Automated identification and characterisation of secondary and tertiary γ′ precipitates in nickel-based superalloys

Abstract

The use of different electron loss edges in energy-filtered transmission electron microscopy (EFTEM) has allowed researchers to capture images of the morphology and size of precipitates in nickel-based superalloys. In this work, the authors discuss a computational methodology for automated detection of secondary and tertiary γ′ precipitates in EFTEM images. The optimum parameters for the automated region growing technique were identified using a combination of visual inspection and intensity information from the EFTEM images. The microstructural statistics obtained from the segmented γ′ precipitates agreed with those of the manually segmented precipitates. Then, automated segmented precipitates are used to extract microstructural information about the distributions of equivalent diameters of 656 tertiary precipitates along with the distances to the nearest secondary precipitates. The significance of this technique is its ability to automate segmentation of precipitates in a reproducible manner for acquiring microstructural statistics that relate to both processing and properties.     

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.     

Effect of stress on μ phase formation in single crystal superalloys

Precipitation behavior of the μ phase under applied stress was systematically investigated in two single crystal superalloys CMSX-4 and R0. The investigation has shown that both the sign and magnitude of the applied stress can affect the precipitation behavior of the μ phase with a certain degree of regularity. At first, the precipitation of the μ phase varied significantly when a small amount of stress was applied, regardless of whether the specimen is under tension or under compression. Under most conditions however, precipitation of the μ phase was restrained by this low application of stress. After increasing the applied stress, precipitation of the μ phase increased relative to the amount of tension or decreased under compression.     

Evolution and analysis of γ′ rafting during creep of single crystal nickel base superalloy

Abstract

By means of TEM observation and finite element analysis, an investigation has been made into the directional coarsening of the γ′ phase for a single crystal nickel base superalloy with [001] orientation during creep at 1040°C. The results show that the strain energy change related to the elastic strain is to be the driving force for γ′ rafting. The extruded strain of the lattice in the cuboidal γ′ interfaces results in a supersaturation of the elements Ta and Al of larger atomic radius. The extrusion or expansion strain in the lattice of the cuboidal γ′ planes may repel or trap these atoms to promote the directional growth of the γ′ phase into a needle-like raft structure along the direction parallel to the stress axis under an applied compression stress, or into a meshlike raft structure along the direction perpendicular to stress axis under applied tensile stress. The normal direction of the expanding lattice is supposed to be the one in which the γ′ rafts grow. The rate of γ′ rafting is enhanced by increasing viscoplastic flow in the γ matrix and elastic strain in the γ′ phase. Therefore, there is a smaller rate of growth under compressive than under tensile stress as a result of the smaller expansion strain and viscoplastic flow occurring in the former.     

Review Progress in Ostwald ripening theories and their applications to the γ′-precipitates in nickel-base superalloys Part II Nickel-base superalloys

The Lifshitz-Slyozof-Wagner (LSW) theory, which corresponds to a zero volume fraction approximation, was developed to model kinetics of precipitate growth from supersaturated solid solutions. The subsequent modifications of the LSW theory for the incorporations of various factors including volume fractions of precipitates to fit the experimental data from the coarsening precipitates were made by various workers during last twenty five years. The LSW theory and its modifications have been applied on the diffusion-controlled Ostwald ripening of the precipitate particles [Ni₃(Al,Ti)] in nickel-based superalloys. The important Ostwald ripening theories were reviewed in the part I of this paper, and the coarsening characteristics of the -precipitates in Ni-base high temperature superalloys are presented in detail in relation to these theories in the present part. A model developed by D. McLean can be used to predict the particle growth over service lifetimes in the case of relatively Al-rich nickel-base superalloys. Additional fundamental data (such as the precipitate-matrix interfacial energy, diffusivity of the component species of the particle, and the equilibrium solubility with a particle in nickel-based superalloys) can be obtained from experimental results for coarsening, if the concentration changes during coarsening can be measured precisely, using the methods developed by A. J. Ardell. Furthermore, the factors affecting the shape changes and splitting of the precipitate particles during the coarsening were also considered seperately since the classical Ostwald ripening theories can not explain the morphological changes.     

Phase field simulation of morphology evolution and coarsening of γ′ intermetallic phase in Ni–Al alloy

Abstract

The morphology evolution and coarsening behaviour of γ′ (Ni₃Al) phase in Ni–Al alloys were investigated by using the Cahn–Hilliard and Ginzberg–Laudan phase field equations, and the composition, order parameter and average particles size evolution coupling with the internal elastic strain were studied. Cubic γ′ phase was presented firstly and then the rectangle particles were formed by means of coarsening; the mechanisms of γ′ phase coarsening are Ostwald ripening or coalescence of neighbouring particles. The average particle size of γ′ phase increases rapidly at the initial stage, and it slows down at the coarsening dominated stage. The average particle size follows a cubic law of ‘Lifshitz–Slyozov’ encounter modified theory in the coarsening regime.     

Stereological characterization of γ′ phase precipitation in CMSX-6 monocrystalline nickel-base superalloy

The purpose of this investigation was to study in detail the means to quantitatively evaluate γ′ phase precipitation. Many of the mechanical properties of superalloys are directly influenced by the presence of the γ′ (gamma prime) precipitate phase dispersed in a γ matrix phase. The γ′ precipitates act as effective barriers to dislocation motion and restrict plastic deformation, particularly at high temperatures. Due to this, it is essential to accurately quantify the γ′ precipitate size, volume fraction and distribution. Investigations based on quantitative metallography and image analysis were performed on a monocrystalline nickel-base superalloy taking into consideration various γ′ precipitate sizes present in that alloy microstructure. The authors of the present paper propose a new method of quantifying the total volume fraction of the γ′ phase applying images of the microstructure with γ′ phase precipitates registered using light microscopy, scanning electron microscopy (at two different magnifications) and scanning transmission electron microscopy.     

Enlightening from γ, γ′ and β phase transformations in Al-Co-Ni alloy system: A review

Alloys based on the ternary Al-Co-Ni system have found their broad applications in high-temperature structural materials as well as functional materials due to the existence of three major phases-γ, γ′, β and the phase transformations between them. The knowledge of phase equilibria and fundamental understanding of phase transition behaviors are of great importance to the research and design of industrial Al-Co-Ni alloys. This paper provides a comprehensive review and discussion of recent progresses in Al-Co-Ni system including the phase equilibria investigations, catalytic properties, shape memory effect and mechanical properties of Al-Co-Ni alloys consisting of different intermetallic phases. The differences between experimental determinations of the phase equilibria and calculated results using CALPHAD method and first-principles based thermodynamic model are evaluated, where the existence of Nishizawa horn is also discussed. Finally, prospective research topics in Al-Co-Ni alloys regarding further experimental and theoretical studies are proposed. The review provides general references and suggestions for developing novel Ni-based alloys with desired microstructure and performance through phase transformations.     

Molecular Dynamics Simulation of Tensile Deformation and Fracture of γ-TiAl with and without Surface Defects

Molecular dynamics simulation of uniaxial tension along [001] has been performed to study the influence of various surface defects on the initiation of plastic deformation and fracture of γ-TiAl single crystals.The results indicate that brittle fracture occurs in perfect bulk; surfaces and edges will be detrimental to the strength of materials and provide dislocation nucleation site. The defects on surfaces and edges cause further weakening with various effects depending on defect type, size, position and orientation,while the edge dimples are the most influential. For γ-TiAl rods with surface dimples, dislocations nucleate from an edge of the rod when dimples are small, dimple dislocation nucleation occurs only when the dimples are larger than a strain rate dependent critical size. The dislocations nucleated upon [001]tension are super dislocations with Burger vectors 011] or 1/2 112] containing four 1/6 112 partials. The effects of surface scratches are orientation and shape sensitive. Scratches parallel to the loading direction have little influence, while sharp ones perpendicular to the loading direction may cause crack and thus should be avoided. This simulation also shows that, any type of surface defect would lower strength,and cause crack in some cases. But some may facilitate dislocation nucleation and improve ductility of TiAl if well controlled.     

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     

Coarsening kinetics of γ′ precipitates in Ni-Al and Ni-Al-Mo alloys

The ordered L1₂ precipitate coarsening kinetics without the influence of an external stress were studied in a Ni-Al(13.5 at%) alloy at 1413 K and a Ni-Al(13.8 at%)-Mo(5.9 at%) alloy at 1443 K. The Ni-Al-Mo alloy has a lattice mismatch of about –0.3% at the ageing temperature while the Ni-Al has a positive lattice mismatch of about 0.25% at the ageing temperature. For both alloys, the precipitates were initially cuboidal. After ageing for 3–10 min, the precipitates in the Ni-Al-Mo alloy split mostly into two parallel plates (doublets) or eight sub-cubes (octets), but the initial cuboidal precipitates in the Ni-Al alloy only showed the tendency to split into doublets. After further ageing, the precipitates in both alloys eventually aligned and agglomerated into groups consisting of many particles separated by a small distance of 30 nm, and the distribution of the precipitates became inhomogeneous. There was no linear relationship between the cube of the average precipitate size and the ageing time as predicted by the classical Lifshitz-Slyozov-Wagner theory. Instead, a retardation of the coarsening process is found.     

A correlative multidimensional study of γ'precipitates with Ta addition in Re-containing Ni-based single crystal superalloys

The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tan-talum(Ta)content(2Ta,5Ta and 8Ta in wt％)was investigated.Ta addition significantly affected the γ'precipitate morphology,γ/γ'lattice misfit and microstructural stability during long-term aging.Results showed that the partitioning behaviors of solutes were enhanced by Ta addition,meanwhile,the reversal partitioning behavior of W was triggered which partitioned from γ'precipitate to y matrix.The elemental concentration redistribution caused variations in lattice misfit from positive to negative,the values of lat-tice misfit were measured to be 0.16％for 2Ta alloy,then decreased to-0.07％for 5Ta alloy and negatively increased to-0.23％for 8Ta alloy.These variations in the lattice misfit were reflected on the transition of γ'morphology from round-cornered cuboidal shape to cuboidal with sharp corners,accomplished with increasing shape parameter ratio η.Consequently,the optimal γ'shape could be obtained at lattice misfit of approximately 0.3％.The γ'coarsening investigation at 900℃(up to 2000 h)indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy,accompanied by the enhanced capability of resisting γ'coalescence.By incorporating the calculated interfacial energy,computational modeling,Thermo-Calc and PrecipiCalc,were employed to elucidate the γ'kinetic pathways,the simulation results agreed with experiments,indicating that the model and parameters were reasonable.Additionally,it was found that there was no overlap between γ'nucleation and coarsening when the γ/γ'interfacial energy increased to a critical value.     

The molecular weight distribution of polymerized iso-butylsilane and γ-methacryloxypropylsilane in aqueous solutions

The condensation of iso-butyltrimethoxysilane and -methacryloxypropyltrimethoxysilane (-MPS) in aqueous solutions has been studied. The silanes were hydrolysed in acidic solutions followed by adjustment of the pH of the solutions to allow condensation at different pH conditions. The molecular weights of the polymer mixtures obtained were determined by using gel permeation chromatography (GPC) with a light scattering detector. The GPC results suggest that the polybutylsilsesquioxanes differ in molecular weight distribution to the polymethacryloxypropylsilsesquioxanes. In general, polymers with the highest molecular weight were obtained in mild alkaline conditions. Strongly acidic as well as strongly alkaline conditions gave low molecular weight oligomers. The effect of fluoride ions on polymer molecular weight was also investigated. The polymers from the hydrolysis and condensation of iso-butylsilane and -MPS were monodisperse when the condensation was catalysed by fluoride ion. The molecular weights of the polymers were lower than those from the analogous base-catalysed reactions and the molecular weights increased with an increase in fluoride concentration.     

Crushed sand in concrete – Effect of particle shape in different fractions and filler properties on rheology

Studies of sand particle characteristics (shape, voids content, filler particle size distribution) and rheology of concrete (slump-flow, yield stress and plastic viscosity) show that the 0.125/2 mm particle shape and ≤0.125 mm filler properties are the most important factors for concrete workability when the sand grading 0/8 mm is kept constant. By normalizing the maximum variation of rheology (slump-flow value in mm) obtained in mixes where different size fractions are exchanged, the fraction ≤ 0.125 mm was found to have around 6–8 times larger effect on rheology per unit volume % exchanged, compared to the coarser sand fractions: ≤0.125 mm = 35.2 mm/%; 0.125/2 mm = 4.9 mm/%; 2/5 mm = 6.0 mm/% and 5/8 mm = 3.8 mm/%.     

Phase boundaries of single-phase ɛ- and γ′-fields in Fe–C–N ternary phase diagram

Abstract

The activity of N in α-Fe is derived from the solubility at fairly low temperature (580°C) in the Fe–N diagram. The interaction coefficients between Fe and N, Fe and C, and C and N are obtained and the phase boundaries of single-phase ?- and γ′-fields are subsequently calculated. The calculated results show that the γ′ -region of the Naumann and Langenscheid Fe–C–N ternary phase diagram is correct, but the ?-region requires revision. The calculations suggest that recent experimental data on the ?-phase appear reasonable. Calculations taking graphite to be a standard state confirm this conclusion.

MST/385     

Activation energy for growth in single size distribution and the dissolution features of γ′ precipitates in the superalloy IN738LC

Activation energy for the growth of γ′ precipitates in single size unimodal distribution has been determined by annealing the solution-treated and quenched alloy with fine 70 nm size cooling precipitates at 1,040°, 1,080°, and 1,100 °C for different periods of time up to 100 h. Results obtained using the LSW matrix diffusion model concur with the deductions of earlier work that the activation energy for growth of precipitates in single size distribution is not a constant, but increases with increasing size of precipitates. Also, the activation energy plotted against the corresponding precipitate sizes yields a straight line with a positive slope. During long-time annealing, precipitate particles line up to reduce interactive free energy and grow to fairly coarse sizes along the matrix grain boundaries. Some of the particles in isolated islands or those lined along the matrix grain boundaries attain critical maximum sizes and begin to dissolve into the matrix in four different ways, designated as Modes 1 through 4. The various modes of dissolution are described and the possibility of repeated growth and dissolution in cycles is envisaged.