition may be harmful for the mechanical properties of single crystal superalloy.The effects of Ru addition on the solidification behavior of the Ni-based single crystal superalloy will be also discussed for comparison.