Topologically Close-packed Research Articles

New insights of the microcrack initiation induced by Mo addition in Ni-based single crystal superalloys during creep at 900 °C

Mo has received much attention in Ni-based single crystal superalloys as a main strengthening element at elevated temperatures above 1000 °C. However, the effects of Mo addition in new generation single crystal superalloys at medium temperature conditions are still not clear. This paper investigated the creep property at 900 °C and 392 MPa in a 4th generation Ni-based single crystal superalloy with different Mo contents, indicating that Mo addition decreased the creep life and induced large amounts of microcracks. Higher Mo content could weaken the interface strength by inducing more stacking faults, causing crack initiation along the interface. The elemental segregation along the stacking faults triggered the nucleation of other precipitates, which could further lead to cracks. The precipitation of topologically close-packed (TCP) phases introduced by more Mo addition caused crack initiation along the interfaces between the TCP phases and γ′ phase. By investigating the microcrack formation induced by Mo during creep at 900 °C, we provided a new perspective for the alloy design.

Comparison of fatigue characteristics and failure analysis of Ni-based superalloy subjected to thermomechanical fatigue under different phase conditions

In this research, the thermomechanical fatigue (TMF) characteristics of a second-generation Ni-based single-crystal superalloy were investigated. The combined effects of temperature cycling and mechanical stress were examined using a strain-controlled approach under different TMF testing conditions. The results indicated that cyclic softening occurred during the early stages during in-phase (IP) loading. In contrast, out-of-phase (OP) tests exhibited cyclic hardening, as evidenced by an increase in the stress range, which intensified at higher mechanical strain amplitudes. This resulted in a shorter fatigue life compared to the IP TMF case. The crack distribution patterns and damage mechanisms on the cross-sectional and fracture surfaces of the failed specimens were analyzed. In the OP TMF tests, oxidation-assisted cracking was identified as the primary damage mechanism. The dominance of oxide layers was a key distinguishing feature. Additionally, twinning-related deformation and topologically close-packed (TCP) phase particles were observed in specimens subjected to a higher mechanical strain of 0.6%, where they ran parallel to the main crack, contributing to a reduction in lifetime. Conversely, the IP TMF cases were mainly dominated by creep damage mechanisms, with some signs of oxidation penetration observed during testing. The application of the Ostergren model to the fatigue life prediction of a Ni-based single-crystal superalloy under both conditions was evaluated. As this model was insufficient, a modified Ostergren model was proposed by incorporating the maximum and amplitude stresses in the power law forms.

Enhancing strength-ductility combination of Fe–Ni–Cr–Al–Ti high-entropy alloys via co-precipitation of sigma and L21 phases

The sigma phase is a well-known harmful phase in transition metal alloys, leading to severe embrittlement due to its inherent brittleness. However, in this work, we develop a new strategy to obtain enhanced strength-ductility combination via the synergistic effects of interlocking co-precipitation of the sigma and L21 phases in Fe40Ni30Cr20Al5Ti5 (at. %) high-entropy alloy (HEA). This HEA exhibits a good strength-ductility combination after aging treatment (800 °C for 4h), with a yield strength (YS) of about 920 MPa and an ultimate tensile strength (UTS) of about 1300 MPa while maintaining a total elongation (TE) of 24 %. It is attributed to the interlocking co-precipitation of the sigma and L21 phases with equal volume fractions into the face-centered cubic (FCC) matrix through thermomechanical treatment. During the tensile deformation, the softer L21 phase coordinates the deformation around the sigma phase, suppresses the local stress concentration, and achieves a stable high work hardening capability, as well as stacking faults (SFs) networks and density Lomer-Cottrell (L-C) locks, contributes to the excellent strain hardening capacity and thus obtains the enhanced strength-ductility combination in the HEA. This study will provide a new strategy to obtain enhanced strength-ductility combination for alloys containing brittle and hard topologically close-packed (TCP) phases, such as the sigma phases.

Molecular dynamics simulation of the microstructural evolution in FeCuNi metallic glass under neutron irradiation

Investigating the microstructural characteristics of metallic glasses (MGs) under high-energy irradiation is of great importance for their industrial applications. In this study, molecular dynamics simulations of FeCuNi MG under neutron irradiation have been carried out, and the microstructural evolution has been systematically investigated by the largest standard cluster analysis (LaSCA). It is found that both topologically close-packed (TCP) and icosahedral structures in MG exhibit significant self-healing ability under irradiation. However, the TCP structures are quantitatively more dominant and thus can more rationally explain the structural self-healing mechanism of irradiated MG. Furthermore, during the irradiation process, the large-sized TCP clusters in the MG split into many small-sized clusters. However, due to the inherent self-healing ability of the TCP structures, these split clusters recombine into large-sized clusters after the irradiation ends. In summary, the results of this study are expected to provide theoretical guidance for the development of new high-performance reactor materials.

Long-term aging behavior and mechanism of CMSX-4 nickel-based single crystal superalloy at 950 ℃ and 1050 ℃

Nickel-based single crystal superalloys CMSX-4 have been widely used to manufacture aeroengine turbine blades because of their excellent high-temperature comprehensive properties. The microstructure evolution behavior, mechanism, and influence on micromechanical behavior of the second-generation nickel-based single crystal superalloy during long-term aging at 950°C and 1050°C for 10–2000 h were investigated. The results show that, with the extension of aging time, the average size of γ' phase increased, whose morphology changed from square block to strip and L shape and rafting phenomenon was observed. The coarsening of γ' conforms to Lifshitz–Slyozov–Wagner (LSW) theory and the coarsening rates are 4.241×10−5 and 9.954×10−5μm3/h at 950°C and 1050°C, respectively. After aging at 950°C and 1050°C for 1000 h, segregation of refractory elements was formed in the samples. There are hard topological close-packed (TCP) phases surrounded by γ' envelope precipitates, which destroyed the continuous structure of γ'. The precipitation of TCP caused the depletion of refractory element Re in the γ' envelope phases, resulting in softening effect. It was also found that high temperature can not only accelerate the coarsening of γ' phase but also promote the precipitation and growth of TCP phases.

The effect of stress gradient on the formation of topologically close packed phases in a Ni-based single crystal superalloy at 1050 °C

The formation of topologically close packed (TCP) phases is detrimental to the mechanical properties of Ni-based single crystal (SX) superalloys, which are used in harsh environments with high temperature and gradient stress caused by centrifugal force during service. In order to study the precipitation behavior of TCP phases in coupling with temperature and stress gradient, a third generation Ni-based SX superalloy is selected to conduct force holding experiments at 1050 °C for 8 hours. Stress gradient can be realized under constant load by designing tensile samples with cross-sectional area continuously changed. The results indicate that the area fraction of TCP phases varies at different positions of the sample, where the effective tensile stress is different. The area fraction of TCP phases decreases at the thinner region of the sample, which means that the amount of TCP phases decreases as applied stress increases. At the thinnest position, where the stress is estimated to be 400 MPa, almost no TCP was observed. The inhibition of stress on the formation of TCP phase may be due to the reduction of Re concentration in the γ matrix, because of the segregation of Re at γ/γ′ phase interfacial dislocation cores and the dissolving of γ′ phase.

Investigation on the interdiffusion mechanism between single-crystal superalloy and FeCrNiAl-based medium entropy alloys

High and medium entropy alloys (HEAs, MEAs) have emerged as promising candidates for bond-coat materials in the thermal barrier coating systems due to their exceptional oxidation resistance. However, understanding the interdiffusion behavior between these alloys and single-crystal superalloys (SXs) substrates requires further investigation. In this study, diffusion couples were created between SXs and two types of MEAs, namely FeCrNiAl and FeCrNi0.4Al0.4, and the interdiffusion behavior was simulated using the commercial DICTRA code. The results reveal significant interdiffusion at 1100 °C in the couples, attributed to the substantial elemental differences between SXs and MEAs. This led to the formation of the interdiffusion zone (IDZ) and the secondary reaction zone (SRZ). Within the SRZ, phase transformation of SXs occurred, resulting in the precipitation of topologically close-packed (TCP) phases. Three types of TCP phases (σ, μ, and P) were observed. Along the {11¯02} plane of the μ phase, twins and stacking faults have formed. The glide of P phase contributed to the bending of bar-like precipitates. Notably, the amorphization of the σ phase was observed for the first time, suggesting an intermediate state during the phase transformation. Despite elemental differences between the two MEAs, the growth mechanism of IDZ and SRZ, as well as the precipitation of TCP phases, exhibited similarities. This underscores the importance of Cr and Ni diffusion alongside Al diffusion. The pronounced interdiffusion behavior raises concerns about the suitability of 3-d transition metal MEAs as bond-coat materials due to the lack of slow diffusion effect.

The origin of the straight propagation of the σ phase in a Ni-based single crystal superalloy at elevated temperature

Topologically close-packed (TCP) phases, although inherently hard, are commonly perceived as detrimental in various alloys. In Ni-based single crystal superalloys (Ni-SXs), the primary adverse impact of TCP phases on material mechanical properties stems from their direct propagation, disrupting the original γ/γ′ dual-phase structure and leading to cleavage along the TCP phases under applied stress. Understanding the specific reasons for the straight propagation of TCP phases through the complex phase structure is essential for mitigating or potentially reversing their detrimental effects on material performance, a challenge that remains elusive. This study addressed this challenge by employing ultra-high spatial and energy resolution transmission electron microscopy (TEM) characterization, coupled with in situ scanning transmission electron microscopy (STEM) heating experiments in a fourth-generation Ni-based single crystal superalloy. Our research unveiled a significant phase-separation process during the incubation stage preceding σ phase formation at elevated temperatures, driven by the diffusion of atoms. Notably, domains of the γ′ phase, characterized by a scale of tens of nanometers, emerged within preexisting γ phases, and vice versa. This phenomenon reconstructed the original γ/γ′ dual-phase structure, leading to a finely refined dual-phase structure. The refined structure effectively reduced the diffusion distance of alloy elements necessary for forming σ phases. Consequently, the σ phases nucleated randomly and propagated directly through the newly refined γ/γ′ dual-phase structure. Throughout the thickening process, the notable difference in the diffusivity of alloying elements eventually resulted in the growth of a highly defective σ phase structure.

Structural pathway for nucleation and growth of topologically close-packed phase from parent hexagonal crystal

The solid diffusive phase transformation involving the nucleation and growth of new phase particles is universal and frequently employed but has not yet been fully understood at the atomic level. Here, our first-principles calculations reveal a structural formation pathway of a series of topologically close-packed (TCP) phases within the hexagonally close-packed (hcp) matrix. The results show that the nucleation follows a nonclassical nucleation process, and apart from atomic diffusion, the entire hcp→TCP transformation only involves shuffle-based displacements, with a specific 3-layer unstable hcp-ordering serving as the basic structural transformation unit (BSTU). The thickening of plate-like TCP phases relies on the formation of these unstable hcp-orderings at their coherent TCP/matrix interface to nucleate a ledge, but the ledge lacks the dislocation characteristics which is commonly considered in the conventional view. Additionally, the atomic structure of the critical nucleus for the Mg2Ca and MgZn2 Laves phases was predicted in terms of Classical Nucleation Theory (CNT). The formation of polytypes and off-stoichiometry in TCP precipitates is found to be related to the nonclassical nucleation behavior. These insights contribute to a deeper understanding of solid diffusive phase transformations at the atomic level and provide a foundation for investigating other technologically significant transformations.

Microstructure and mechanical assessment of Inconel 625 and AISI 904L dissimilar joints using high-density welding technique for pressure vessels

The dissimilar bimetallic joints of 5 mm thick Inconel 625 and AISI 904L were accomplished through the high-density fiber laser welding (LBW) process. The metallurgical investigation ensured the micro-segregation of Mo and Nb in the dendritic and inter-dendritic regions along with the formation of secondary topologically closed packed (TCP) phases. In addition to the metallurgical studies the Kernal average misorientation (KAM), Schmid factor (SF), and nucleation of the new grains were investigated through Electron backscattered diffraction (EBSD) analysis. The tensile strength opined the occurrence of failure on the 904L side, away from the weld seam ensuring that the joints have a higher strength. The notch tensile showed a higher strength of 678 ± 4 MPa in the weld zone than that of the 904L base alloy. The Charpy V notch studies showed a drastic decrease in the impact toughness with 38 ± 2 J as compared to the base alloys under sudden impact loading at room temperature (RT).

Preparation of Binary and Ternary Laves and µ-Phases in the Ta–Fe(–Al) System for Property Analysis at the Microscale

Topologically close-packed (TCP) intermetallic phases are attractive candidates for adapting the property profile of both structural and functional materials, but their mechanical behavior, in particular below their brittle-to-ductile transition, is generally still poorly understood. The systematic analysis of the underlying deformation mechanisms requires the fabrication of homogeneous samples with sufficiently high purity and large enough grain size. Here, we describe identified pathways on the example of Laves and µ-phases from the binary Ta–Fe and ternary Ta–Fe–Al systems with regard to input materials, liquid metallurgy synthesis, heat treatment, and metallographic preparation methods. Preparation routes for structural analysis by electron backscatter diffraction and mechanical analysis by nanomechanical testing, as well as the transferability of our approach to other alloy systems containing TCP phases, are outlined and discussed.

Effect of Ru on the precipitation of the TCP phase from the γ-γʹ microstructural degradation analysis

This work investigates the effect of Ru on the precipitation of the topologically close-packed (TCP) phase from the γ-γʹ microstructural degradation analysis. The addition of Ru is found to delay the precipitation time of the TCP phase from 100 h to 1000 h after thermal exposure at 950 °C. The effect of Ru on the γ-γʹ microstructural degradation is also investigated. Adding Ru slightly decreases the coarsening rate of the γʹ size and promotes the reduction of the volume fraction of γʹ phase. The reduction of the volume fraction of γʹ phase causes the "inverse partitioning" effect and increases the solution limit of the TCP forming element in the matrix. These help inhibit the precipitation of the TCP phases in Ni-based single crystal superalloys during thermal exposure.

Nucleation and transition sequences of TCP phases during heat-exposure in a Re-containing Ni-based single crystal superalloy

The nucleation and transition sequences of topologically close-packed (TCP) phases in a Re-containing Ni-based single crystal superalloy were systematically investigated using in-situ transmission electron microscopy (TEM) and three-dimensional atom probe technology (3D-APT). During the initial stage of heat-exposure at 1100 °C, the TCP phase forming elements (Re, Co, Cr, etc.) segregated at the γ/γ′ interface near the γ matrix side to provide the concentration undulations for the nucleation sites of TCP phases, following which the σ and P phase coherently nucleated along the (111‾)γ and (022)γ planes from the γ/γ′ interface near the γ matrix side, respectively. With prolonged heat-exposure time, transitions from σ phase to P phase, σ phase to μ phase, and P phase to μ phase occurred. Besides, the orientation relationships of TCP phase intergrowth structures indicated that the P phase grew along the (1¯01)σ plane of the σ phase by co-lattice precipitation, meanwhile, the µ phase grew with smaller lattice misfits along the (04¯0)σ plane of the σ phase and the (400)P plane of the P phase. Additionally, the result by first-principles calculation evidenced that the μ phase had the lowest system energy to make the transition of σ phase and P phase to μ phases inevitable, therefore, the TCP phase ultimately existed as the most stable μ phase. Finally, the transition sequences of TCP phase during heat-exposure could be summarized into three types: γ matrix→σ→μ, γ matrix→P→μ, and γ matrix→σ→P→μ.

Quantitative analysis of interfacial microstructural degradation in MCrAlY-coated superalloys during varied service conditions

MCrAlY coatings are extensively utilized to shield turbine blades from oxidative and erosive circumstances. Nonetheless, the interdiffusion between the coating and the superalloy substrate results in the genesis of topologically close-packed (TCP) particles, thereby reducing the longevity of components exposed to high temperatures during operation. In this investigation, the progression of the microstructure in MCrAlY-coated Ni-based single crystal superalloys was meticulously quantified using digital image analysis, considering a spectrum of pre-exposure temperatures, durations, and stress levels. Through digital image analysis, we observed a variation in the length of TCP particles, ranging from 3.25 to 9.8 μm, under different conditions. Interestingly, the width of TCP particles remained consistent at 0.6 μm under all tested conditions. An innovative microstructure forecasting model was formulated, encompassing standardized parameters to evaluate the impact of pre-exposure conditions. This model demonstrated a robust correlation with experimental results. This study contributes to the refinement of our understanding of interfacial microstructure deterioration caused by operational conditions and aids in the progression of coating-substrate chemical compatibility.

Formation and significance of topologically close-packed Laves phases in refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) inherently have a high potential to form topologically close-packed (TCP) Laves phases. Phase engineering to control the amount and type of TCP-Laves phases plays an important role in physical and mechanical properties of RHEAs. The present investigation addressed key parameters to TCP-Laves phase formation in RHEAs by considering thermodynamic calculations and empirical relations. Two novel TiVCrZrCo and TiVCrZrFe RHEAs were designed and fabricated as alloy models with high melting points (>2000 K) and relatively low density (∼6.5 g.cm−3) with multiphase microstructure including BCC matrix together with C14 and C15 Laves phases. Existence of atoms such as V, Co, Cr and Fe together with Zr, Ti, Nb and Hf refractory elements promote TCP-Laves phase formation. Very negative values for mixing enthalpy of atom pairs (ΔHmix) and remarkable differences between atomic sizes of the constitutive elements (δ) are key factors in the Laves phase formation in multicomponent alloys. The parameters including ΔHmix and δ must be in the defined range of −20 ≤ ΔHmix ≤ −3 kJ.mol−1 and 4 <δ < 10 to appear Laves phase in the microstructures of HEAs. TCP-Laves phase formation led to increase and decrease in hardness and fracture toughness, respectively. The understanding of key factors in RHEAs design can lead to phase engineering and the fabrication alloys with a desirable amount of TCP-Laves phase, which are suited for hydrogen storage applications or for applications in harsh operating environments exposed to high temperatures, irradiations, wear and erosion.

Effect of Ti content on achieving development of mechanical properties and wear resistance of CoCrFeNiMoTix high entropy alloy

To achieve the synergy among density, strength, ductility, wear performance and thermal stability, a series of newly developed CoCrFeNiMoTix (x = 2 and 16.666 at. %) high entropy alloys (HEAs) via multiple phases nano crystalline structure were successfully fabricated using mechanical alloying (MA) following by spark plasma sintering (SPS). The results demonstrated that the SPSed Ti2-HEA and Ti16.6-HEA have a multiple phases structure consisting of FCC structure (Ni-type as a major phase), two types of BCC structure (Mo-type and Ti-type), topologically close packed (TCP) phases (laves, sigma (σ) and R phases) and geometrically close packed (GCP) phase (L12 precipitates). The micro hardness, strength and specific strength were increased from 711 HV, 1770 MPa and 197 MPa.Cm3/g for the Ti2-HEA to 817 HV, 1830 MPa and 236 MPa.Cm3/g for the Ti16.6-HEA, respectively, which were affected by more percentages of precipitates, intermetallic and BCC phases. Besides, fracture strain, coefficient friction, wear rate and the surface roughness were decreased from 16%, 0.19, 4.9*10−4 mm3 N−1 m−1 and 54.2 for the Ti2-HEA to 14.5%, 0.17, 2.7*10−4 mm3 N−1 m−1 and 31.6 for the Ti16.6-HEA. The ultimate shear strength (USS) values at 300 °C, 400 °C, 500 °C and 600 °C were increased from 960 MPa, 820 MP, 646 MPa and 24 MPa for the Ti2-HEA to 1180 MPa, 1010 MP, 802 MPa and 94 MPa for the Ti16.6-HEA, respectively. The Ti16.6-HEA illustrated higher thermal stability at all temperatures, where the USS slightly was decreased from 300 °C to 500 °C with a sudden drop at 600 °C.

Crystallographic texture and multiscale boundaries mediated creep anisotropy in additively manufactured Ni-based Hastelloy C276 superalloy

The microstructure and high-temperature creep mechanisms of Ni-based Hastelloy C276 superalloy fabricated using wire and arc-based directed energy deposition were investigated systematically and innovatively. The microstructural investigation revealed that the as-fabricated samples comprise γ-Ni matrix and topologically close-packed (TCP) P phase precipitates. The γ matrix subgrains and grains are spread over multiple highly textured columnar dendrites, with a majority of γ <001> crystallographic orientations closely aligned along the deposition direction. Moreover, the interdendritic regions exhibit severe Mo segregation, P phase particles, and dislocation bands. Creep tests were conducted on miniature samples under various temperature and stress conditions, loaded either in the deposition direction (DD) or travel direction (TD). DD samples exhibit lower minimum strain rates, greater strains-to-failure, and longer creep rupture lifetimes than TD samples, indicating significant creep anisotropy. Dislocation creep was identified as the primary creep mechanism for both DD and TD conditions. During creep, dynamic precipitation of TCP phases occurred in the interdendritic regions, resulting in varying creep resistance between interdendritic and dendritic core regions. Isostress and isostrain models, considering both crystallographic texture and precipitation strengthening, reasonably predicted the observed creep anisotropy during the secondary creep stage. Additionally, variations in the Schmid factor led to significant deformation incompatibility among dendrites in TD samples. Dislocation accumulation in TD sample interdendritic regions promoted new grain nucleation, triggering dynamic recrystallisation, facilitating grain boundary sliding, and accelerating tertiary creep. Furthermore, TCP phase particles in the interdendritic regions contributed to microcrack development, further accelerating creep fracture, especially in the TD condition.

Microstructures Evolution of a Second Generation Single Crystal Superalloy after Long Term Aging at 980°C

The microstructure evolution of the second-generation single crystal superalloy DD412 aged at 980°C for 1000 h were investigated. The results show that needle-shaped topologically close packed (TCP) phases rich in Re and W were firstly found in the dendritic core region after aging at 980°C for 400h, while the size and amount of TCP phases increased gradually with aging time, which is due to the fact that high melt point elements Re and W segregated to the dendritic core region. With the increment of aging time, the size of γ′ phase increased and volume fraction of γ′ phase decreased. The γ′ phase in dendrite core and inter-dendrite region remained cuboidal basically, and the γ′ phase in second dendritic arm showed directional coarsening after aging for 800 h. The MC carbides in inter-dendritic region decomposed gradually and transformed to the M6C carbides partially during long term aging, while few fine blocky M6C carbides precipitated in dendritic core after aging for 400h. The different microstructure evolution in dendritic region is due to the dendrite segregation of alloying elements and elements diffusion during aging.

On phase stability and phase relations of topologically close packed phases in binary Cr-Os system over the temperature range of 900–1300 ℃

Osmium (Os) addition has been found to be beneficial in enhancing the creep resistance and microstructure stability of nickel-based single-crystal superalloys. The phase stability and phase relationship of topologically closely packed (TCP) phases in a binary Cr-Os system is thus essential for developing a better understanding of the constitutional effects of Os in multicomponent nickel-based single-crystal superalloys, but are still not fully understood in the literature. In this paper, a series of Cr-Os alloys were prepared, and the phase constituents and the microstructure of both as-cast and annealed alloys over a temperature range of 900–1300 ℃ were analyzed using the X-ray diffraction (XRD) and electron probe microanalysis (EPMA) techniques. The experimental results show that (Cr) solution, (Os) solution and Cr2Os phases are formed during solidification. After long-term annealing at 900–1300 ℃, the phase constituents and the microstructure of the alloys display obvious changes. Based on the measured data, the phase stability and phase equilibria of TCP between 900 and 1300 ℃ in the binary Cr-Os system are accordingly updated. The stability results for the two TCP phases in the binary Cr-Os system demonstrate that the Cr3Os phase exists stably, while the Cr2Os phase decomposes into Cr3Os and (Os) at 900–1300 ℃.

Atomic-scale study of linear reciprocating friction of TCP/γ phase in nickel-based single crystal alloy

In order to systematically investigate the role of TCP (topologically close-packed) phases in the fretting wear process of nickel-based single crystal alloys (NBSC), this study employed molecular dynamics to conduct comparative analyses of mechanical properties, atomic displacements, wear depth, defects, dislocation density, and the influence of temperature under constant load on the friction process in material wear. The research revealed that during the repetitive friction process, the friction force exhibited a peak at the extreme positions of reciprocating friction on the workpieces, and this peak increased with the number of friction cycles. The dislocation density in the worn area increased, resulting in hardening, and the removal rate of material decreased. At the initial stages of friction, the presence of interfaces notably hindered the transfer of temperature, defects, and atomic displacements in the workpiece, and this inhibitory effect weakened with an increasing number of friction cycles. The TCP phases experienced stratification due to the overall deformation they underwent. Furthermore, as the relaxation temperature increased, the workpiece exhibited enhanced plastic deformation capacity, an increase in dislocation density, and adhesion between abrasive particles and the grinding ball occurred.