Base Superalloy Research Articles

Stacking faults and their effects on improving plasticity in a Co–Al–W–base superalloy at 800 °C

Stacking faults and their effects on improving plasticity in a Co–Al–W–base superalloy at 800 °C

B2 to ordered omega transformation during isothermal annealing of refractory high entropy alloys: Implications for high temperature phase stability

Al containing refractory high entropy alloys or complex concentrated alloys (RHEAs or RCCAs) often exhibit a patterned BCC + B2 microstructure, resembling the FCC + L12 microstructure typically observed in many Ni/Co base superalloys. There is a rapidly growing interest in these BCC+B2 microstructures, especially due to their promising balance of elevated temperature mechanical properties. Unfortunately, the stability of such microstructures on long-term exposure to elevated temperatures is not well-understood and prompts the motivation of the present study. The current study traces the phase transformation pathway during the intermediate temperature annealing of such microstructures for extended time periods, and more specifically the formation of ordered omega type phases. In the case of a high strength RHEA, Al0.5Mo0.5NbTa0.5TiZr the relationships between the two structures strongly suggest a strong possibility that such ordered omega phases are essentially derived from a parent B2 phase. At the early stages of annealing, the solutionized condition of this alloy consists of a co-continuous mixture of BCC and B2 phases which transforms into a continuous B2 matrix with discrete BCC precipitates presumably via spinodal decomposition concomitant with ordering. With increasing annealing time at 800 °C, isolated islands of BCC phase embedded with discrete, fine-scale pockets of the B2 and hP18 (P63/mcm) phase develop, where the hP18 phase is an ordered derivative of the omega phase, commonly found in titanium and zirconium base alloys. The transformation of the parent B2 phase to the hP18 ordered omega type phase can be rationalized via a combined replacive and displacive transformation, including simultaneous rejection and enrichment of specific alloying elements to achieve the desired stoichuometry. First-principles based DFT calculations were also carried out to compare the stability of the B2 versus hP18 phases, as a function of composition. While long term annealing and CALPHAD based calculations both indicate towards an equilibrium phase field comprising of hP18, B2 and BCC phases, hP18 does not directly nucleate homogeneously within the BCC grains. Instead, it appears to form heterogeneously along grain boundaries and more importantly within the grains with B2 phase acting as an intermediate phase, both of which indicates a high nucleation barrier for its homogeneous nucleation. The universality of this phenomenon has been demonstrated using examples from other RHEAs, and the resultant impact on the high temperature stability of the BCC+B2 microstructure has been discussed.

Effect of B and Zr additions and homogenization treatment on the carbide morphology of a vacuum cast Co-Cr-W-Ni superalloy

The influence solution heat treatment (1200 °C) on the evolution of carbide morphology and creep tensile properties of a vacuum cast Co-Cr-W-Ni base superalloy with and without of B and Zr additions has been investigated. The experimental evidence, shows that compared to the as cast microstructure, the creep life at a temperature of 750 °C, the carbide morphology and distribution are affected by the B and Zr additions as well as the solution treatment. After solution treating the alloy without additions, a finer carbide distribution along the interdendritic arms was observed. On the contrary, the additions of B and Zr impact the carbide morphology as well as its distribution, precipitating within the dendrites and at interdendritic regions. Due to this, the creep life and creep deformation of sample with additions are 12.5 and 2.7 times higher compared to the sample without additions respectively.

Improving Intermediate Temperature Plasticity of Co–Al–W–Base Superalloy: Based on Optimizing MC Carbides

The brittleness of the new Co–Al–W–base superalloys is said to be a fatal weakness for their application in the intermediate temperature range, especially for polycrystal Co–Al–W–base superalloys. MC carbide controlling is used to improve the intermediate temperature plasticity of the Co–Al–W–base superalloy in the present study. Both microstructure characterization and interrupted tensile tests were performed to investigate the mechanism of improving the plasticity of the superalloy by MC carbides at an intermediate temperature of 800 °C. The results show that the plasticity of the superalloy is mainly dependent on MC carbides. MC carbide breakers appear after yielding, which closely depends on the morphology and size of MC carbides. Based on the experimental and calculation results, it is found that the intermediate temperature plasticity of the superalloy can be guaranteed by controlling the critical mean size of MC carbides, and the desired mean size of MC carbides should be less than 47 μm.

Effect of Mo and W interlayers on microstructure and mechanical properties of Si3N4–nickel‐base superalloy joints

AbstractSi3N4/nickel‐base superalloy (Inconel‐625) and Si3N4/Si3N4 joints with refractory metal (W and Mo) interlayers were vacuum brazed using a Ti‐active braze Cu‐ABA (92.75Cu–3Si–2Al–2.25Ti) at 1317 K for 30 min with the following interlayered arrangements: Si3N4/Mo/W/Inconel and Si3N4/Mo/W/Si3N4. The joints exhibited Ti segregation at the Si3N4/Cu‐ABA interface, elemental interdiffusion across the joint interfaces, and sound metallurgical bonding. Knoop microhardness profiles revealed hardness gradients across the joints that mimicked the interlayered arrangement. The compressive shear strength of Si3N4/Si3N4 joints both with and without W and Mo layers was ∼142 MPa but the strength of Si3N4/Inconel joints increased from ∼9 MPa for directly bonded joints without interlayers to 53.5 MPa for joints with Mo and W interlayers.

Effect of γ' phase and microtwins on the microstructural evolution and mechanical properties of a novel Ni–Co base superalloy

A new Ni–Co base precipitation-hardened superalloy designed using the low stacking fault energy (SFE) was used primarily in the hot components of aeroengines designed with low SFE exposed to high temperatures. Different microstructures of the new alloys were obtained by controlling the solution treatment temperature. The corresponding mechanical properties were tested at 25 °C and 750 °C. The primary γ' precipitates dissolved with increasing solution temperature, causing grain growth and an increasing microtwin fraction. Fine-grained, transient, and coarse-grained zones were formed in sequence because of the interaction between the γ′ phase and microtwins. The tensile strength decreased as the grain grew when tested at room temperature, and the strengthening mechanism was dislocation strengthening. However, when the tensile experiment temperature was increased to 750 °C, the tensile properties increased from the fine-grained to the transient zone and then decreased to the coarse-grained zone. Moreover, the fracture mechanism changed from ductile fracture to a mixed ductile and brittle fracture. The size of the secondary γ′ precipitates increased by 32 nm because the dissolved primary γ′ precipitated when the solution temperature was increased from 1090 °C to 1120 °C, which improved the tensile properties. The electron channeling contrast imaging and transmission electron microscopy results confirmed that microtwins were formed during the tensile process at 750 °C. The microtwins hindered the dislocation movement. Therefore, the transient zone of the low-SFE alloy demonstrated the best mechanical properties, which guided the optimization direction of the alloy design and service.

Tensile properties and deformation mechanisms of a new Ni–Co base superalloy from room temperature up to 750 °C

In this article, we developed a new Ni–Co base superalloy with excellent mechanical properties in the temperature region from room temperature to 750 °C. The yield and tensile strengths of the alloy at room temperature are ∼1212 MPa and ∼1638 MPa, respectively, and they can still reach ∼1077 MPa and ∼1233 MPa at 750 °C, maintaining a good intermediate temperature strength. Microstructure characterizations show that the plastic deformation is mainly dominated by anti-phase boundary (APB)-coupled dislocation pair shearing γ′ precipitates at room temperature. With increasing the temperature, although the dislocation pair shearing still dominates, stacking fault (SF) shearing and nanotwinning also appears and contributes minor to the plastic deformation. The interactions among dislocations, SFs and nanotwins finally results in the decent working hardening ability even at such a high temperature up to 750 °C, which in turn gives rise to the ultrahigh ultimate tensile strength of the Ni–Co base superalloy.

Effect of Al addition on intergranular embrittlement behavior of GH3535 alloy induced by fission product Te

The effect of Al addition on Te diffusion behavior of a Ni-16Mo-7Cr base superalloy was investigated at 800 °C for 100 h. The results show that Ni3Te2 and Cr3Te4 are the mainly reaction products formed on the surface of Al-modified alloy. Although no aluminum telluride is detected both on the surface and in the alloy, the addition of Al promotes the formation of compact and continuous Cr3Te4 layer on the surface, which effectively inhibits the diffusion of Te into the alloy. Moreover, Al can also promote the γ' phase formation, and thus improve the tensile property of the alloy significantly.

EFFECT OF HOT CORROSION AND OXIDATION ON COBALT - BASE SUPERALLOYS COATED BY ALUMINUM WITH PRESENCE OF THERMAL BARRIER ZRO2 AND MIXTURE OF (NACL) AND (NA2SO4) STEAMSALTSAT HIGH TEMPERATURES

The aim of the study was to investigate the Mechanisms properties of thermal barrier coatings (TBCs) to enhance of performance evaluation characteristics and develop TBCs.Cobalt –base superalloy has been used as a substrate and zirconium stabilized Aluminum as ceramic topcoat , in addition the study include degradation behavior of system during thermal cycling (3hr per cycle in furnace) the failure of the aluminized was due to thermally grown oxide (TGO) interface. The fractures propagatethrough the interface and produce a deformation of the bond coating . the effect of cycle will result a spallation failure of the TBCsand this is also corresponding to a slightdegradation .The steam of salt (Nacl)and(Na2So4) mixture will affecton the coating lifetimes .The high temperature have a strong effect thermally grown oxide (TGO) which consistent with a first order growth of scale failer variation.

Mo-Silicide Alloys for High-Temperature Structural Applications

The challenges of a high-temperature environment (T > 1,400°C) impose severe material performance constraints in terms of melting point, oxidation resistance, and structural functionality. A number of ceramic materials, intermetallic compounds, and refractory metals with high melting temperatures are available as material choices. However, in a single-component single-phase form, these materials do not satisfy all the aforementioned requirements. One clear message from the evolutionary development of high-temperature alloys is the importance of developing multicomponent alloys with multiphase microstructures and the capability to control phase fractions and morphologies to satisfy a number of mechanical property requirements. Besides the essential structural requirements, elevated temperatures often also involve aggressive environments that require a material to display an inherent oxidation protection that can be further enhanced by coating. Among the leading candidates to advance beyond the capability of the current nickel (Ni)-base superalloys, the multiphase microstructures that can be developed in the molybdenum-silicon-boron (Mo-Si-B) system involving a high melting temperature (>2,100°C) ternary-based intermetallic Mo5SiB2 (T2) offer an attractive performance. Most of the attention has been on three-phase alloys comprised of Mo(ss), T2, and Mo3Si that offer high-temperature stability and robust microstructures, but new alloy designs are in development. In this review the recent advances in the development of Mo-silicide alloys are discussed in terms of alloy design, microstructure control, structural performance, environmental resistance, and component analysis.

Synthesis and Characterization of aluminium based multicomponent alloys

In general, the traditional alloys are essentially based on the domination of a single principle element, such as the aluminium (Al)-base alloys, magnesium (Mg)-base alloys, iron (Fe)-base alloys, nickel (Ni)-base super alloys, etc. For the traditional lightweight alloys, such as the alloys of aluminum, the concentration of minor alloying elements added to the major element (Al) is typically less than or equal to [ ≤] five atomic percent. In this research paper, the intricacies specific to the development of aluminium-based multicomponent alloys as a viable option to engineer the development of advanced lightweight alloys is presented and adequately discussed. The alloys, Alx(LiMgCuZn)100-x were designed based on the multi-element alloying approach in which the concentration of each alloying element was either greater than or equal to [≥] five atomic percent, except for the element aluminum [70 atomic percent and 75 atomic percent]. The multicomponent alloys, Al70Li10Mg10Cu5Zn5 and Al75Li10Mg5Cu5Zn5, were synthesized using the technique of disintegrated melt deposition [DMD]. The calculated mixing entropy of the Al70Li10Mg10Cu5Zn5 alloy was > 1R [R is gas constant, 8.314 J/K mol], classifying it to be a medium entropy alloy (MEA). For the Al75Li10Mg5Cu5Zn5 alloy, the mixing entropy was less than 1R, classifying it to be a low entropy alloy (LEA). Characterization studies were carefully done on the as-synthesized multicomponent alloys. The microstructure and mechanical properties of the two alloys is presented and briefly discussed. Particular emphasis will be given to both examining and understanding microstructural evolution and the resultant influence of microstructure on mechanical properties of the engineered alloys. A comparison of the properties of the two alloys [Medium Entropy Alloy and Low Entropy Alloy] is highlighted.

Intergrowth structure of Laves within [formula omitted] phases in Co–Al–W base superalloy

In a Co–Al–W base superalloy, two types of topologically close-packed (TCP) phases, μ and Laves phases, were investigated by high-resolution transmission electron microscopy, scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. It is found that an intergrowth structure of Laves within μ phases can be formed and both have a completely coherent interface relationship. With the aid of image simulation, both μ and Laves phases are identified as the quaternary compounds containing Co, Cr, W and Mo. Compared to Co7W6-type μ phase, W1 and W3-sites are the preferred sites of substitution by Mo atoms and Co1-site is a preferred site of substitution by Cr atoms in the quaternary μ phase, leading to the atomic displacement and an alteration of lattice parameter. In contrast to Co2W-type Laves phase, the newly formed quaternary Laves phase can maintain the C14-type Laves structure although Co and W are substituted partially by Cr and Mo respectively.

Effect of grain boundary carbides on the diffusion behavior of Te in Ni-16Mo-7Cr base superalloy

Fission product Te could result in intergranular embrittlement of Ni-16Mo-7Cr superalloy and threaten the safety of molten salt reactor. Although embrittlement of Te has been reported widely, there are few researches about the influence of precipitated grain boundary carbides (M12C) on the diffusion behavior of Te in the alloy with long term service. In this study, the effect of M12C on the diffusion behavior of Te in Ni-16Mo-7Cr alloy was studied. The results showed that Cr3Te4 and Ni3Te2 were formed along M12C at the grain boundary of alloy after long-term thermal exposure. However, the formation of M12C carbides and tellurides had little impact on the diffusion of Te in the alloy.

Effects of Mo on the evolution of microstructures and mechanical properties in Co-Al-W base superalloys

The solidification paths, microstructures and mechanical properties of two Co-Al-W base superalloys with different additions of tungsten and molybdenum were investigated. Mo addition does not influence the volume fraction of the γ′ phase; however, the morphology of the γ′ phase changes from cuboidal to near cuboidal. The μ phase and β phase were identified in both alloy systems. Although the melting point of the μ phase is decreased by Mo addition, the solidification paths remain the same: L→ L1 + γ → L2 + γ + μ → γ + μ + β → γ + μ + β + γ′. The precipitation of a γ′ phase envelop around the interdendritic regions in Co-Al-W alloy systems has never been reported. The formation of a surrounding γ′ phase was caused by the diffusion of elements between secondary phases and the substrate during the heat treatments. The microhardness of the μ phase, which can be easily influenced by Mo addition, is the highest in both alloys, followed by the microhardnesses of the β phase and the substrate. In the fracturing process, microcracks were generated in the μ phase, the number of which increased with Mo addition.

Hydrogen embrittlement susceptibility of a Ni-16Mo-7Cr base superalloy

This study investigated the hydrogen embrittlement sensitivity of a Ni-16Mo-7Cr base superalloy under different hydrogen pre-charging states. Detailed electromicroscopy characterization has been employed on the hydrogen pre-charged alloy after tensile testing to understand the mechanism of hydrogen-assisted cracking. The results show that the tensile strength of the alloy is remarkably decreased with the increasing of hydrogen pre-charging time, whereas the yield strength remains stable all the time. The fractographic study combined with ECCI analysis by Scanning Electron Microscopy demonstrates that the cracks propagated predominantly along the grain boundary in the presence of hydrogen, and a few can propagate along planar dislocation slip bands (DSBs) especially along the intersections between nonparallel DSBs. The proposed hydrogen embrittlement mechanism for the Ni-16Mo-7Cr base superalloy is that the coalescence and widening of the nano-voids, which are resulted from the interaction between the hydrogen and dislocations, cause the crack initiation and propagation along the grain boundary, and further cause the alloy intergranular fracture.

Role of base-metal composition in isothermal solidification during diffusion brazing of nickel-based superalloys

ABSTRACTThe key feature of diffusion brazing, also referred to as transient liquid phase bonding, is isothermal solidification which precludes the formation of intermetallic in the joint centreline. Analysing the available data published in the literature showed that the composition of the nickel-based superalloys plays a strong role in determining the required time for obtaining intermetallic-free joint during diffusion brazing. This effect is not predictable by the standard conventional models. It is proposed that increasing the boride-forming elements in the base superalloy which promotes in situ boride precipitation at the diffusion-affected zone can accelerate the diffusion flux of the boron into the base superalloy, leading to faster isothermal solidification. The higher the Cr + Mo + Nb + Ta + W content in base superalloy, the shorter the isothermal solidification time.

Composition design, phase transitions of a new polycrystalline Ni-Cr-Co-W base superalloy and its isothermal oxidation dynamics behaviors at 1300 °C

The composition, phase transitions and isothermal oxidation dynamics of a new polycrystalline Ni-Cr-Co-W base superalloy are investigated using CALculation of PHAse Diagrams (CALPHAD), Differential Scanning Calorimetry (DSC), ThermoGravimetric analysis (TG/dTG), Scanning Electron Microscope and Energy Dispersion Spectrum (SEM/EDS) methods. The microstructure of the designed alloy shows a uniform γ matrix with the precipitates of rich-Ti cellular η and rich-Nb acicular platelet δ phase, and no Topologically Close-packed Phases (TCPs) observed, and the oxide behavior obeys a parabolic law with a parabolic constant (kp) 2.945mg2/(cm4·h) at isothermal 1300°C for 30 hours. The oxide scales are composed of an outer loose oxide mixture containing Cr, Ni, Ti and Nb/Ta and generous pores presented for a volatilization of Cr2O3, and a dense aluminum-rich oxide layer.

The influence of high temperature on DV-2 jet engine Ni-based superalloy turbine blade degradation

Blades and vanes made from Ni – and Co – base superalloys that are used in the hot section of all gas turbine engines are coated to enhance resistance to hot corrosion. The most widely used coatings are those based on the intermetallic compounds NiAl and CoAl, which are formed by diffusion interaction of aluminium with surfaces of the nickel and cobalt alloys, respectively. Simple aluminide coatings resist high – temperature oxidation by the formation of protective layer of alumina and can be used up to about 1 150°C. The coatings degrade by loss of aluminium due to spalling of oxides under thermal cycling conditions. HPT blades of DV – 2 jet engines are made from Ni – based superalloy ŽS6K. For improving alloy’s high temperature resistance are blades coated with Al – Si diffusion layer. Depending on jet engine flying regime an overcrossing of working temperature may occur. It may result into coarsening of the gamma prime particles, thus decreasing its strengthening effect. A real case study of DV-2 jet engine turbine blades after high temperature exposition is discussed in this article.

Creep Processes in MAR-M247 Nickel-Base Superalloy

Creep processes in MAR-M247 Nickel - Base Superalloy were studied at elevated temperatures. The stress exponents of creep rate n and time to fracture m indicate power-low (dislocation) creep regime and suggest the same controlling mechanism for the creep deformation and fracture. The variation of values of parameter n within the interval of applied stress may indicate changes in the rate-controlling creep deformation mechanism. The possibility of using of Monkman-Grant relationship for creep life prediction was demonstrated.

Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

This work discusses the effects of alloying on the coarsening behaviour of the L12 ordered γ′ phase and the structure of the γ/γ′ interfaces in three Co-Al-W base superalloys aged at ∼90 °C below the respective solvus temperatures: Co-7Al-7W, Co-10Al-5W-2Ta and Co-7Al-7W-20Ni (at.%). The coarsening kinetics are adequately characterised by the classical Lifshitz-Slyozov-Wagner model for Ostwald ripening. Co-7Al-7W exhibited much slower coarsening than its quaternary derivatives. Alloying can be exploited to modify the coarsening kinetics either by increasing the solvus temperature by adding tantalum, or by adding nickel to shift the rate controlling mechanism towards dependence on the diffusion of aluminium rather than tungsten. Lattice resolution STEM imaging was used to measure the widths of the order-disorder (structural) and Z-contrast (compositional) gradients across the γ/γ′ interfaces. Similarly to nickel base superalloys, the compositional gradient was found to be wider than the structural. Co-7Al-7W-20Ni had much wider interface gradients than Co-7Al-7W and Co-10Al-5W-2Ta, which suggests that its γ′ phase stoichiometry is less constrained. A possible correlation between temperature and misfit normalised r vs. t1/3 coarsening rate coefficients and the structural gradient width has also been identified, whereby alloys with wider interfaces exhibit faster coarsening rates.