Alan J. Ardell

Microstructures of the commercial 7075 Al alloy in the T651 and T7 tempers

The microstructures of the commercial 7075 Al alloy in the peak aged (T651) and overaged (T7) tempers were studied using transmission electron microscopy. The microstructure of 7075-T651 is characterized predominantly by the presence of a fine dispersion of the ή transition phase; the primarily plate-shaped ή particles have diameters ranging from 3 to 10 nm. The microstructure of 7075-T7 mainly consists of a bimodal size distribution of ή phase variants,i.e., plate-shaped ή1 and ή2 particles ranging from 12.5 to 30 nm in diameter and finer ή1 particles 5 to 10 nm in diameter. The ή transition phase is identified as having a hexagonal crystal structure with lattice parameters, α = 0.496 nm andc = 1.403 nm. The maximum hardness of the 7075 alloy is believed to arise mainly from the fine dispersion of small ή particles.

Precipitation of Al3Sc in binary Al–Sc alloys

Abstract The precipitation of coherent Al 3 Sc particles in Al–Sc alloys containing 0.06, 0.12 and 0.18 at.% Sc was investigated. The alloys were aged at 350°C for times up to 4663 h and the kinetics of particle growth, the particle size distributions and the evolution of particle morphology were measured and evaluated using transmission electron microscopy. Al 3 Sc precipitates did not nucleate homogeneously in the most dilute alloy; this result was unexpected because 0.06 at.% Sc exceeds the solubility limit at 350°C. Persistent dislocation networks were observed in the alloy containing 0.12 at.% Sc under normal solution treatment conditions (e.g. 1 h at 600°C) and the dislocations acted as heterogeneous nucleation sites. The dislocations were ultimately eliminated using a very long solution treatment time of ∼70 h near the melting temperature. Aging of both of the more concentrated alloys produced coherent precipitates. At short aging times the particles in the alloy containing 0.12% Sc were cauliflower-shaped and became spherical at longer times. At 4663 h some of the precipitates in this alloy were cuboidal, while others appear to have become semicoherent. The precipitates in this alloy were highly resistant to coarsening, and their size distributions were for the most part narrower than that predicted by the classical theory of Lifshitz, Slezov and Wagner (the LSW theory). The shapes of the precipitates in the alloy containing 0.18% Sc evolved from spherical to cuboidal with increasing aging time. The kinetics of growth of the precipitates in this alloy were consistent with the predictions of the LSW theory, the average size, 〈 r 〉, increasing with aging time, t , according to an equation of the type 〈 r 〉 3 ≃ kt . The experimentally measured rate constant, k , was in very good agreement with that calculated theoretically for this alloy.

Lattice misfits in four binary Ni-Base γ/γ1 alloys at ambient and elevated temperatures

High-temperature X-ray diffractometry was used to determine thein situlattice parameters,aγ andaγ′, and lattice misfits, δ = (aγ′, -aγ)/aγ, of the matrix (γ) and dispersed γ′-type (Ni3X) phases in polycrystalline binary Ni-Al, Ni-Ga, Ni-Ge, and Ni-Si alloys as functions of temperature, up to about 680 °C. Concentrated alloys containing large volume fractions of theγ′ phase (∼0.40 to 0.50) were aged at 700 °C to produce large, elastically unconstrained precipitates. The room-temperature misfits are 0.00474 (Ni-Al), 0.01005 (Ni-Ga), 0.00626 (Ni-Ge), and -0.00226 (Ni-Si), with an estimated error of ± 4 pct. The absolute values of the lattice constants of theγ andγ′ phases, at compositions corresponding to thermodynamic equilibrium at about 700 °C, are in excellent agreement with data from the literature, with the exception of Ni3Ga, the lattice constant of which is much larger than expected. In Ni-Ge alloys, δ decreases to 0.00612 at 679 °C, and in Ni-Ga alloys, the decrease is to 0.0097. In Ni-Si and Ni-Al alloys, δ exhibits a stronger temperature dependence, changing to-0.00285 at 683 °C (Ni-Si) and to 0.00424 at 680 °C (Ni-Al). Since the times required to complete the high-temperature X-ray diffraction (XRD) scans were relatively short (2.5 hours at most), we believe that the changes in δ observed are attributable to differences between the thermal expansion coefficients of theγ andγ′ phases, because the compositions of the phases in question reflect the equilibrium compositions at 700 δC. Empirical equations are presented that accurately describe the temperature dependences ofaγ,aγ′, and δ over the range of temperatures of this investigation.

Interfacial free energies and solute diffusivities from data on Ostwald ripening

Using recent theoretical modifications of the kinetic constants characterizing Ostwald ripening, it is demonstrated that accurate values of the interfacial free energy, λ, and solute diffusivities, D, can be obtained from experimental data when the kinetics of particle growth are measured in conjunction with independent measurements of either the decrease of the matrix supersaturation or the increase in volume fraction with aging time. The accuracy of λ is limited only by the assumption that the matrix phase is an ideal solid solution, and is effectively independent of the influence of equilibrium volume fraction, Φe, on the kinetics of coarsening. Analyses of the available data on the coarsening of γ′-type (Ni3X) precipitates in binary Ni−Al, Ni−Si and Ni−Ti alloys yield values of λ=6.9±0.3, 10.2±3.0 and 13.0 mJ/m2, respectively, assuming ideal solution thermodynamics; a more realistic thermodynamic model for the Ni−Al solid solution raises the value of λ in Ni−Al alloys to 8.1±0.2 mJ/m2. Proportional increases probably obtain in the other two alloys. The accuracy with which D can be evaluated from comparable data depends theoretically on Φe. However, analyses of the same data yield values of D in very good agreement with the results of conventional diffusion experiments. This is consistent with the absence of an effect of Φe on the kinetics of Ostwald ripening in these alloys over the ranges of Φe investigated.

Precipitation hardening of Ni-Al alloys containing large volume fractions of γ′

Precipitation hardening of oriented Ni-Al single crystal specimens, mostly containing from 7.80 to 8.12 wt pct Al, was studied by compression testing of samples aged at 600 to 625‡C for times ranging from l/4 to 1505 h. Values of the increment in the CRSS, δΤ, over that of the matrix solid solution were determined in samples tested at 77, 183, 297 and 373 K. The volume fraction,f, of γ′ (Ni3Al) precipitates in these samples was in the vicinity of 0.35. As demonstrated by analysis of the data, the conventional theory of order-strengthening, applicable to alloys in whichf is small, cannot predict the magnitude of the measured values of δΤ. A modified theory, which is valid for the larger values off encountered in this investigation, is presented. It is demonstrated that the modified theory agrees quantitatively (to within 5 pct) with the measured values of δΤ for the underaged samples. The agreement between theory and experiment for the samples aged for longer times is not quite as good, but is nevertheless not too bad (to within about 15 pct). Some reasons for this are discussed. The dislocation structures observed by transmission electron microscopy done on slip-plane sections taken from a limited number of samples are discussed in light of theoretical expectation. They travel as highly parallel pairs, as demanded by theory, but with spacings about two times larger than expected.

Correlation between microstructure and calorimetric behavior of aluminum alloy 7075 and AlZnMg alloys in various tempers

Abstract The results of detailed microstructural analyses of the commercial aluminum alloy 7075 in several tempers, combined with consideration of similar data on pure AlZnMg alloys, are shown to provide a sound basis for the interpretation of various calorimetric studies reported in the literature. Particular emphasis is placed on the analysis of differential scanning calorimetry (DSC) data on the peak-aged microstructure of both types of alloy, and of the effect of retrogression and reaging (RRA) treatments on the aluminum alloy 7075. It is demonstrated that all the salient features of the DSC curves can be rationalized with the precipitate microstructures produced by the various heat treatments, specifically η′ precipitates in the peak-aged alloysand mixtures of η′ and η precipitates in the slightly overaged and RRA alloys.

Dislocation images in quartz and the determination of Burgers vectors

Abstract It is demonstrated that the Burgers vectors of dislocations in quartz can be positively identified from the results of conventional diffraction contrast experiments in the electron microscope. Dislocations having an a Burgers vector were found in experimentally-deformed natural and synthetic crystals, and dislocations with a c Burgers vector were also observed in a synthetic crystal. The factors that complicate the successful execution of diffraction contrast experiments on quartz are discussed.

Elastic interactions and their effect on γ' precipitate shapes in aged dilute Ni-Al alloys

The shape of coherent gamma prime precipitates in aged Ni-Al alloys was investigated experimentally using transmission electron microscopy. It is found that the shape of the gamma prime precipitates depends not only on their size but also on their volume fraction. The higher the volume fraction of gamma prime precipitates, the greater is the tendency for their elastic strain fields to overlap and to become cuboidal in shape, with the individual precipitates acquiring convex interfaces. In the case of small volume fractions, the gamma prime precipitates become nearly perfectly cubic as they grow to a certain size. 13 refs.

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, fe, 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 fe, i.e. they both decrease slightly as fe increases. Average values of the chemical diffusion coefficient and the Ni 3Ge/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.  2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Scaling characteristics of dislocation link length distributions generated during the creep of crystals

Abstract The distribution of dislocation link lengths comprising the dislocation network generated during the elevated temperature deformation of NaCl and Al monocrystals has been evaluated at several values of the applied stress, σ, and temperature, T, and at various stages of primary creep up to the steady state condition. The properties of the normalized function, Φ(u) = β2ρ−2φ(L, t), where φ(L, t) dL is the number of dislocation links per unit volume of length between L and L + dL at time t, ρ is the dislocation density, u = L 〈L〉 , 〈L〉 is the average link length and β is a constant defined by the relationship 〈L〉 = βρ − 1 2 , are analyzed. Within experimental error Φ(u) is independent of t during the primary creep of NaCl. The same appears to be true for Al, although the greater scatter render this conclusion less convincing. For these two materials Φ(u) has the attributes of a dynamic scaling function. It is tentatively concluded that Φ(u) is at most weakly dependent on σ and T and depends primarily on crystal structure. Specifically, Φ(u) ∝ um as u → 0, with m ~ 1.5 for NaCl and MgO and m ~ 1.33 for Al. Since the data on Al were taken at stresses spanning the Harper-Dorn and power-law creep regimes the results suggest that m is independent of the mode of creep, although it may increase with increasing σ for NaCl. Evidence is presented that β is dependent on the mode of creep. It is smaller (0.75–0.8) for Al deformed in the H-D regime than for materials, including Al, deformed in the power-law regime, where the value of β typically exceeds 0.85 and may also increase with increasing σ. The limiting behavior of Φ(u) is related to the nature of the equation governing the rate of growth g = dL dt , of individual links. In particular, g(L, t) must approach L−m as L → 0. The implications of these observations are discussed.