Phases In Superalloys Research Articles

Roles of Refractory Solutes on the Stability of Carbide and Boride Phases in Nickel Superalloys

Nickel-base superalloys contain high amounts of solutes like Cr, Mo, W, Nb, Ti, etc. These solutes promote the formation of different types of carbide and boride phases that may contain multiple elements. Researchers have mostly discussed the roles of primary elements responsible for the formation of a given carbide/boride phase, often ignoring the role of other solutes on its stability. In the present work, thermodynamic stability of carbide and boride phases in seven commercial superalloys, namely, Alloy 625, Alloy 690, Alloy 718, MAR M246, Rene 100, Udimet 710 and Nimonic 80A, has been studied using the CALPHAD based Thermo-Calc software. The aim of the study was to understand the role of different alloying elements on temperature stability and chemical compositions of equilibrium phases in superalloys. As the accuracy of CALPHAD based predictions depends upon the database used, a detailed examination of its inadequacies has also been carried out to ascertain the limitations of the predicted data. From the calculated equilibrium chemical compositions, major and minor constituents promoting the formation of carbides and borides have been identified. The individual effect of a given solute as well as the synergistic effect of two solutes on the relative thermodynamic stability of carbide/boride phases has been identified using property diagrams and isothermal sections of the temperature-composition diagrams. Most of the simulated results have been found to be consistent with the experimental data available in the literature. From a comparison of the experimental literature and the simulated data of the stable carbide and boride phases in the studied alloys, the interplay of different solutes has been deduced to define conditions under which these phases form, within the limitations of the database used. This study has helped in better understanding of general tendencies of solutes to form different carbide and boride phases in nickel-based superalloys.

Elucidating the deformation behavior of as-cast B2 CoTi intermetallic using in-situ tensile testing

This study deals with microstructural aspects of the deformation behavior of as-cast homogenized CoTi intermetallic by using in-situ tensile testing coupled with EBSD analysis. CoTi exhibits excellent wear resistance and noticeable room temperature ductility. Therefore, it is commonly used as coating material or a reinforcing phase in superalloys. In-situ study of this intermetallic is pertinent as it will provide information about the microstructural evolution during deformation under tensile loading. During the initial stage of deformation, the plastic strain localization was observed to occur at softer grains due to the activation of {011}⟨100⟩ slip systems, which resulted in the accumulation of GNDs and increase in fLAGBs (fraction of low angle grain boundaries). However, fLAGBs did not increase significantly at higher strains. In addition, the contribution of texture towards deformation behavior was found to be weak. The intermetallic exhibited a mixed-mode failure, where both transgranular and intergranular features were observed on the fracture surface. A slightly higher strain to failure in the intermetallic has been attributed to both the increase in the GNDs density at the initial stage of deformation and increase in SSDs at the later stage of deformation, leading to strain hardening.

Plasticity of the Nb-rich μ-Co7Nb6 phase at room temperature and 600°C

The µ phase is a common precipitation phase in superalloys and it exists in a wide composition and temperature range. As such, we study the influences of composition and temperature on its plasticity by micropillar compression tests and transmission electron microscopy. The micropillars of the μ-Co7Nb6 phase deform plastically by basal slip at room temperature and 600 °C. At room temperature, the Co-49Nb and Co-52Nb micropillars show high yield stresses and an abrupt large strain burst at the onset of yielding regardless of orientation, whereas the Co-54Nb micropillars oriented for basal slip yield at much lower stresses and show intermittent small strain bursts during plastic deformation. While the Co-49Nb micropillars deform by full dislocation slip on the basal plane at room temperature, the Co-54Nb micropillars deform by partial dislocation slip on the basal plane. At 600 °C, the Co-49Nb micropillars oriented for basal slip show stable and continuous plasticity and their critical resolved shear stresses decrease dramatically. In contrast to the full dislocation slip at room temperature, the plastic deformation of the Co-49Nb micropillars occurs via partial dislocation slip on the basal plane at 600 °C. Based on the geometric γ-surfaces for all potential basal slip planes, we explore where and why the glide of full and partial dislocations on the basal plane occurs in the μ-Co7Nb6 phase.

The element occupation preference of μ phase in ternary Ni-Cr-Mo model superalloy

Topologically close-packed (TCP) phases seriously endanger the properties of Superalloy. Investigating the microstructure of the TCP phases will provide a crystallographic reference for the design of superalloys with better properties. The μ phase is a common TCP phase in superalloys. It belongs to the space group R-3m, and has a hexagonal structure with five Wyckoff sites of 3a, 18h, 6c1, 6c2 and 6c3. In this paper, occupation of the three kinds of atoms at five Wyckoff sites of the μ phase in Ni-Cr-Mo model superalloy was determined using aberration-corrected High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), QSTEM software simulation and first-principles calculations. It was found that Mo atoms with larger size and smaller electronegativity tend to occupy 6c3, 6c1 and 6c2 sites in order of priority. Ni atoms mainly occupy the sites with coordination number (CN) of 12 at 3a and 18 h, and 3a sites contain more Cr atoms than 18 h sites.

The site preference and doping effect on mechanical properties of Ni3Al-based γ′ phase in superalloys by combing first-principles calculations and thermodynamic model

The fundamental aspects of site preference of alloying elements on sublattice of the strengthen γ′ phase with L12 structure have not been well understood, which hinders the optimized design of advanced Ni-based high-temperature alloys. In this contribution, the temperature- and composition-dependent site occupying preferences of the binary, ternary, and quaternary of Ni3Al-based γ′ phase alloyed with Mi where Mi represents the additional transitional metals Co, Cr, Cu, Fe, Mn, Mo, Re, Ta, Ti, V, or W atoms (arranged in alphabetical order) chosen frequently, were studied using a two-sublattice thermodynamic model (Ni, Al, Mi)1a(Ni, Al, Mi)3c. The site occupying fractions (SOFs) were calculated based on a thermodynamic database established in this work, where the thermodynamic data of the end-members involved were obtained using first-principles calculations and phonon spectrum calculations. The calculated SOFs results show that there is an obvious site preference for stoichiometry binary Ni3Al, and its site configuration changes from (Al)1a(Ni)3c at room temperature to (Al0.9984Ni0.0015)1a (Al0Ni0.9994)3c at 1273 K. For the γ′ phase with the composition 78Ni-26Al-4Mi (atom ratio and xNi/xAl = 3:1), Mo atoms always preferred to occupy the 1a sublattice (Al site), Co, Mn, and Ti atoms always prefer the 3c sublattice (Ni site) in the whole temperature range, while the site preference of Cr, Cu, Fe, Re, Ta, V, or W atom is affected by temperature. For example, when the heat treatment temperature is lower than 700 K, Cr, Cu, Fe, Ta, V, and W atoms occupy the 1a and 3c sublattice randomly, and Re atoms prefer to 3c sublattice, while when the heat treatment temperature is higher than 1273 K, Cr, Cu, and W atoms prefer 3c sublattice, Fe and Ta atoms prefer to 1a sublattice, while all Re atoms occupy the 3c sublattice exclusively, and all V atoms occupy the 1a sublattice exclusively, respectively. Likewise, the site preference of the quaternary system with selective compositions 78Ni-26Al-2 M1-2 M2 was also predicted. Based on calculated SOFs results, the mechanical and thermodynamic properties were studied at the ground state. It has been revealed that Cr, Re, and V doping can improve the microhardness of Ni3Al alloys; in particular, the effect of Cr is extraordinary; and all elements, except Mn, Mo, and Ti, would enhance the bulk modulus of Ni3Al-based γ′ phase, in which Mn have the greatest influence on reducing the bulk and shear modulus, respectively. Furthermore, all the B/G ratios of the computed Ni3Al-based γ′ phase are >1.75, showing inherent ductility. Only Cr doping significantly enhances the Debye temperature of the Ni3Al-based γ′ phase.

Influence of carbon on the formation of γ/γ′ microstructure and κ-phase in the WC/Co–Ni–Al–W system: ab initio calculations and experimental studies

Modifications of the binder phase (γ) of cemented carbides have the potential to increase the hardness and wear resistance of the whole material. Partially, coherent precipitations with L12 structure (γ’) promise these improved properties without sacrificing tensile strength or toughness. γ’ is a metastable phase in the Al–Co–W ternary system in the form of Co3(Al,W) which is stabilized by the substitution of cobalt with nickel. Superalloys of the composition Co–(30Ni)–9Al–7 W with different carbon contents were prepared by inductive melting, and the resulting microstructures were analysed using SEM–EDS, XRD and Vickers hardness. Cemented carbides with γ/γ’ binder microstructure were prepared via DTA, and the phase equilibria in the composite material were investigated experimentally and in silico. It was shown that nickel stabilizes the γ’ phase in superalloys as well as in cemented carbides. Carbon leads to the formation of an additional phase with E21 structure (κ). DTA measurements of cemented carbides with different aluminium–cobalt–nickel mixtures as binder gave an overview of the compositional influence. Enthalpies of formation for compounds with L12 and E21 structure were calculated using ab initio methods and compared to experimental results.Graphical

Effect of gradient microstructure on elevated temperature mechanical properties of Ni-based superalloy ATI 718Plus

The effect of the ultrasonic surface rolling process (USRP) and post-treatment on the microstructure and elevated-temperature tensile properties of ATI 718Plus were investigated. The gradient distribution of the η-Ni3(Al0.5Nb0.5) phase and recrystallized grains was generated on the surface of the USRP-treated samples after heat treatment. The amount of η phase in the gradient layer decreased with increasing depth, whereas grain size showed the opposite trend. These η phase particles, distributed in the grain boundaries (GBs) of the gradient microstructure with refined grains, stabilized the trans-scale grain hierarchy at a high temperature of 704 °C. This heterogeneous microstructure promotes the accumulation of high-density geometrically necessary dislocations (GNDs) near the GBs and η phase during plastic deformation, resulting in additional hardening. The central coarse grains in the heterogeneous structure were subjected to the most plastic deformation and conserved ductility during the tensile tests. Compared with the standard sample, the yield strength (YS) and ultimate tensile strength (UTS) of the ultrasonically rolled samples increased by approximately 12% during tensile tests at 704 °C, without distinct plastic deterioration occurring. This study helps to improve the understanding of the functional mechanism of the equilibrium secondary phase in superalloys and the influence of the distribution state of the equilibrium phase on the mechanical properties.

Effect of nickel on the formation of γ/γ’ microstructures in WC/Co-Ni-Al-W

Cuboidal and coherent precipitates form in ternary cobalt-aluminium‑tungsten systems and are described to increase high temperature toughness of superalloys for gas turbines. γ’ is a metastable phase in the form of Co3(Al,W) which is stabilized by the substitution of cobalt with nickel. Strengthening of cobalt binder (γ) in cemented carbides by precipitation hardening is of interest as it potentially increases hardness and wear resistance without sacrificing tensile strength or toughness of the composite materials. Infiltration of preliminary produced cobalt-base and cobalt‑nickel-base superalloys into tungsten carbide powder beds yielded a γ/γ’ binder microstructure. It was shown that nickel stabilizes the γ’ phase in superalloys as well as in cemented carbides, but carbon may lead to the formation of an additional κ phase with E21 structure.

A new refractory Ni7Nb2 phase identified in Laves eutectic regions by TEM study

The dissolution behavior of γ/Laves eutectic within a typical Ni-based superalloy (Inconel 718) was investigated by in-situ and conventional transmission electron microscopy (TEM) techniques. The results show that the Laves phase dissolved firstly at 700°C, but a new Ni7Nb2 phase possessing similar composition with δ-Ni3Nb phase precipitated near Laves phase did not dissolve at temperatures up to 950°C. The new Ni7Nb2 phase with lattice parameters: a=0.473 nm, b=0.794 nm, c=1.21 nm and β=91.3°, shows a C2/m space group symmetry. The crystal structure of the new Ni7Nb2 phase can be understood by “triple layers” as the Ni7Zr2-type structure does, stacking perpendicular to the (001) planes. Instead of stacking with one single shift vector s corresponding to the Ni7Zr2-type structure, the stacking sequences of the new Ni7Nb2 can be characterized by alternate occurrence of +s and -s. Moreover, the structural change of γ/Laves eutectic constituent before and after heat treatment was investigated. The Ni7Nb2 could remain after heat treatment. Since the experimental Ni7Nb2 structure was shown to be unstable in the Ni-Nb binary system by calculation, it is suggested that Zr, Si and Co promote the formation of Ni7Nb2 phase in superalloys according to the segregation behaviors of elements. Moreover, as Ni7Zr2 is considered to be detrimental to the mechanical properties, Ni7Nb2 phase should also have negative influences and should be eliminated through possible approaches. The finding of the new phase should shed light on the microstructural controlling and heat treatment process formulating of superalloys.

Deformation mechanisms of D022 ordered intermetallic phase in superalloys

High-temperature alloys in general and superalloys in particular are strengthened by ordered intermetallic phases that are relatively stable at elevated temperatures. Because of their low symmetry, however, these ordered intermetallic phases have rather complicated deformation mechanisms that are difficult to uncover by experiment alone. In this study we use a combination of ab initio calculation and phase field simulation at the elementary defect level to illustrate how dislocations interact with precipitates of an ordered intermetallic phase, γ″ (D022, tetragonal), the primary strengthening phase in Ni-Nb-Fe-Cr-Ti-Al-Mo alloy (Inconel 718 or IN718 superalloy). A rich variety of new and sophisticated deformation mechanisms are discovered, including a novel mechanism of dislocation generation (accompanied by a spontaneous stacking fault (SF) transition), formation of superlattice intrinsic SF ribbons (SISF-ribbons) and 1/3<112>-type compact super-dislocations, along with ISF shearing and Orowan looping. The predicted deformation microstructures seem to agree with recent electron microscopy observations in IN718. The detailed deformation mechanisms uncovered can be incorporated in constitutive microstructure-property relationships for advanced crystal plasticity modeling and the approach developed can be used to study plastic deformation of other intermetallic phases in different alloy systems.

Regulating the coarsening of the γ′ phase in superalloys

The properties of superalloys are typically deteriorated by the coarsening of the nano-sized γ′ phase, which is the primary strengthening component at high temperatures. Stabilizing the γ′ phase represents one of the key challenges in developing next-generation superalloys. Herein, we fabricate a cobalt-nickel-based superalloy with a nanoscale coherent γ′ phase, (Ni,Co)3(Al,Ti,Nb), which is isolated by stacking-fault ribbons in the alloy matrix as a result of the Suzuki segregation of alloying atoms. Additionally, we demonstrate that this new nanostructure can slow down the coarsening of the γ′ phase at high temperatures. As a result, the cobalt-nickel-based superalloy displays considerably high tensile yield points, exceeding 1650 MPa at room temperature and 1250 MPa at 973 K, which are markedly higher than those of the commonly used nickel- and cobalt-based superalloys. This study thereby paves a new path for developing superalloys with exceptional mechanical performance and thermal stability. By using a novel nanostructure, a team has made a cobalt-based superalloy with a high tensile yield point whose γ' phase resists coarsening. Coarsening of the nanoscale γ' phase of superalloys — the process by which large particles grow at the expense of smaller ones — tends to degrade the properties of superalloys since the γ' phase is the main strengthening component at high temperatures. By isolating the γ' phase through forming stacking-fault ribbons in the alloy matrix via Suzuki segregation of the alloying atoms, Yunping Li of Central South University in China and collaborators at Tohoku University in Japan realized a cobalt-nickel-based superalloy with a tensile yield point of 1,650 megapascals at room temperature (1,250 megapascals at 973 kelvin). They consider this method to be promising for realizing superalloys with superior mechanical properties and thermal stability. A large number of multilayered stacking faults are detected along {111} planes during aging (arrows in a) of present superalloy. Every γ′ phase particle is isolated by multilayered stacking-fault ribbons and grows slightly. The coarsening rate of the γ′ phase decreases significantly after plastic deformation, implying that the formation of multilayered stacking-fault ribbons as a consequence of Suzuki segregation can obviously retard the coarsening of γ′ phase in this novel alloy.

MCrAlY coating design based on oxidation-diffusion modelling. Part I: Microstructural evolution

To improve the efficiency of modern gas turbines, it is highly desired to develop durable MCrAlY alloys, to be used as protective coatings against oxidation and corrosion for superalloys which are the base materials for some hot components like turbine blades and vanes. In this paper, an oxidation-diffusion model was used to simulate the diffusion of alloying elements and the corresponding microstructural changes in different superalloy-coating systems at high temperature. Two important processes are considered in this model: oxidation of the coating and interdiffusion between the superalloy and the coating. The model showed an accurate predictability of the diffusion and microstructural evolution in real superalloy-coating diffusion couples studied at high-temperature exposure. The model was further applied to investigate the elemental effects of Ni, Co, Cr and Al on the microstructural evolution, considering the development of two important phases in superalloys and coatings, i.e. FCC-γ′ and BCC-β, at different temperatures. The results in this paper deepen the knowledge of the MCrAlY coating design for superalloy-coating systems in high-temperature applications.

Study of the Secondary Phases in Inconel 718 Aged Superalloy Using Thermodynamics Modeling

The evaluation of stability of secondary phases in superalloys is of great importance because of its application under high homologue temperatures. The precipitation hardening Inconel 718 superalloy has high stress rupture and creep resistance, good LCF behavior and high corrosion resistance. Those properties can be obtained due to the high stable matrix based on Ni, Fe and Cr, as well as secondary phases promoters elements such as Ti, Al and Nb. This study aims to analyze the phase stability among a range of temperature from 500°C to approximately 1400°C. The thermodynamic modeling has a high importance to predict the behavior of alloys among equilibrium transition. The microstructure of the system was evaluated using Thermo-Calc software and the selected databases: SSOL4 and TTNI6. This analysis made possible the prediction of the phases presented, compositions and amounts. Results from SEM analysis were used to compare the results obtained, showing good coherence between them.

Effect of Re on the precipitation behavior of μ phase in several single crystal superalloys

Three single crystal superalloys were designed based on the composition of superalloy CMSX-4 and effect of Re on the thermo-stability of these designed alloys has been investigated. Addition of the Re increased the phase instability of the alloys but did not change the type of the precipitation. All the topologically close packed (TCP) precipitation found in the three alloys was characterized as μ phase. Addition of the Re increased the formation temperature of μ phase, accelerated the precipitation of μ phase near the nose temperature of the temperature–time–transformation (TTT) curve and at low temperature (850°C), but relatively depressed the precipitation of μ phase between these temperatures. It is also found that the Re could stabilize or increase the segregation abilities of other alloying elements in the γ/γ′ phases. With the addition of Re, the W content increased in the γ phase, while the Cr content decreased, which is the reason of that addition of the Re can enhance the phase instability of μ phase in superalloys.

Phases Micromechanical Properties of Ni-base Superalloy Measured by Nanoindentation

This investigation describes the changes in the phase’s micromechanical properties as a result of alloying elements moving at interdiffusion. The induction of the interdiffusion between different phases of a Ni-based single crystal superalloy still is a challenging issue in this field of the materials science. For this we used novel technique - hard cyclic viscoplastic deformation at room temperature. The chemical compositions of the phases were determined by the cumulative bulk deformation. For detailed knowledge of this material behavior during life time the micromechanical properties of phases were investigated by nanoindentation and the results were analyzed by scanning electron microscope and the corresponding chemical content was investigated by X-ray microanalysis. It was established, that the calculated results values depend on indentation load stepwise increase from 5 to 10 and 20 mN, respectively. The hardness, contact compliance and elastic recovery parameters decrease, while indentation modulus, elastic- and plastic parts of indentation work increase with load increase. In initial material the hardness, indentation modulus and elastic recovery parameter for the γ/γ’-microstructure with coarse intermetallic γ’-precipitates were lower with compared to microstructure with fine γ’-precipitations. As a result of interdiffusion the chemical content was changed and these parameters for the coarse γ/γ’-microstructure increased as the γ’-Ni3Al content was increased, while these values in the γ-γ’-eutectic and Nb/Ta-rich phases decreased as the niobium and rhenium contents decreased, respectively. By this the lakes of γ-γ’-eutectic phase have maximal plastic part of indentation work with compare to other phases of superalloy. The cumulative bulk deformation increases lead to decrease of dendrites dimensions.DOI: http://dx.doi.org/10.5755/j01.ms.18.1.1337

Nonlocal damage‐viscoplastic model for high temperature creep of single crystal superalloys

AbstractA new nonlocal damage‐viscoplastic model for high temperature creep of single crystal superalloys is developed. It is based on the variational formulation consisting of free energy, plastic and damage dissipation potentials. Evolution equations for plastic strain and damage variables are derived from the minimum principle for dissipation potentials [1]. The model is capable of describing different stages of creep in a unified way. The evolution of dislocation densities of gamma and gamma prime phases in superalloys incorporates plastic deformation. It results in the time‐dependence of the creep rate in primary and secondary creep. Tertiary creep is taken into account by introducing local and nonlocal damage variables. Herein the nonlocal one is considered as numerical treatment to remove mesh‐dependence. Numerical results and comparisons with experimental data of the single crystal superalloy LEK94 are shown. (© 2011 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Evolution and analysis of TCP phase in a single crystal nickel-based superalloy containing Re during aging

By means of the aging treated for longer time at 1100 °C and microstructure observation, an investigation has been made into the evolution of TCP phase in a superalloy. Results show that the elements W and Re may promote the precipitation of TCP phase in the superalloy. After aged for longer time at 1100 °C, TCP phase is precipitated in the form of the slice-like along {111} planes and 〈110〉 orientation, and the TCP phase is determined as μ phase. The observed strip-like μ phase may be at the angle of 60° or vertical to each other on the different crystal planes. The slice-like μ phase coarsens gradually as the aging time prolongs, and the groove appears in the two sides of the strip-like μ phase until the dissolving up to abruption, in the further the μ phase are transformed into the spheroidized morphology. The free energy of the elements congregated during aging reduces with the increase of the element Re, which makes that the strip-like μ phase coarsens and the unevenness feature appears in the interfaces of the μ/γ′ phases. In the further, the additive pressure is generated in the regions with the smaller curvature radius to enhance the chemical potential of the solute elements in the μ phase. This is thought to be the main reason of resulting in the gradual decomposition and spheroidized of the μ phase.

Local order in industrial and model γ phases of superalloys

Different γ phase model alloys of nickel base superalloys are investigated as a function of temperature using elastic diffuse neutron scattering, macroscopic tensile tests, TEM post mortem observations and in situ straining experiments. Short Range Order (SRO) of (1 1/2 0) type resulting from nano-domains with correlation lengths of 0.5–1 nm is observed in all alloys. The dislocation distribution confirms the presence of short-range ordered nano-domains with a size of about 0.7 nm. In all the alloys, the influence of SRO on the deformation mechanisms is clear at low and medium temperature and thus explains the similarities of the deformation behaviour observed in this temperature range. From 750°C to 1000°C, the deformation microstructure becomes progressively homogeneous which suggests the decrease of SRO as illustrated by a decrease of diffuse scattering intensities at the (1 1/2 0) positions on heating. The various techniques applied prove to be complementary in the investigation of complex solid solutions.

Development of a Nickel Based Superalloy (Putalloy) with Rhenium and Platinum

A new nickel-based superalloy (Putalloy) containing rhenium and platinum has been designed by a computer program. The program uses an iterative process to ensure that the alloy composition has an electron hole number Nv of less than 2.0. Values of Nv less than 2 are expected to reduce the formation of topologically-close-packed and geometrically-close-packed phases in superalloys after prolonged exposure in service. This in turn should significantly improve creep strength. Stress rupture tests at several temperatures indicate that the creep strength of this new alloy is indeed superior to that of some of the existing polycrystalline Ni-based superalloys. Other mechanical properties such as tensile strength, and yield strength, of the as-cast and solution treated Putalloy samples were found to be comparable to conventional and existing Ni-based superalloys at room temperature and at 500 and 800°C. The oxidation resistance of putalloy was found to be superior to the existing nickel based superalloys.

On the microstructure and its diffraction anomaly of the μ phase in superalloys

acad sinica,inst met res,atom imaging solids lab,110015 shenyang,peoples r china.;zhu, j (reprint author), cent iron & steel res inst,beijing,peoples r china