Single Crystal Nickel Base Research Articles

Rafting in single crystal nickel-base superalloys — An overview

Currently nickel-base single crystal (SX) superalloys are considered for the manufacture of critical components such as turbine blades, vanes etc., for aircraft engines as well as land-based power generation applications. Microstructure and high temperature mechanical properties are the major factors controlling the performance of SX superalloys. Rafting is an important phenomenon in these alloys which occurs during high temperature creep. It is essential to understand the rafting mechanism, and its characteristics on high temperature properties before considering the advanced applications. In this review article, the thermodynamic driving force for rafting with and without stress is explained. The nature and influence of rafting on creep properties including pre-rafted conditions are discussed. In addition, the effect of stress state on γ/γ′ rafting, kinetics and morphological evolution are discussed with the recent experimental results.

In-situ observation of deformation micromechanisms in a rafted γ/γ′ superalloy at 850 °C

In-situ straining experiments were performed in a transmission electron microscope at temperatures ranging from 800 to 850 °C on a MC2 γ/γ′ nickel-based superalloy single crystal with a rafted microstructure. Perfect and partial dislocations were observed in motion. It is shown that deformation starts in the γ phase, which is the weaker phase at this temperature, although interaction of dislocations with point defects is still effective. The γ/γ′ interfaces play a different role depending on the propagation direction of the dislocations. From γ to γ′, interface crossing often acts as an additional obstacle that can be overcome by pile-ups of dislocations. From γ′ to γ, an increase of the dislocation speed is almost always observed, with no noticeable waiting time at the interface. Some dislocation multiplication processes are observed when interfaces are crossed from γ′ to γ. Unusual dissociation of superdislocations involving super stacking faults is observed in γ′, along with microtwinning. This may be taken as an indication of the low stacking fault energy of the MC2 alloy.

Gradient-dependent deformation of two-phase single crystals

In this work, a gradient- and rate-dependent crystallographic formulation is proposed to investigate the macroscopic behaviour of two-phase single crystals. The slip-system-based constitutive formulation relies on strain-gradient concepts to account for the additional strengthening mechanism associated with the deformation gradients within a single crystal with a high volume fraction of dispersed inclusions. The resulting total slip resistance in each active system is assumed to be due to a mixed population of forest obstacles arising from both statistically stored and geometrically necessary dislocations. The non-local theory is implemented numerically into the finite element method and used to investigate the effect of the relevant microstructural (i.e., size and volume fraction of precipitated inclusions) and deformation-gradient-related length scales on the macroscopic behaviour of a typical nickel-based superalloy single crystal. An analytical framework to link the strain-gradient effects at the microscopic level with the macroscopic behaviour of an equivalent homogeneous single crystal is also proposed.

Creep deformation modelling of superalloy single crystals

A model of creep deformation in arbitrarily oriented nickel-base superalloy single crystals has been developed. This model extends the preceding model of the present authors [Svoboda, J. and Lukáš, P., Acta mater., 1998, 46, 3421] covering the case of creep in 〈001〉 oriented crystals. The calculated creep curves were compared with the creep curves of single crystals CMSX-4 of the orientations 〈001〉, 〈011〉 and 〈111〉 measured at 750°C; a fair agreement was found.

Wave propagation in an anisotropic nickel-based superalloy

The effects of elastic anisotropy on ultrasound propagation in a nickel-based single crystal test component are studied using a 25 MHz focused probe in a water immersion system. Anisotropy gives rise to directionally dependent acoustic wavespeeds, beam steering, acoustic energy focusing and mode conversion for normal incidence. Transverse mode echoes are particularly strong in the vicinity of crystallographic directions in which the Gaussian curvature of the slowness surface is zero and divergence of the echo amplitude is predicted on the basis of the stationary phase approximation. There are other directions where the transverse mode echoes vanish for symmetry reasons. The longitudinal mode echo amplitude also shows significant variation with direction. Overall there is good agreement between the echo signal arrival times and amplitudes we measure and calculation. Progress in applying this technique to gas turbine blades is reported.

Modelling the orientation and direction dependence of the critical resolved shear stress of nickel-base superalloy single crystals

The orientation and direction dependence of the critical resolved shear stress was determined experimentally for the chromium-rich superalloy SC 16 at 650, 750 and 850°C and a constant strain rate of 10 −3 s −1. The results are used to establish an extended Schmid law for octahedral slip in the temperature and orientation range in which cross-slip pinning of dislocation pairs in the γ′ phase takes place. Normal Schmid behaviour was assumed for orientations near [111], for which cube slip was activated on a macroscopic level. Differences between some commercial superalloys are worked out and can be attributed to morphology and volume fraction of the γ′ phase. The orientation dependence and asymmetry effects increase in the order NIMONIC 105, SC 16, René N4. The orientation range where macroscopic cube slip can be expected increases in the same order. A close inspection of the parameters which are responsible for non-Schmid behaviour suggests that, in addition to cross-slip pinning, a matrix effect must be operating as well, partly counteracting the behaviour expected for mono-phase γ′ crystals.

Formation and role of dislocation networks during high temperature creep of a single crystal nickel–base superalloy

An investigation has been made into the formation and role of dislocation networks during high temperature creep in a single crystal nickel-base (Ni-Al-Mo-Ta) superalloy. The results show that the three-dimension networks in the matrix channels are formed by the reaction of dislocations during primary creep. It is found that these three-dimension networks formed at the gamma'/gamma interfaces play an important role in accommodation of the strain hardening and the recovery softening during steady state creep. However, in the tertiary creep stage, the main deformation feature of the alloy is the shearing of the gamma' rafts by the moving dislocations in matrix from the gamma'/gamma phase interfaces where the dislocation networks are damaged. (C) 2000 Elsevier Science S.A. All rights reserved.

Fatigue crack path prediction in UDIMET 720 nickel-based alloy single crystals

The effect of orientation, applied stress state, environment, and temperature on the crack growth and crack-path behavior of single-crystal specimens of UDIMET 720 has been examined. Stage I cracking has been promoted by mixed-mode loading, plane stress, and vacuum conditions; increasing the test temperature to 600 °C does not suppress stage I crack growth in vacuum. Consideration of the local resolved shear-stress intensity and local resolved normal-stress intensity for each slip system as it intersects the nominal crack-growth plane allows the prediction of stage I crack paths, clarifying the importance of secondary single-crystal testing orientation (i.e., nominal crack-growth direction as well as effective tensile axis). A combination of both opening and shearing are found to promote stage I crack growth, and boundary conditions have been established within which stage I cracking is promoted. Highly deflected stage I cracking gives rise to significant shielding effects, but under suitable mixed-mode loading, highly oriented, coplanar stage I crack growth can be produced. Intrinsic stage I cracking under mixed-mode loading appears to be greatly accelerated compared with mode I-dominated stage II crack growth for comparable stress-intensity levels.

Transmission electron microscopy contrast simulations of -superdislocations in the L1 2 ordered structure

Typical characteristics for creep fracture cleavage plane of nickel-based single crystal were investigated through the experimental observations. The results suggest that the topological shape of cleavage planes is related to the crystal graphical orientation. The cleavage surface exhibit four-fold symmetric in [001] orientation, two-fold symmetric in [011] orientation, and triple-symmetric in [111] orientation. It is proved that the topological morphology is associated with the stress field near void through crystal plasticity finite element method. Void size dependence and external stress magnitude dependence of cleavage plane morphology was studied as well. The void size and external stress magnitude have a significant effect on the magnitude of the axial strain, but do not change visually with respect to the distribution of axial strain. The formation of typical characteristics of cleavage plane was discussed by the slip systems activation. The characteristics of dislocation motion around the void were described by molecular dynamics simulation.

Model of creep in 〈001〉-oriented superalloy single crystals

A model of creep in 〈001〉 oriented nickel-base superalloy single crystals applicable both for low and high temperatures has been developed. This model extends the preceding model of the present authors [Svoboda, J., Lukáš, P., Acta mater., 1997, 45, 125] covering only the case of low temperature creep.The present model takes into account five mechanisms, namely (i) dislocation slip in γ channels and concurrent multiplication of dislocations, (ii) dislocation slip in γ′ particles, (iii) dynamic recovery of the dislocation structure, (iv) morphological changes of γ′ particles by migration of γ/ γ′ interfaces and (v) coarsening of the rafted structure. The calculated creep curves were compared with the creep curves measured on single crystals CMSX-4 at 750°C and 1000°C; a good agreement was found.

Activation energy of creep in 〈001〉-oriented superalloy CMSX-4 single crystals

The available experimental data on the creep behaviour of the two-phase nickel-base superalloy CMSX-4 single crystals show that the activation energy of the steady-state creep is much higher than the activation energies of self-diffusion in both the phases. A model of creep in 〈001〉-oriented nickel base superalloy single crystals taking into account dislocation slip, dynamic recovery, migration of the γ/γ′ interfaces and coarsening of the rafted structure has been developed. The simulated creep curves and the predicted values of the activation energy are in reasonable agreement with the experimental findings of Schneider and Mughrabi [1].

Modelling of recovery controlled creep in nickel-base superalloy single crystals

A model of the kinetics of recovery controlled creep in 〈001〉 oriented nickel-base superalloy single crystals has been developed. Two basic deformation mechanisms have been considered, namely (i) deformation of γ channels by slip in discrete slip systems connected with the generation of dislocations and their deposition at the γ γ′ interfaces; and (ii) dynamic recovery of the dislocation structure due to non-conservative motion (a combination of slip and climb) of dislocations along the γ γ′ interfaces and their annihilation. The climb of dislocations is conditioned by the diffusive transport of vacancies generated and annihilated at the climbing dislocations. In the steady-state creep the rate of the slip deformation in all the γ channels is in equilibrium with the recovery induced diffusional deformation. The model predicts realistic values of the steady-state creep rates and their dependence on the applied stress, as well as the strains corresponding to the end of the primary creep stage, dislocation densities at the γ γ′ interfaces and resolved shear stresses both in the γ channels and in the γ′ particles.

Microstructure-based 3D finite element modelling of lattice misfit and long-range internal stresses in creep-deformed nickel-base superalloy single crystals

In the present paper, residual stresses and direction-dependent lattice parameters and lattice misfits in the γ′-hardened, tensile creep-deformed monocrystalline nickel-base superalloy SRR 99 have been studied by three-dimensional finite element modelling based on a “simplified creep process”. The main results are as follows. (i) After creep and cooling to room temperature, the internal strains and stresses in both phases γ and γ and γ′ in the directions [001]/[010] are different. From these, differences between the lattice parameters and lattice misfits in the [001] and the [100]/[010] directions follow. During cooling to room temperature after creep, the change in thermal stresses can cause plastic back flow of the γ-phase. The internal stress state is relaxed accordingly. (ii) During the subsequent heating (after creep and cooling to room temperature), the differences of the lattice parameters in the 〈100〉 directions decrease with increasing temperature. (iii) During final cooling from a stress-free state at 105 °C, the lattice misfit and the internal stresses between the [001] and the [100]/[010] directions increase with decreasing temperature. The influences of the creep-induced dislocation networks at the γ/γ′ interfaces, of the coherency stresses and of thermal stresses on the lattice misfit and internal stresses are discussed in detail. The differences between the values of the lattice misfit measured by TEM and by X-ray diffraction are reconciled. The differences between two- and three-dimensional finite element modelling are also discussed. The results are in reasonable agreement with the experimentally measured data.

Stem analysis of the local chemical composition in the nickel-based superalloy CMSX-2 after creep at high temperature

High temperature creep of nickel-based superalloy single crystals is characterized by directional coalescence of the {gamma}{prime} reinforcing precipitates. The morphology of the coalesced structures depends on experimental parameters such as the sense and direction of the creep stress as well as on intrinsic parameters such as the sign of the misfit between matrix and precipitates. For single crystals of the commercial superalloy CMSX-2 submitted to creep at 1,323K, the initially cuboidal {gamma}{prime} precipitates coalesce into platelets arranged perpendicular or parallel to the stress direction when the stress is in tension or in compression, respectively. So far, however, the evolution of the local chemical composition around dislocations had not been experimentally investigated in nickel-based superalloys.

The growth and elevated temperature stability of high refractory nickel-base single crystals

Additions of refractory alloying elements to single-crystal and polycrystalline nickel-base alloys are known to provide potent high temperature strengthening. However, high levels of elements such as Re, W and Ta can result in unexpected modes of degradation during solidification and subsequent long-term exposures at high temperatures. Here the contributions of refractory alloying additions to various mechanisms of high temperature strengthening are discussed. In addition, the results of experiments which have identified two new modes of degradation associated with the presence of high levels of refractory alloying elements in nickel-base single crystals will be presented. Nucleation and growth of isolated columnar grains during solidification and discontinuous reactions at defects in the single-crystal structure will be shown to be extremely sensitive to the level of refractory alloying additions in a number of nickel-base single-crystal alloys.

Atomic force microscopy imaging to measure precipitate volume fraction in nickel-based superalloys

In nickel-based superalloys, quantitative analysis of scanning electron microscopy images fails in providing accurate microstructural data, whereas more efficient techniques are very time-consuming. As an alternative approach, we propose to perform quantitative analysis of atomic force microscopy images of polished/etched surfaces (quantitative microprofilometry). This permits the measurement of microstructural parameters and the depth of etching, which is the main source of measurement bias. Thus, nonbiased estimations can be obtained by extrapolation of the measurements up to zero etching depth. In this article, we used this approach to estimate the volume fraction of γ′ precipitates in a nickel-based superalloy single crystal. Atomic force microscopy images of samples etched for different times show definition, homogeneity, and contrast high enough to perform image analysis. The result after extrapolation is in very good agreement with volume fraction values available from published reports.

A dislocation based criterion for the raft formation in nickel-based superalloys single crystals

A dislocation based criterion is proposed for describing the raft formation in single crystals of nickel-based superalloys subject to creep at high temperature. Those of the precipitate faces exhibiting dislocations after the onset of creep are thought to expand by directional coarsening of γ′ precipitates. The proposed criterion predicts correctly the raft morphology for creep along cristallographic directions of high and low symmetry for alloys exhibiting a positive or a negative misfit. A diffusional mechanism based on the effect of creep dislocations on the local γ phase chemical composition is suggested to account for the observed relationship between dislocation activity and directional coarsening of precipitates.

Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

The thermal-mechanical fatigue behavior of AM1 nickel-base superalloy single crystals is studied using a cycle from 600 °C to 1100 °C. It is found to be strongly dependent on crystallo-graphic orientation, which leads to different shapes of the stress-strain hysteresis loops. The cyclic stress-strain response is influenced by variation in Young’s modulus, flow stress, and cyclic hardening with temperature for every crystallographic orientation. The thermalmechanical fatigue life is mainly spent in crack growth. Two main crack-initiation mechanisms occur, depending on the mechanical strain range. Oxidation-induced cracking is the dominant damage mechanism in the lifetime of interest for turbine blades.

Vibration of Nickel-Base Single Crystal and Directionally Solidified Superalloy Plates

We investigate the free vibration mode for Nickel-base single crystal and directionally solidified (DS) superalloy cantilever plates using three-dimensional finite-element analyses. The elastic anisotropy had a significant effect on the eigenfrequency and eigenmode of plates. The rotation of the crystal axis around the [001] axis had a smaller effect at low frequencies but a significant effect at high frequencies. The deviation of the crystal axis in the plate had a significant effect on the eigenfrequency at low frequencies as well as at high frequencies. The bulk elastic modulus of DS plates derived by means of the Reuss average was effective in estimating the eigenfrequencies of DS plates. Eigenfrequencies of DS plates could also be predicted by averaging the eigenfrequencies of single crystals which comprise DS plates.

Directional coarsening in nickel-base single crystals with high volume fractions of coherent precipitates

Directional coarsening has been investigated experimentally in two nickel-base single crystal alloys with high volume fractions of precipitates. The initial elastic stresses due to the positive or negative misfit of the coherent precipitates biased the orientation of the precipitates developed during coarsening. However, directional coarsening of the precipitates did not occur until some limited amount of creep deformation was initiated. With the application of an external stress, dislocations penetrated preferentially into the most highly stressed matrix channels and directional coarsening occurred by coalescence of the γ′ precipitates in the plane of the initially less highly stressed channels, where the misfit stresses were not altered by the presence of misfit relieving dislocations. The diffusional processes responsible for rafting apparently operate on the local scale of the precipitates, with preferential local dissolution of the γ′ and diffusional flow of alloying elements around the periphery of the precipitates.