Coarsening of γ′ in Ni–Al alloys aged under uniaxial compression: I. Early-stage kinetics

The kinetics of coarsening of γ′ precipitates under applied compressive stress were investigated in monocrystalline Ni–Al alloys (nominal composition 13.36 at.% Al) aged at 640 °C. The specimens tested were doubly tapered cylinders or right circular cylinders. The maximum stress used was ~150 MPa; plastic deformation was less than 4% at the longest aging time (1021 h). The scatter in the data was large and is attributed to experimental factors discussed in the text. We find that compressive stress retards the kinetics of coarsening by 20–25% at 150 MPa. The particle size distributions become broader as the stress increases. We attribute the slower kinetics under compressive stress to its effect on the coefficient of diffusion of Al. A semiquantitative model in support of this idea is the subject of a companion paper.     

Matrix-Diffusion-Controlled Coarsening of the γ′ Phase in Waspaloy

Metals and Materials International - This paper discussed the coarsening behavior of the γ′ phase in Waspaloy during heat treatment at different aging times under the temperatures of...     

Trans-interface-diffusion-controlled coarsening of γ′ precipitates in ternary Ni–Al–Cr alloys

Published data on the coarsening behavior of γ′ precipitates in three ternary Ni–Al–Cr alloys are examined in light of the theory of trans-interface-diffusion-controlled (TIDC) coarsening, in which the kinetics is controlled by diffusion through the coherent precipitate–matrix interface. The experimental data are independent of the equilibrium γ′ volume fraction, as expected for TIDC coarsening. Kinetics of the type 〈r〉ⁿ ∝ t for the growth of precipitates of average radius 〈r〉, and X_∞Al and X_∞Cr ∝ t^–1/n for the variations of the far-field matrix solute concentrations, X_∞Al,Cr, with aging time, t, are characteristic of TIDC coarsening. The temporal exponent n ≈ 2.4 was obtained from the fitting of published particle size distributions. Based on correlation coefficients, the dependencies of 〈r〉ⁿ on t and X_∞Al,Cr on t^?1/n were comparable for n = 2.4 and n = 3 (the temporal exponent for matrix-diffusion-controlled coarsening). The dependencies of volume fraction, f, and number density, N_v, on t are also compared with theoretical predictions. Using a thermodynamic model of the Ni–Al–Cr phase diagram, the interfacial free energy, σ, was estimated from analysis of the data; σ varies from ～14.5 to 19 mJ m^–2 in the three alloys. Interfacial diffusion coefficients, also obtained from analysis of the data, are greater than those in the γ′ phase but smaller than those in the γ phase, which is consistent with the demands of the TIDC theory. Comparisons with the results from previously published work are noted and all discrepancies are discussed.     

Coarsening behavior of γ′ particles in a nickel-base superalloy

The coarsening behavior of γ′ particles in a nickel-base superalloy FGH95 was investigated by means of experimental observations and growth kinetics calculations. The results show that when aging at 1000, 1080 and 1140°C for different times, the relation of average particle size to time obeys the cube law (ā/2)³ = kt, where k is 15.49 × 10³, 77.5 × 10³ and 230.04 × 10³ nm³/min, respectively. The particle size distributions are better fit to the LSW theoretical distributions when aging at 1000°C within 1440 min. The activation energy for γ′ particles coarsening is determined to be 288.20±1.79 kJ/mol, which correlates well to the diffusion activation energies of Al, Ti, and Nb in the nickel matrix. This indicates that the coarsening of γ′ particles is controlled by the diffusion of Al, Ti, and Nb in the nickel matrix. The coarsening kinetics of γ′ particles in FGH95 is predicted as _t ³ = 1.04×10¹⁶ t exp[−(288200±1790)/RT]     

Coarsening of γ′ in Ni–Al alloys aged under uniaxial compression: II. Diffusion under stress and retardation of coarsening kinetics

An elasticity-based model is developed for predicting how the coefficient of diffusion, D, changes under an applied compressive stress. Stress affects the activation energy for motion, Q_M, of an atom, as well as its jump distance, ℓ. Since a cubic crystal is slightly distorted tetragonally under uniaxial stress, Q_M and ℓ, hence D, both become different in directions normal and parallel to the stress axis. Q_M is calculated using a dynamical model of diffusion due to Flynn [Point defects and diffusion. London: Oxford University Press; 1972]. Using the elastic constants for Ni–Al alloys, and taking into account the effect of the hydrostatic component of the applied stress on D, the model predicts that D decreases by about 6% under an applied stress of 150 MPa. This reduction does not account entirely for the retardation of coarsening kinetics observed experimentally, but given the simplicity of the model the result is satisfactory.     

Coarsening of γ′ in Ni-Al alloys aged under uniaxial compression: III. Characterization of the morphology

The morphological evolution of γ′ precipitates during coarsening at 640 °C under applied compressive stress in the range 0–150 MPa was investigated in a monocrystalline Ni-Al alloy containing nominally 13.36 at. % Al. The strain was primarily elastic. The microstructures were examined by transmission electron microscopy, primarily in the section (100) perpendicular to the applied stress. The aspect ratio of a γ′ precipitate, as well as a shape parameter which provides a measure of how cuboidal it is, were used to characterize the γ′ morphology. As the stress increases precipitates of a given size generally tend to become more non-equiaxed and their interfaces are more planar, though this depends on the γ′ volume fraction. The applied compressive stress also promotes the coalescence of γ′ precipitates. This tendency is more pronounced the higher the γ′ volume fraction and appears to be its main influence on directional coarsening during elastic deformation.     

Effect of Thermal Aging on Coarsening Kinetics of γ′ in Alloy 617

The effect of thermal aging on coarsening kinetics of alloy 617, a candidate material for heat exchanger of the very high temperature reactor, was experimentally studied at 750 and 950 °C for up to 5300 h. Formation of various precipitates such as μ-phase, M₂₃C₆ and γ′ phases and significant coarsening of the γ′ phase have been observed in the microstructure of the aged samples. Experimental observation was compared to alloy thermodynamic calculation and γ′-phase precipitation kinetics simulation. Thermal aging effect on the microstructural evolution and mechanical behavior of alloy 617 was then discussed based on experimental and microstructural modeling results.     

A technique for size determination of γ′ precipitates in nickel-base superalloys

A method is described for the preparation of extraction replicas from the cast superalloy IN-738, employing techniques which are generally applicable to nickel-base superalloys. Incomplete γ′ precipitates on the sample surfaces are removed prior to replication, thus obviating the need for geometrical correction to the mean measured precipitate size. the size distribution obtained from measurements of the small spheroidal γ′ precipitates (≤0.2 μm) is shown to correlate well with the form predicted by diffusional coarsening theory. Comparisons are made with other methods, including bright and dark field transmission electron microscopy of thin foils.     

Quantitative analysis of δ′ precipitation kinetics in Al–Li alloys

The Rietveld method has been applied to X-ray spectra in order to study the precipitated mass fraction of δ′ and δ in Al–Li alloys. The method allows us to obtain quantitatively the δ′ and δ precipitate mass fractions, and their evolution with aging time. Furthermore, this method also gives directly the cell parameter evolution of the matrix phase and indirectly the mean half radius of δ′ precipitates through an appropriate calibration curve. Experimentally, this calibration has been approached by previously studying the evolution of the mean half radius of δ′ by transmission electron microscopy (TEM). Thermoelectric power has also been shown to be a powerful technique to study the microstructural evolution of Al–Li alloys, being sensitive to the different stages of precipitation associated to the δ′ and δ phases. The comparison of the different experimental results allow us to stablish a clear difference between the precipitation kinetics and the hardening kinetics.     

Coarsening of Ni3Ge in binary Ni–Ge alloys: microstructures and volume fraction dependence of kinetics

Coarsening of Ni₃Ge precipitates in binary Ni–Ge alloys containing 12.15, 13.01 and 14.03 at.% Ge, aged at 724 °C (equilibrium volume fractions, f_e, equaling 0.022, 0.105 and 0.202, respectively) was investigated using transmission electron microscopy and magnetic analysis. The rate constants for the kinetics of particle growth and depletion of supersaturation depend anomalously on f_e, i.e. they both decrease slightly as f_e increases. Average values of the chemical diffusion coefficient and the Ni₃Ge/matrix interfacial free energy, derived from analysis of the data, are in reasonable agreement with previously reported values. Alignment of the Ni₃Ge precipitates parallel to cube directions is strong, but coalescence into plate shapes is never observed. Precipitates of the solid solution γ phase nucleated in large, coherent, concave-cuboidal Ni₃Ge precipitates. This behavior is expected, considering the phase boundary between the two-phase and Ni₃Ge regions in a recently published phase diagram.     

Degradation and recovery of the γ′-phase in heat-resistant nickel alloys

High-alloy nickel alloys are the principal structural material for parts of hot channels of gas turbine engines (GTE), which withstands high temperatures and loads and has a long service life. The high high-temperature strength is a result of complex alloying and phase composition that ensures the stability of the alloy structure in a field of high-temperatures and stresses. The -phase (Ni₃Al-base intermetallic), whose content is determined mainly by the content of Al and amounts to 60–65% in modern alloys, plays a special role in the strengthening process. The article presents results on the phase composition of some high-temperature nickel alloys after service. The degree of degradation of the -phase in a field of high temperatures and stresses and the degree of its recovery under the effect of regenerative heat treatment are investigated.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 29–32, April, 1995.     

Lattice misfit measurement in Inconel 706 containing coherent γ′ and γ″ precipitates

Lattice misfit is an important parameter in Ni–Fe based alloy IN706 as coherency strain influences the strength of the alloy and the stability of the microstructure. X-ray and synchrotron measurements were performed on bulk samples with precipitates in constrained lattice. Lattice parameters of different phases and the lattice misfit could be determined even though the peaks were completely overlapped.     

The incoherent γ/γ′ solvus in Ni-Al alloys

Analysis of the kinetics of solute (Al) depletion during the coarsening of γ′ (Ni ₃ Al)precipitates has been used in earlier investigations to provide values of the coherent solubility of the γ phase in binary Ni-Al alloys. As demonstrated in recent experimental investigations, the coherent solubility is a function of the total concentration of Al in the two-phase alloy. Using the model of Ardell and Maheshwari to analyze the data on coherent equilibrium, the authors estimated the incoherent equilibrium solubility of the γ′ phase over the temperature range 400 to 800 ° C. The calculated incoherent solubility limits are 3 to 5 % smaller than previously published measurements, the differences decreasing as the temperature increases. The authors argue that the calculations in this article provide the first estimates of the incoherent solvus of the γ′ phase in Ni-Al alloys.     

Comparative study of Oswald ripening and trans-interface diffusion-controlled theory models: Coarsening of γ′ precipitates affected by elastic strain along a concentration gradient

According to Lifshitz, Slyozov, and Wagner (LSW) and Trans-Interface Diffusion-Controlled (TICD) theoretical models, this paper reports the microstructure and its coarsening behavior of γ′ metastable-coherent precipitates in concentration gradient of Ni-13.75Ti (at%)/Ni generated by diffusion couple. The coarsening of precipitates was evaluated in two different Ti contents (R1-11.4Ti (at%) and R2-13Ti (at%)) generated along the concentration gradient and includes average size, size distributions and growth rate. The solvus and metastable-coherent bimodal lines as determined at 850 °C of 9.16 (at%) and 9.92Ti (at%) respectively by scanning electron microscopy. This paper suggests that elastic strains produced by the matrix/precipitate lattice mismatch caused significant deviations between the experimental results and those predicted by the LSW or TIDC theories. Activation energies for TIDC (Q _i ) and LSW (Q _r ) are Q _r : 219.69 and 172.61 kJ mol^-1 for R1 and R2 regions, respectively, and Q _i : 218.46 and 164.56 kJmol^-1 for R1 and R2 regions, respectively. A concentration gradient allows the study of various alloys with different concentration and volume-fraction in a single sample.     

Co-strengthening contribution of δ′and T_1 precipitates in Al-Li alloys 2090 and 2090+Ce

1. Introduction The spherical δ′(Al3Li) and plate-shaped T1(Al2CuLi) are dominant precipitation strengthen-ing phases in Al-Li alloy 2090. The main mecha-nism of δ′ phase contribution is order strengthening, but Miura et al. suggested that the interaction be-tween dislocation and δ′ particles depends on the chemical composition and aging condition [1-2]. Howe et al. have clearly observed T1-plates can be sheared by dislocations using HRTEM, so chemical strengthening and Orowan mech…