Topologically Close-packed Research Articles

The role of TCP structures in glass formation of Ni50Ag50 alloys

A molecular dynamics simulation of rapid cooling at a cooling rate of 1011 K/s for Ni50Ag50 alloys was conducted. The rapid cooling process was analyzed by using the system energy, the pair distribution function, and the largest standard cluster analysis. It is found that with a crystallization-like energy curve the solidification of Ni50Ag50 alloy experiences a liquid-liquid and a liquid-solid transition (LLT and LST), finally result in an amorphous solid. In LLT stage the topologically close-packed (TCP) short-range order structures of both crystal (S666) and glass (S555) increase; whereas only glass medium-range order structures are formed. TCP-based long-range order (LRO) structures of glass do not increase until the beginning of the LST. Even in solid (T < Tg) the LRO parameter based on ICO structures fluctuates violently, while that based on TCP structures is quite stable. Therefore, TCP not only is the basic structural characteristics of disordered system but also reasonably represents the metastability of metallic glasses. These findings not only clarify the importance of LLT for the glass forming ability of alloys, but also fully demonstrate the effectiveness and necessity of TCP structures for describing the structure of amorphous systems.

Competition between TCP and crystalline clusters during phase transition of rapidly super-cooled aluminum

Undercooling is usually a necessary condition for the starting of crystallization of molten metal under cooling. What structure hampered crystallization is however still unclear yet. The rapid cooling of pure aluminum has been conducted by a large-scale molecular dynamics simulation at different cooling rates. The evolution of local structures and phase transition mechanism are investigated in terms of the atom energy and the largest standard clusters. It is found that with high local density and low average potential energy (APE), the topologically close-packed (TCP) clusters in liquids increase exponentially with decreasing temperature, and have a number saturation stage whose survival time is related to cooling rate. When the crystalline clusters’ APE is lower than that of the system, crystal nucleation does not begin until the TCP clusters’ APE become higher than the system. Thus the competition between TCP and crystalline clusters results in the saturation of TCP clusters. These findings will provide new understandings on solidification and nucleation theory.

A new method to develop a NiRe-Al2O3 diffusion barrier between single crystal Ni-base superalloy and NiAl coating

Formation of a thick interior interdiffusion zone (IDZ) dispersing brittle topologically close packed (TCP) precipitates is a key factor of unacceptably damaging the high temperature creep resistance of advanced single crystal (SX) Ni-base superalloy, when it is coated with conventional NiAl coating and exposed to high temperature environment. In this work, a NiRe-Al2O3 layer between the NiAl coating and the SX Ni-base superalloy was developed by electrophoretic deposition of α-Al2O3 nanoparticles followed by NiRe alloy electrodepostion. The 100 h oxidation at 1050 °C indicates that the intermediate layer worked well as a diffusion barrier (DB), not only decreasing the thickening rate of deterioration zone in the superalloy, but also totally preventing the coating from being penetrated by refractory elements from the underlying alloy and DB, which is always observed for typical metallic DBs. The beneficial effect of the new DB on interdiffusion is discussed.

Formation mechanism and thermal stability of C15-Laves phase in a Hf-containing Co-based superalloy

Topologically close-packed (TCP) phases such as μ and Laves phases formed during solidification has an important influence on the mechanical properties of superalloys. Understanding the formation and elimination mechanism of TCP phases is the key to promote the development of superalloys. In this study, C15-Laves phase was first found in a Hf-containing Co-based superalloy, meanwhile, the formation mechanism and thermal stability of the C15-Laves phase were clarified. The results showed that the formation of C15-Laves phase was associated with the segregation behavior of alloying elements during solidification: Co, W and Cr segregated to the dendrite core, and Hf, Ta, Ti, Mo, Ni, Al, Si segregated to the interdendritic region. In the final stage of solidification, the C15-Laves phase enriched in Hf, Ta and Ti formed, and it had similar lattice parameters to the C15-Co2Hf phase. The incipient melting point of C15-Laves phase was between 1170 ℃ and 1180 ℃, and it gradually decomposed to form the Hf-rich MC carbide during solution treatment. The large formation energy differences between the MC carbide and C15-Laves phase promoted a rapid elimination of the C15-Laves phase, which reduced solution treatment time of the alloy. The results can provide a theoretical basis and experimental data support for the composition design and heat treatment process optimization of multicomponent novel Co-based superalloys.

On the interdiffusion and precipitation of topologically close-packed phases in (Ni,Pt)Al coating/superalloy system: Effects of Pt and Ni contents in coating

In combination with the electroplating and electron beam physical vapor deposition (EB-PVD), four types of (Ni,Pt)Al coatings, namely Ni45Al, (Ni,Pt)45Al, Ni50Al, and (Ni,Pt)50Al were prepared. In order to study the interdiffusion and topologically close-packed (TCP) phases precipitation behavior, annealing experiments were carried out at 1100 °C for 20 h in vacuum. The addition of Pt layer has little effect on the thickness of interdiffusion zone (IDZ) because the inhibition effect of Pt on the Al diffusion weakens with the continuous diffusion of Pt. Meanwhile, the increase of Pt content in substrate promotes the Al diffusion. Besides, the effect of Pt on the TCP phases precipitation depends strongly on the Ni content in coating-For Ni45Al coating, adding Pt makes the volume fraction of TCP phases reduces from ~16.35 % to ~3.15 %, whereas this value is comparable in both Ni50Al and (Ni,Pt)50Al coatings (18.3 % and 17.9 %). This phenomenon is attributed to the competition mechanism between the acceleration effect of Pt on Al diffusion and the inhibition effect of Pt on TCP phases growth. The results provide a better understanding about the microstructure, especially the IDZ and TCP characteristics of (Ni,Pt)Al coating, which is significant for controlling the interdiffusion and TCP precipitation behavior in (Ni,Pt)Al coating.

The topologically close-packed Fe70Cu15Ni15 nanoparticles- A simulation study

The classical molecular dynamics simulation is performed to investigate the frozen structure obtained by rapid cooling Fe70Cu15Ni15 nanodroplets, in terms of the system energy, the pair distribution function (PDF), and the largest standard cluster analysis (LaSCA). Interestingly, all frozen nanodroplets have a core-shell structure with copper atoms on the surface, whether they are amorphous or crystalline nanoparticles. Besides the familiar body-centered cubic (BCC) structure, a topologically close-packed (TCP) nanoparticle is observed for the first time. For a type of nanodroplets, the larger the size, the higher the onset temperature of crystallization, and the lower the potential energy of the nanoparticles at 300 K. For BCC nanoparticles crystallinity increases with size, but this is not the case for TCP nanoparticles. These findings indicate a simple and efficient way to produce core-shell nanoparticles.

Microstructural degradation after thermal exposure of a Re-containing single crystal superalloy

Microstructural degradation after thermal exposure of a 5% Re -containing single crystal (SX) superalloys has been studied. The alloy was subjected to thermal exposure at temperature of 1100 °C for periods of 100, 500, and 1000 h. The microstructure of the alloy was degraded after long-term thermal exposure, mainly in γ' coarsening, the precipitation topologically close packed (TCP) phases. Moreover, the stress-rupture properties of the samples before and after long-term thermal exposures were characterized. The results showed that the γ' coarsening and TCP formation after exposed for 500 h, which led to a damaged of the stress-rupture life time. Besides, the growth of pores during exposure also influences the stress-rupture life. • γ' coarsening and TCP phase precipitation are characterized after long-term exposure. • The TCP phase does not embrittle the alloy. • The severe γ' coarsening is the main factor in the degradation of the life time.

Dual plateau stress of C15-type topologically close-packed lattice structures additive-manufactured by laser powder bed fusion

Imitating crystal structures offers new avenues to design lattice structures composed of unit cells with nodes (act as atoms) and struts (act as atomic bonds). We found out unique dual plateau regions in the stress–strain curves of lattice structures inspired by topologically close-packed (TCP) structure of C15 Laves phase (AB2-type intermetallic compounds) and manufactured via the laser powder bed fusion. X-ray computed tomography of compressively deformed lattice structures as well as finite element analysis revealed that the existence of two kinds of units with different relative density caused the dual plateau regions. In addition, the C15 lattice structures exhibited a hybrid deformation mode of bending- and stretch-dominated deformations and can absorb high total energy of 5.0 MJ·m−3 in two stress levels.

Laves phase crystal analysis (LaCA): Atomistic identification of lattice defects in C14 and C15 topologically close-packed phases

The identification of defects in crystal structures is crucial for the analysis of atomistic simulations. Many methods to characterize defects that are based on the classification of local atomic arrangement are available for simple crystalline structures. However, there is currently no method to identify both, the crystal structures and internal defects of topologically close-packed (TCP) phases such as Laves phases. We propose a new method, Laves phase crystal analysis (LaCA), to characterize the atomic arrangement in Laves crystals by interweaving existing structural analysis algorithms. The new method can identify the polytypes C14 and C15 of Laves phases, typical crystallographic defects in these phases, and common deformation mechanisms such as synchroshear and non-basal dislocations. Defects in the C36 Laves phase are detectable through deviations from the periodic arrangement of the C14 and C15 structures that make up this phase. LaCA is robust and extendable to other TCP phases.Graphic abstract

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa. Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared. For specimen with higher inhomogeneity, stronger segregation of refractory elements reinforces the local strength in dendrite core, but damages the strength in interdendritic region. Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing in γ′ phase, faster degradation of dislocation networks and facilitated topological inversion of rated structures. Although the segregation of refractory elements produces a high density of topologically close-packed (TCP) phase in dendrite core, faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen. In another specimen, increased solid solution time gives rise to overall reduced inhomogeneity. Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region. The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity. Importantly, the balance of local strength between dendrite core and interdendritic region results in over 40% increase of creep rupture life of the second specimen.

Thermal Exposure Effect on the Microstructural and Mechanical Properties of a Laser-Welded Inconel 617 Joint in an Air Environment

A laser-welded Inconel 617 superalloy was thermally exposed at 750 °C for up to 7000 h. It was found that carbides (M23C6, M6C-type Ti(C,N)), topological close-packed (TCP) μ phase and γ' phase (Ni3Al) were formed in the laser welds of Inconel 617 during thermal exposure. The spherical γ' phase in laser-welded Inconel 617 coarsens due to Ostwald ripening, while it shows better thermal stability under long-term exposure at 750 °C. The relationship between the carbides (the globular M6C, the short rod-like M6C and the block-shaped M23C6 carbide) and the matrix γ is a cube-on-cube orientation, while the relationship between the long rod-like M6C carbide and matrix γ is a noncoherent orientation. An increase in the volume fraction and average diameter of the M23C6 and M6C carbides is controlled by the diffusion of C, Cr and Mo atoms. The precipitation of the block-shaped Mo-rich μ phase is promoted in the carbon-depleted region around the M6C carbide by the segregation of Co and Mo. The mass precipitation of the γ' phase, M6C carbide and M23C6 carbide results in an increase in the hardness and a significant drop in the impact toughness of the exposed welded metals after 7000 h of exposure.

The Site Preferences of Transition Elements and Their Synergistic Effects on the Bonding Strengthening and Structural Stability of γ′-Ni3Al Precipitates in Ni-Based Superalloys: A First-Principles Investigation

Advanced mechanical properties of Ni-based superalloys strongly depend on the site preferences of alloying X elements in γ′-Ni3Al-X precipitates, which are associated with the partial bonding characteristics between Ni, Al, and X atoms. Therefore, in the current work, the site occupancy tendencies of transition X metals were revealed via first-principles ab initio calculations at 0 K. Bonding features of Ni-Al, Ni-X, and Al-X pairs were simulated by using the charge density difference (CDD), electron localization function (ELF), and density of states (DOS) methods, respectively. According to simulations, higher atomic size X elements preferably occupy Al sites of γ′-Ni3Al-X intermetallics and lead to strong covalent-like directional bondings between themselves and their nearest neighbor (NN) Ni atoms along 〈110〉 directions. However, if these larger X metals substituted for Ni sites, the bonding properties would differ by plane due to the nature of the L12-type crystal structure of γ′-Ni3Al-X precipitates. Considering all transition elements, refractory metals (i.e., X = Re, W, Mo, Ta, or Nb) appear as the most effective strength inducers, improving the structural stability of γ′ phase, even if Ni site substitution of X = Re atoms would start to increase structural instability. On the other hand, relatively small alloying X elements having electron configuration similarities with Ni (i.e., X = Co, Cu, Rh, Pd, Ag, Ir, Pt, or Au) are more likely to worsen bonding strengthening. Instead, these transition X metals creating metallic bondings with NN Ni atoms would contribute to ductility and malleability of Ni-based superalloys. Furthermore, depending on the relative atomic size of γ′-former and refractory elements, the phase and site preferences of refractory atoms would alter in multicomponent systems. As a result of the attractive or weak repulsive forces between Re-Re, Re-Mo, and Re-W pairs, the structural stability of the constituent phases would deteriorate and harmful topologically close-packed (TCP) phases would precipitate.

Influence of long-term aging on microstructural stability and performance of DD6 superalloy

In order to investigate the deterioration of microstructure in service, the intrinsic microstructural stability and performance of DD6 during long-term aging without an external load are investigated. During aging at 980°C, γ′ rafting in 〈100〉 and 〈010〉 directions show no anisotropy due to small driving forces. γ′ coarsening and rafting lead to a monotonic decrease in hardness between 200 and 1000 h. However, precipitation of topologically close-packed (TCP) phases occurs significantly during aging at 1050°C. The amount of TCP phases increases initially, and then almost saturates, which results in a decrease in hardness. However, owing to the reduction in the growth of the TCP phases and coarsening of γ′, the hardness saturates for an aging time greater than 500 h.

Study on Si-like and topologically close-packed structures during rapid solidification of Au-Si alloys

A molecular dynamics (MD) simulation has been conducted to investigate the Si-like and topologically close-packed (TCP) structures in AuxSi(100−x) alloys during rapid solidification at the cooling rate of 1 × 1011 K/s. A new method named largest standard cluster analysis (LSCA) is performed to analyze and characterize the microstructural evolution during solidification. Results indicate that it is difficult to crystallize for each kind of Au-Si alloy, but eventually freeze into amorphous solids. The Si-like structures with low coordination number are significantly formed in the Si-rich alloys with Au content CAu≤30%, while TCP structures with higher packing density prefer to form in the Au-rich alloys with CAu≥80%. Moreover, the Si-like Medium-range order (MRO) structures are more likely to link with each other by 1100 bond-type, while the more denser TCP MROs are inclined to combine by 1551 bond-type. This provides new analysis ideas and methods for studying the amorphous structure between transition metal and semiconductor.

AlCoCrNiMo high-entropy alloy as diffusion barrier between NiAlHf coating and Ni-based single crystal superalloy

To solve the issue of elemental interdiffusion between protective coating and superalloy substrate during high-temperature service, an AlCoCrNiMo high-entropy alloy (HEA) diffusion barrier was deposited by direct current magnetron sputtering between NiAlHf protective coating and Ni-based single crystal superalloy (René N5). The interdiffusion behavior and high-temperature oxidation resistance of the coating system were investigated via isothermal oxidation at 1373 K. After long-term oxidation at 1373 K, no interdiffusion zone (IDZ), secondary reaction zone (SRZ) and rod/needle-like topologically close-packed (TCP) precipitates were observed in the coating system with HEA diffusion barrier. Besides, the NiAlHf coating exhibited an improved oxidation resistance with the addition of HEA diffusion barrier. The results indicate that the HEA diffusion barrier can suppress the interdiffusion of alloying elements between substrate and coating effectively, which is attributed to the sluggish diffusion effect of HEA and the in-suit formed alumina layers at the substrate/HEA and HEA/NiAlHf interfaces during high-temperature oxidation. Moreover, it was found that the Co element in HEA layer also plays an important role in suppressing the formation of SRZ and TCP precipitates in the superalloy.

Effect of post-treatment on the microstructure and tensile properties of Ni–Co-based superalloy manufactured by selective laser melting

ABSTRACT A newly designed Ni–Co-based superalloy was manufactured by selective laser melting (SLM) and the effect of hot isostatic pressing (HIP) and heat treatment on the microstructure and tensile properties was investigated. The as-built sample showed epitaxial grains including sub-structures, whereas recrystallisation occurred in the HIP-treated samples. Furthermore, carbide particles appeared at the grain boundaries, and the γ’ phase formed extensively. The sample that was subjected to HIP-solution-treatment and aging (STA) contained a larger fraction of carbide and γ’ phase compared to the HIP sample, with a needle-shaped topologically close-packed (TCP) phase also generated inside the γ matrix. The yield and tensile-strengths increased by approximately 60% after HIP, and the elongation increased by 0.8%. However, the HIP-STA sample underwent a significant decrease in elongation by TCP phase. The correlation between the microstructure and mechanical properties of the SLM-built superalloy is also discussed on the basis of the fractography results.

Development of arc welding technique to preclude microsegregation in the dissimilar joint of Alloy C-2000 and C-276

Hastelloy C-2000 and C-276 are widely used in Flue Gas Desulphurization (FGD) system and Chemical Processing Industries (CPI). Current work is focused on weld microstructure, and mechanical properties (structure-property relationship) of the dissimilar combination of alloy C-2000 and C-276. Multi-pass Pulsed Current Gas Tungsten Arc (PCGTA) welding was adopted for joining the dissimilar alloys using the filler ERNiCrMo-17. Microstructural characteristics of the weld joint were assessed by Optical and Scanning Electron Microscope (SEM). Weld interface microstructure examination revealed the presence of grain coarsening near the Heat Affecting Zone (HAZ) of the alloy C-276 side. SEM analysis shows the absence of secondary Topologically Closed Packed (TCP) phases in the Inter-Dendritic (ID) regions of the dissimilar weld. Micro-segregation of alloying elements in the weldment was assessed by Energy-Dispersive X-ray Spectroscopy (EDS). X-Ray Diffraction (XRD) analysis had been carried out to identify the phase constitution and average grain size. Strength, toughness, and hardness of the dissimilar weld were evaluated with the support of the tensile test, Charpy impact test, and Vicker’s hardness test. Tensile study showed that all the tensile fracture occurred at the base metal side of alloy C-276. The average toughness of the dissimilar alloy joint was noted about 84 J. Hardness test results indicated that fusion zone (FZ) hardness value was 6.19% and 2.27% superior to the candidates’ material (C-276 and C-2000) employed in this study. The refined grain structure and absence of microsegregation resulted in the highest hardness in the dissimilar weld FZ. Results revealed the substantiated use of PCGTA welding for the effective joining of dissimilar alloys of C-2000 and C-276 through the evaluation of metallurgical and mechanical characteristics.

Thermodynamic assessment of the Co–Cr–Ni, Co–Cr–W and Co–Ni–W

The thermodynamic description of the Co–Cr–Ni, Co–Cr–W and Co–Ni–W systems is the basis of material design for Co-WC cutting tools and superalloys. A common feature of the three ternary systems is that the detrimental topologically close packed (TCP) phases (μ and σ phases) have large ternary extensions, which makes it crucial to provide a reliable description of the TCP phase boundaries. In order to obtain an accurate description of the TCP phase boundary, the effect of magnetism on the Gibbs energy of the TCP phases containing ferromagnetic elements must be considered. In the present work, the effect of magnetism on the Gibbs energy for the Ni-related TCP phases has been discussed. The magnetic related parameters TcNiσ, βNiσ, GoNidis_σ, TcNiμ, βNiμ, GoNidis_μ were evaluated. CALPHAD (CALculation of PHAse Diagrams) assessment for the ternary systems has been performed. The present CALPHAD description can well describe the experimental data.

Creep prediction model for nickel-based single-crystal superalloys considering precipitation of TCP phase

The microstructure of nickel-based single-crystal (SC) superalloys has a pivotal influence on their creep properties. The addition of the Re element not only enhances the long-term creep properties of nickel-based SC superalloys, but also results in the formation of a topologically close-packed (TCP) phase which is a harmful and brittle hard phase. Here, high-temperature creep interruption tests of a nickel-based SC superalloy that contains 4.8 wt% Re were performed under various temperatures and stress conditions, and the evolution of microstructure during creep was observed by scanning electron microscopy (SEM). The volume fraction of the TCP phase was also extracted to explore the mechanism that controls the impacts of the TCP phase on the creep properties. According to the microstructure evolution mechanism, the influence of the TCP phase was attributed to the initial damage and critical shear stress of the material. A creep performance prediction model for nickel-based SC superalloys considering the precipitation of the TCP phase that is based on the crystal plasticity theory and a modified creep damage model was established. The simulation curves fit well with the experimental results and the errors between prediction creep life with test results are within 5%.

Eliminating topologically closed-packed phases in deteriorated nickel-based superalloy by pulsed electric current

This work investigates the reverse thermodynamic dissolution of the topologically closed-packed (TCP) phases in the deteriorated nickel-based superalloy using the pulsed electric current. The mechanical properties cannot be restored directly by the conventional heat treatment because the dissolution temperature (1130 °C) of the TCP phases is higher than the initial solvation temperature (950 °C) of the γ′ phase. Here it was found that the TCP phases can be selectively dissolved under the pulsed electric current at a relative low temperature (800 °C), and the volume fraction of γ′ phase is unchanged. It means the pulsed processing is a lossless performance regeneration method. The count of TCP phases decreased from 3.49 × 109/m2 to 1.24 × 109/m2, accounting for only 35.5% of the thermal exposure sample. Correspondingly, the mechanical properties change toward the young stage. By the HRTEM, the TCP phases initially interrupted at stacking faults have been observed, and the stacking faults are basically aligned along the (111)TCP direction. Subsequently, the interrupted TCP phases dissolved from both ends to the middle. The solute elements can be rapidly diffused into the γ-matrix and γ′ phase using the high current density regions that caused by conductivity discrepancies.