Phases In Nickel-base Superalloys Research Articles

In Situ Study of Temperature Stability and Precipitation Kinetics of $$\delta $$ and $$\eta $$ Phases in Nickel Base Superalloys

In Situ Study of Temperature Stability and Precipitation Kinetics of $$\delta $$ and $$\eta $$ Phases in Nickel Base Superalloys

Effect of Co and Cr on the Stability of Strengthening Phases in Nickelbase Superalloys

Nickel-base superalloys such as VDM 780 may possess a high content of Cr and Co. This influences solution energies of phase-forming elements such as Al and Ta (γ′-phase), Nb (γ″- and δ-phase), and Ti (η-phase). We perform density functional theory studies of a nickel matrix at 0 K with high concentrations of either Co and Cr and calculate the influence of these elements on solution energies. In the case of Co, the solution energy can be predicted well by the nearest-neighbor interaction in the Co-rich matrix. For Cr, the effect is more complicated because Cr has a larger ionic radius and changes the magnetic state of the material. The effect of a Cr-rich matrix on the energy of Co is dominated by magnetic effects and interactions with the other elements by elastic deformation of the lattice. A high content of Co or Cr will thus increase the solvus temperature of the strengthening phase in nickel-base superalloys, in agreement with the literature and thermodynamic calculations.

Characterisation of short-range order using dislocations

Abstract The mobile dislocation pile-ups observed during in situ straining experiments have been used as probes to describe the short-range order (SRO) present in the γ phase of nickel-base superalloys. An experimental approach, based on the measurement of the dislocation positions, to characterise the SRO has been developed. This paper focuses on the main results we have obtained on this topic. A quantitative analysis allows to get the chemical forces associated with SRO, and also the SRO degree. For the first time, the energies associated with SRO are evaluated along a dislocation pile-up as a function of temperature. More generally, a description of the SRO is proposed: SRO is seen as a heterogeneous distribution at the micrometer scale of nanometric areas. Another characteristic has been evidenced: the reversible character of the SRO creation.

The nucleation and growth of η phase in nickel-based superalloy during long-term thermal exposure

The microstructure degradation and subsequent phase transformations in Waspaloy nickel-based superalloy during thermal exposure at 780 °C for 10,000 h were investigated. Two paths of η phase formation in the centre of extra-large γ’ (EL-γ’) following the formation of EL-γ’ were observed: (i) η phase directly precipitated within EL-γ’ when the coalescence of γ’ reached a critical stage; (ii) η phase precipitated at the interface of small size MC carbide and EL-γ’, with both MC and η embedded inside EL-γ’. The phase transformation process including the formation of EL-γ’ were experimentally observed and the formation sequences were schematically suggested. Two criteria of η formation and growth within EL-γ’ were established: (i) stacking faults formation in the nucleation site and (ii) sufficient atom diffusion during nuclei growth. The study of kinetics of η formation through two different paths revealed the critical role of small size carbides in promoting η nucleation and growth. It is concluded that η formation may be suppressed by controlling the size and density of MC carbides during materials processing.

Slip and fracture behavior of δ-Ni3Nb plates in a polycrystalline nickel-based superalloy during fatigue

The fatigue deformation behavior of δ-Ni3Nb (D0a intermetallic phase in nickel-based superalloy) was analyzed in detail to determine its fracture characteristics and possible slip systems. Strong arrangement-dependence (i.e. arrangement direction of plate length relative to loading direction) deformation characteristics were observed. When δ plates were arranged not paralleling to the loading direction, they exhibited certain degree of fatigue resistance. Slip traces were found in the non-parallel δ plates after some fatigue cycles. By analyzing the traces and habit plane of δ, the possible operative slip systems were confirmed to be (010)δ[100]δ and (010)δ[102]δ.

Accurate determination of lattice parameters based on Niggli reduced cell theory by using digitized electron diffraction micrograph

In this paper, we used Niggli reduced cell theory to determine lattice constants of a micro/nano crystal by using electron diffraction patterns. The Niggli reduced cell method enhanced the accuracy of lattice constant measurement obviously, because the lengths and the angles of lattice vectors of a primitive cell can be measured directly on the electron micrographs instead of a double tilt holder. With the aid of digitized algorithm and least square optimization by using three digitized micrographs, a valid reciprocal Niggli reduced cell number can be obtained. Thus a reciprocal and real Bravais lattices are acquired. The results of three examples, i.e., Mg4Zn7, an unknown phase (Precipitate phase in nickel-base superalloy) and Ba4Ti13O30 showed that the maximum errors are 1.6% for lengths and are 0.3% for angles.

Prediction of TCP Phases in Nickel-Base Superalloys

PHACOMP, Equilibrated alloying method and d-electrons alloy design theory have been employed for the prediction of TCP phases in nickel-base superalloys. The prediction of d-electrons alloy design theory was agreement with the samples aged at 900°C/1000h.

Morphological evolution of γ′ precipitates in a nickel-based superalloy during various solution treatments

The morphological evolution of the γ′ phase in nickel-based superalloy жc6y during various solution heat treatments was investigated. The significant changes of the γ′ precipitates were observed in the solution-treated samples. The coarsening and dissolution of γ′ phase simultaneously occurred at intermediate temperatures. In some areas, the primary precipitates became blunt and the adjacent ones were interconnected with each other via a diffuse neck, indicating a coarsening process of the primary γ′ population. The coarsening was dominated by the precipitate agglomeration mechanism (PAM) rather than by the well-known Ostwald ripening mechanism. In other areas, the partial dissolution of the γ′ precipitates began to occur, spreading gradually from dendrite cores to interdendritic regions. In addition, a flower-like γ′ structure was developed during the subsolvus solution treatments. The observable long filaments composed of erraticly shaped precipitates were caused by the heterogeneous nucleation of the cooling precipitates during water quenching.

Influence of rhenium and ruthenium on the local mechanical properties of the γ and γ′ phases in nickel-base superalloys

The effect of rhenium and ruthenium on the hardness of the γ′ precipitates and the γ matrix in nickel-base superalloys was investigated using a nanoindenting atomic force microscope. The partitioning behaviour of the alloying elements and the lattice misfit between the γ and γ′ phase were determined in fully homogenised samples to explain the alloying effects. Rhenium strongly strengthens γ as it predominantly partitions to γ and has a strong solid solution-hardening effect. Ruthenium strengthens both γ and γ′ due to a more homogeneous partitioning behaviour. Ruthenium was found to cause less partitioning of rhenium to γ. This results in a stronger increase of the γ′ hardness. The change in the nanoindentation-derived hardness of both phases could be mainly attributed to the solid solution strengthening of Re and Ru.

Numerical modelling of precipitation of topologically close-packed phases in nickel-base superalloys

A new mesoscale model was developed to understand the precipitation sequence of topologically close-packed (TCP) phases in single-crystal nickel-base superalloys. Nucleation and growth phenomena are described in detail. The model allows the multicomponent, multiphase and multiparticle calculation of TCP phase volume fractions and size distributions from supersaturated solid solution to thermodynamic equilibrium. It is based on the numerical Kampmann–Wagner method and applies CALPHAD (ThermoCalc) and DICTRA methodology for thermodynamics and diffusion. The capabilities of the simulations are demonstrated by the prediction of the size distributions of TCP phases and comparison with experimental data.

Temperature dependence of element partitioning in rhenium and ruthenium bearing nickel-base superalloys

The partitioning behaviour of alloying elements between the γ and γ′ phase in nickel-base superalloys with systematically varied contents of rhenium and ruthenium was investigated in the temperature range between 850 °C and 1300 °C. Ruthenium promotes a reverse partitioning of rhenium to the γ′ phase. This reverse partitioning is found over the whole temperature range up to γ′ solvus temperature. It is caused by an increased solubility of rhenium in the γ′ phase when ruthenium is present.

Phase Field Simulation of Rafting Behavior of γ`Phase in Nickel Base Superalloy

This paper describes phase field simulations of the rafting behavior of γ’ phase with a simple interfacial dislocation network model. The interfacial dislocation network model accounts for the effect of the network on the lattice misfit between γ and γ’ phases and the subsequent rafting behavior. The model is implemented into the phase field simulation to see the dependence of the rafting behavior of γ’ phases on the interfacial dislocation network. Without the dislocation network model, the amount of the rafting was negligibly small. On the other hand, with the dislocation network model, the γ’ phases shows a large amount of rafting, which is in good agreement with the results of the experimental observations. Therefore, the combination of the phase field method and the simple interfacial dislocation network model developed in this work is appropriate for the simulation of the rafting of γ’ phases.

Dissociation of a ⟨100⟩ edge superdislocations in the γ ′-phase of nickel-base superalloys

Dissociation of a⟨100⟩ edge superdislocations in Ni3Al, the hardening γ′-phase of nickel-base superalloys, was investigated using molecular dynamics simulations and theory of elasticity. It was shown that these dislocations dissociate either symmetrically or asymmetrically when they are close to the ⟨011⟩ orientation. The symmetric dissociation, called Hirth lock, has the lowest energy. The reasons for the dissociation are the strong energy reduction due to the core splitting and the relaxation of elastic strains within the dissociation area. The dissociation of a⟨100⟩ edge superdislocations is the reason for their alignment in ⟨011⟩ orientation in the γ′-rafts of superalloys. However, the dissociation does not block the movement of the dislocation because they penetrate the γ′-rafts by climbing. Under loading conditions, typical for creep tests of nickel-base superalloys at high temperatures (≥1000°C), the Hirth lock slightly expands but remains stable. The asymmetric configuration is less stable and can transform into the lower energy Hirth lock.

Characterisation of short-range order using dislocations

Abstract The mobile dislocation pile-ups observed during in situ straining experiments have been used as probes to describe the short-range order (SRO) present in the phase of nickel-base superalloys. An experimental approach, based on the measurement of the dislocation positions, to characterise the SRO has been developed. This paper focuses on the main results we have obtained on this topic. A quantitative analysis allows to get the chemical forces associated with SRO, and also the SRO degree. For the first time, the energies associated with SRO are evaluated along a dislocation pile-up as a function of temperature. More generally, a description of the SRO is proposed: SRO is seen as a heterogeneous distribution at the micrometer scale of nanometric areas. Another characteristic has been evidenced: the reversible character of the SRO creation.

Partitioning of Re between γ and γ′ phase in nickel-base superalloys

The partitioning behavior of the elements W, Mo, Re, Ru and Ir of three experimental nickel-base superalloys was investigated. For comparative studies ternary and quaternary alloys were produced and analyzed. The distribution coefficient of Re is shown to be a function of composition. Ru and Ir do not affect the partitioning behavior of Re.

412 ニッケル基超合金中の転位挙動の分子動力学シミュレーション(OS05-4 超合金・析出物)(OS05 電子・原子シミュレーションに基づく材料特性評価)

Understanding of the dislocation motion in nickel-base superalloy is very important to predict the lifetime of gas-turbine blades. γ' phase in nickel-base superalloy acts an strong obstacle to the dislocations, and, in some cases, stops the dislocation motion. In this paper, we performed a molecular dynamics simulation of dislocations motion in pure nickel matrix containing of one γ' particle. In the simulation, straight edge dislocation glides along the slip plane to contact with the γ' phase under shear stress loading. Subsequently we can observed that the motion and shape of the dislocation are affected by the γ' particle.

On the measurement of lattice mismatch between γ and γ′ phases in nickel-base superalloys by CBED technique

Nickel base superalloys are strengthened by ordered {gamma}{prime} precipitates of L1{sub 2} structure, which are coherently embedded in the {gamma} matrix (fcc). The mismatch in lattice parameter ({delta}) between the {gamma} and the {gamma}{prime} phases influences the mechanical properties of these alloys. The value and the sign of {delta} also control the morphological changes in {gamma}{prime} (rafting), which occur during deformation at high temperatures and at low strain rates. Recently, Convergent Beam Electron Diffraction (CBED) has emerged as a powerful method for lattice parameter measurements and has been used to measure lattice constants in cubic systems including superalloys. In the CBED technique, the lattice constants are determined by analyzing the High Order Laue Zone (HOLZ) line patterns present in the transmitted beam of the CBED pattern. The symmetry and position of the HOLZ lines are very sensitively dependent on the lattice constants, the orientation and the thickness of the specimen as well as on the microscope operating conditions. By comparing the experimental HOLZ line patterns with computer simulated patterns from a known crystal, one can determine the lattice constants very accurately (comparable to X-ray measurements).

Local Order and Associated Deformation Mechanisms in the γ Phase of Nickel Base Superalloys

Using X-ray diffraction, atom probe nanoanalysis, T.E.M. associated with in situ deformation, the presence of local order in the γ matrix of a monocrystalline MC2 superalloy is demonstrated. In Ni 2 CoCr the model alloy of this phase, as well as in the γ MC2 matrix, the existence at room temperature of a strong short range order of {1 1/2 0} special point type is emphasized. It induces high friction stresses opposing the movement of dislocations which have to propagate in planar arrays in the corresponding materials. In addition, long range ordered L1 2 hyperfine precipitates appear in the γ MC1 samples annealed at 750 °C. They induce several pairs of dislocations at the head of the moving pile-ups in order to cut them.

Accurate Measurement of Lattice Misfit Between γ And γ' Phases in Nickel-Base Superalloys at High Temperatures

Accurate measurement of the lattice misfit between the γ and γ phases in Ni-base single crystal superalloys at high temperatures has successfully been achieved using a parallel beam mode X-ray diffractometer with a high temperature specimen stage. The superalloys have γ' precipitates, an ordered FCC structure based on Ni3Al, in a γ-matrix having a disordered FCC structure (LI2). The parallel beam mode, the optics of which is made from a Ge (111) channel cut crystal, generates a superior monochromatic and parallel CuKα1 beam, which makes it possible to resolve a cluster peak in Ni-base superalloys into individual γ and γ ‘ peaks. This peak separation was almost impossible using the CuKα1+2 doublet beam from the standard type para-focusing mode, due to the very small difference in lattice parameters between the two phases.

Enhancements of thermal structure stability in a Ni-Base superalloy

[gamma][double prime](bct DO[sub 22] structure) and [gamma][prime] (fcc L1[sub 2] structure) are typical strengthening phases in nickel-base superalloys. The unique strengthening effect of [gamma][double prime]-Ni[sub 3]Nb and [gamma][prime]-Ni[sub 3]Al helps Incone1718 to have wide industrial application. Because of the structure stability limitation of [gamma][double prime], the highest service temperature of IN718 is 650[degree]C. Microstructure instability of IN718 is mainly controlled by a [gamma][double prime] coarsening and plate-like [delta] phase formation. In order to improve structure stability, Cozar and Pineau published a pioneer paper on a modified alloy with a compact morphology of [gamma][double prime] and [gamma][prime], which showed higher thermal stability at age hardening test results. Recently a series of papers slightly modifying the chemical composition with variation of Al, Tk and Nb contents in IN718 has shown that a compact morphology can be obtained by appropriate heat treatments and that mechanical properties beyond 650[degree]C can be improved. However, structure stability at long time aging beyond 650[degree]C, as well as reasons to improve structure stability have scarcely been published. A detailed study in this paper was undertaken to develop a compact morphology. Cube-shaped [gamma][prime] particles were coated on their six faces with a shell of [gamma][double prime] precipitates, so amore » higher structure stability was created compared to separate precipitation of [gamma][double prime] and [gamma][prime] as the mixture of disk-shaped [gamma][double prime] and round [gamma][prime] particles in conventional IN 718. This investigation concentrates on the long time aging structure stability beyond 650[degree]C for fundamental understanding and explanation of such an improvement in thermal structure stability of compact morphology.« less