Directional coarsening of γ′ phase in single crystal nickel based superalloys during tensile creep

Abstract

The γ′ precipitate rafting kinetics and morphological evolution for two model single crystal superalloys have been studied. The microstructure of the alloys after different stages of tensile creep at 1040°C and under a range of stresses are examined using TEM and SEM. The chemical compositions of both γ and γ′ phases are analysed by energy dispersive spectrometry. Results show that a meshlike γ′ raft structure is formed along the direction normal to the stress axis during primary creep. The applied stress causes a decrease in the coherent strain energy at the γ′/γ interfaces in the (001) crystal plane. The released energy is the driving force for the diffusion of elements, leading to the formation of the γ′ rafts. A longer time is required for the formation of γ′rafts in alloy 2 owing to its higher content of the refractory element W which obstructs the migration of the other elements in the diffusion field of the γ′ rafts.     

Dissolution and precipitation kinetics of γ' in nickel base superalloy Udimet 520

Abstract

The influence of various solution andaging heat treatments on the dissolution and precipitation kinetics of γ' in wrought nickel base Udimet 520 superalloy was investigated, with the aim of determining optimum preforge and post-forge heat treatment schedules. Solution heat treatments at 1070, 1100, 1110, and 1120°C for 1, 2, and 4 h followed by water quenching were applied. Examination by SEM showed that in specimens solution treated at 1070°C the necklace type microstructure remained and partialagglomeration of γ' precipitates was observed even after 4 h holding. By contrast, after solutionising at 1100°C for 4 h, the γ' volume fraction decreased from 28 to 5% and when solution treated at 1140°C, abnormal grain growth was observed. The optimum solution treatment leading to an almost complete dissolution of γ' particles was determined to be 1120°C/4 h. Double aging treatments were carried out at 925°C/4 h/ AC (air cooled) followed by aging at 700 or 800°C for times ranging from 1 to 100 h. The results indicated that the precipitates have almost spherical shapes at the beginning and as the aging time is increased a partial change to a cuboidal shape is observed. This behaviour was more pronounced when aging at 800°C was employed. Microhardness measurements under both aging conditions showed that at 700°C hardness values increase about 60 HV when γ' particles grow 70 nm. On the other hand, in the samples aged at 800°C, hardness values increase with the particle size until a certain value and then drop. The precipitate growth process was followed by image analysis and the activation energy for γ' growth was evaluated to be 104 kJ mol ^-1. The results obtained are discussed in the framework of classic nucleation theory and compared with other workers' results.     

Effect of γ′ size on room temperature low cycle fatigue behaviour of a nickel base superalloy

Abstract

The results of a study on the effect of γ′ particle size on the room temperature (23°C) low cycle fatigue (LCF) behaviour of a Ni base superalloy, Nimonic 90, is reported. The γ′ particle sizes were estimated from transmission electron micrographs. Ranges of particle sizes corresponding to underaged, peak aged, and overaged conditions were identified by examining the age hardening response curve. The solutionised samples had longer LCF lives compared with the aged alloys. Coffin-Manson and cyclic stress–strain plots showed bilinearity at a plastic strain amplitude of around 0.4% in the solutionised, underaged, and peak aged conditions. The observed bilinearity could be attributed to a change in the deformation mode from single slip to multiple slip. The cyclic stress response showed relatively stable behaviour for the peak aged and the overaged specimens compared with the underaged and the solution treated specimens.     

Numerical simulation of γ/γ′ microstructural evolutions induced by TCP-phase in the MC2 nickel base single crystal superalloy

The influence of topologically close packed-phase (TCP-phase) precipitation on the γ/γ′ microstructure evolution of a single crystal superalloy has been studied. Four representative configurations of needle-shaped μ-phase particles observed in the MC2 alloy after creep deformation at 1050 °C/160 MPa have been selected.Finite element modeling was performed on the selected configurations using a crystal plasticity framework and a viscoplastic approach.The local calculated stress/strain fields in the vicinity of TCP-phase precipitates, especially stress concentration, allow an interpretation of the observed microstructural evolutions (γ′-precipitate orientation and thickness).Moreover, our numerical simulation brings a fruitful contribution to the understanding of the phenomenon often observed beyond needle-like μ-phase: the formation of a distorted γ/γ′ microstructure in lieu of the expected γ/γ′-rafted structure.     

Coarsening kinetics of γ' precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r³ vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r² vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.     

Analysis of γ1 shape changes in a single crystal Ni-Base superalloy

Analysis of γ¹ shape changes in a “commercial” nickel-base single crystal superalloy, NASAIR 100, was performed using a novel microstructural lattice model, MCFET. A negative mismatch of −0.3% and a modulos ratio (Eγ¹/Eγ) of 1.12 were used in the analysis. These values were based on actual high temperature measurements for NASAIR 100. Under a uniaxial stress, a regular array of γ¹ particles in the simulated microstructure was predicted to coalesce and form a plate morphology in which the broad faces of the plates and stress axis were perpendicular in tension but parallel in compression. These results were consistent with γ¹ shape changes observed in NASAIR 100 after creep testing at 1000C.     

Effect of cooling rate on the morphology of γ′ precipitates in a nickel–base superalloy under directional solidification

The morphological evolution of γ′ precipitates in a nickel-based superalloy K5 was studied by zone melting directional solidification under vacuum conditions. The results show that at the lower cooling rate of 12.42 K s^–1, γ′ precipitates remand big cuboids, γ′ particles become smaller at the cooling rate ranges from 12.42 to 38.80 K s^–1. For a rather fast cooling rate of 50.16 K s^–1, γ′ particles retain a spherical shape. The experiments show that big cuboids will become unstable and split into several small one at the lower cooling rate of 1.1 K s^–1. The mechanism of the evolution of the γ′ morphologies is also analyzed by introducing a new parameter-shape factor which classifie the total energy into several energy levels. Based on this, the effect of the cooling rate on the γ′ morphology is discussed.     

Influence of local chemical segregation on the γ′ directional coarsening behavior in single crystal superalloy CMSX-4

The influence of local chemical segregation on the directional coarsening behavior of γ′ phase has been investigated in a nickel-base single crystal superalloy during load-free thermal exposure. It is found that the γ′ phase was preferentially oriented with the direction perpendicular to the dendrite arm growth axis. The chemical gradients caused by local chemical heterogeneities were carefully measured on the scale of a dendrite. It showed that the directions of the γ′ rafting and the chemical gradients are consistent and the magnitude of the chemical gradients affects the rafting rates to some extent. It is suggested that the internal chemical gradients can orientate the development of the γ′ rafts by leading to anisotropy in their growth kinetics during Ostwald ripening. Furthermore, influence of the local chemical segregation on the mechanical properties is also discussed.     

Precipitation kinetics of γ′ phase in nickel base superalloy SC16: an in situ neutron diffraction study

Abstract

An in situ study of the kinetics of nucleation and growth of the γ′ phase in the single crystal Ni base superalloy SC16 was carried out by means of neutron diffraction. The investigation was aimed at a better understanding of the fundamental kinetic phenomena, in view of the optimisation of the heat treatments applied prior to the industrial use of the alloy. On heating above the solvus point (1525 K) the γ′ phase decomposes, and when cooling down back towards lower temperatures, it reappears. Measuring superlattice reflections allows the different stages of decomposition, nucleation, growth and concurrent coarsening to be followed. Results show that at temperatures T≤1490 K an increasing volume fraction of γ′ can be found with decreasing temperature, until saturation occurs at around 1100 K. Following classical kinetic theory, an Avrami like growth (time) law was used at each aging temperature and the isothermal reaction rates calculated. They show a maximum at around 1300 K, where the competing nucleation and growth processes find equilibrium. Nucleation and growth can be described with a modified Arrhenius law, analogous to that used for eutectoid reactions. The reaction rate first increases with the undercooling and then decreases. This implies a change in the character of the reaction, which was found to be initially surface and then bulk driven.     

Recrystallisation of single crystal superalloy CMSX–4

Abstract

A series of experiments investigating the recrystallisation of single crystal superalloy CMSX-4 have been carried out. Indentation atroom temperature has been used to study the effects of annealing time and temperature, and it has been found that a very strong dependence upon temperature is evident. Annealing above the γ′ solvus temperature results in very rapid growth of recrystallised grains whereas annealing below the γ′ solvus greatly suppresses the advancing grain boundaries. Additionally experiments have been carried out using an electrothermal mechanical test (ETMT) machine, to study the effects of degree of plastic strain and the temperature at which the strain is introduced. The strain threshold for recrystallisation under various annealing conditions has been determined and it has been found that recrystallisation occurs more readily if strain is introduced above 950°C. Finally, apparent activation energies for recrystallisation have been determined by measuring the change in resistivity that occurs during recrystallisation.     

Effect of very high temperature short exposures on the dissolution of the γ′ phase in single crystal MC2 superalloy

Time-temperature dependence of the γ′ phase volume fraction was investigated for a second generation single crystal nickel-based superalloy exposed to very short high temperature regimes (1,100–1,200 °C). In this temperature range, the dissolution of the strengthening γ′ phase occurs. Evolution of the γ′ volume fraction in transient regimes has been established for each temperature and activation energy of the dissolution phenomenon were determined. They directly attest from the activity of the diffusing species involved during this phenomenon. From these analyses, the volume fraction at equilibrium was established for the entire temperature range where dissolution occurs. A model, based on a time/temperature equivalence, is proposed to quantify the γ′ volume fraction dissolved during short high temperature exposure.     

Elemental partitioning,lattice misfit and creep behaviour of Cr containing γ′ strengthened Co base superalloys

Novel Cr containing Co-Al-W base superalloys were studied by atom probe tomography and neutron diffraction. Cr is found to predominantly partition to the γ matrix and decrease partitioning of W to γ′. Furthermore, Cr significantly enhances the γ′ volume fraction, decreases the γ/γ′ lattice misfit and deteriorates the creep resistance. Addition of Ni to the Cr containing alloys affects partitioning of W and Al, further decreases the lattice misfit and results in the formation of irregularly shaped precipitates. Al, W and Cr tend to occupy the ‘B'sublattice in the γ′-A₃B phase (L1₂ type), while Co and Ni reside in the ‘A' sublattice.     

Control of γ′ morphologyin nickel base superalloys through alloy design and densification processing under electric field

Abstract

The design and development of the future generation of nickel base superalloys for the demanding environments of rocket engines should be based on the metallurgical and structural characteristics of the γ′ phase. The distinctive features of this phase (e.g. strength, stability, size, shape, amount, distribution, uniformity, and order) can be controlled when a superalloy is appropriately designed and subsequently processed by an innovative solidification processing method. This concept has been successfully applied in the design and development of a superalloy. The resulting microstructure is almost perfect requiring no further treatment or processing.

MST/1954     

Long — term growth of superalloy γ′ particles

The microstructures of five commercially available nickel-based superalloys (NIM80A, NIM90, NIM105, IN738, IN939) have been studied after heat-treatments at 4 different temperatures and for times up to 15 000 h (170 samples). In all cases for moderate times and temperatures the mean dimension increased linearly with the cube root of time with an activation energy of 250 to 272 kJ mol^–1 K^–1. However, at high values of time and temperature some deviations from this behaviour were observed on two of the superalloys. These were accompanied by marked morphological changes thought to be due to re-solution effects. Extended analysis of the particle-size distributions suggests a correlation with the distribution functions predicted by the Lifschitz-Slyosov theory modified to take account of encounters between growing particles. The microstructural data so obtained have been used in failure diagnosis. Attempts have been made to explain the changes in shape with respect to long-term composition.     

Effect of post-bond heat treatment on the microstructure and high temperature mechanical property of a TLP bonded γ'-strengthened co-based single crystal superalloy

Post-bond heat treatment (PBHT) applied to a transient liquid phase (TLP) bonding joint is an effec-tive approach to remove the brittle borides and improve its properties.Herein,we proposed two types of PBHT strategies to obtain a TLP bonded γ'-strengthened Co-based single crystal superalloy,and the microstructural characteristics and tensile properties of the two heat treated joints were compared to identify the optimal PBHT strategy.The evolution of the brittle boride in the joint after the PBHT was studied by using in-situ microscopy.The experimental results allowed to provide a theoretic model to quantitatively evaluate the distribution of the brittle phase after the optimal PBHT and analyze the joint fractures to understand the failure mechanisms.The obtained results revealed that a post-bond solid solution treatment performed to the joint at a high temperature (over 1275 ℃) could decrease the area fraction of the boride from 7.2 ％ to 1.4 ％ and increase the elongation from 1.9 ％ to 7.8 ％.This work emphasizes the relevance of solid solution temperature when a PBHT strategy is applied.     

Coarsening of γ′-precipitates in single-crystal superalloy SRR 99

Abstract

The effects of isothermal annealing between 800 and 1200°C on the microstructure of single–crystal SRR 99 have been investigated. During aging at either 800 or 900°C, the precipitates adopt an irregular, rounded, and highly interconnected array which suggests that they have undergone coalescence. Aging at higher temperatures was found to produce a regular cuboidal precipitate morphology, although long–term aging promoted the formation of γ′-‘rafts’. The precipitate size distributions were found to be broader than those predicted by the Lifshitz-Slyosov and Wagner theory, and the coarsening kinetics also showed significant deviations from the power law prediction of this theory. The reasons for these deviations are discussed in the light of the available models, and it is suggested that a progressive transition in the dominant coarsening mechanism takes place.

MST/371