Microstructural observations on the oxidation of γ′-Ni3Al at low oxygen partial pressure

Microstructural development during the oxidation of (001)-oriented γ′-Ni₃Al single crystals at 1223 K under an oxygen partial pressure of 4 × 10⁻¹⁹ atm has been studied by electron microscopy. After 1 min oxidation, TEM cross sections revealed a continuous 4 nm thick film of γ-Al₂-O₃ with equiaxed 20 nm protrusions into the metal. Pre-thinned foils oxidized for 6 min and studied in plan view in the TEM showed that the scale consisted of 20 nm γ-Al₂O₃ grains oriented to the metal such that (111) γ-Al₂O₃ (001) γ′-Ni₃Al. Continued oxidation resulted in thickening of the γ-Al₂O₃ scale, no grain growth, and the development of a plane metal/γ-Al₂O₃ interface. Depletion of Al from the adjoining metal resulted in a well-defined disordered zone of NiAl solid solution between the Al₂O₃ scale and the γ′-Ni₃Al. After 5 h oxidation large, randomly oriented α-Al₂O₃ grains nucleated at the metal/γ-Al₂O₃ interface, growing inward and transforming from the γ-Al₂O₃ outward. The α-Al₂O₃ contained intragranular and intergranular voids. The γ-Al₂O₃ exhibited a high density of planr defects and the interface between the γ-Al₂O₃ and α-Al₂O₃ contained many voids. Voids at the metal/oxide interface were never observed.     

Molecular orbital approach to the chemical bonding at grain boundary in γ′-Ni3Al

The nature of chemical bonding at grain boundary in γ′-Ni₃Al has been analyzed by extended Hueckel molecular orbital method in order to investigate the intrinsic brittleness of grain boundary and the effect of doping element for enhancing ductility of polycrystalline γ′-Ni₃ Al. For doping elements, B, C, P and S positioned at the grain boundary have been examined and following results are concluded. (1) Brittleness of γ′-Ni₃Al grain boundary attributes to the coulombic repulsive force between same atoms that arrange adjacently at the grain boundary. In particular, Al atom repels strongly against each other. (2) Boron can bind with both Ni and Al without decreasing bonding energy between constituent atoms, which would lead to improve the strength of grain boundary.     

On the habit plane of γ″-Ni3V precipitates

The crystalloaraphic habit of coherent γ″-Ni₃V precipitates that have a bet DO₂₂ structure has been investigated. Theoretical calculation based on the elastic energy theory predicts an unusual habit to be close to (102). This calculation is in good agreement with the results of transmission electron microscopy. Four variants of precipitate habit that correspond to a tetragonal direction coexist, leading to an absence of streaking in the diffraction pattern.     

The Site Preferences of Transition Elements and Their Synergistic Effects on the Bonding Strengthening and Structural Stability of γ′-Ni3Al Precipitates in Ni-Based Superalloys: A First-Principles Investigation

Advanced mechanical properties of Ni-based superalloys strongly depend on the site preferences of alloying X elements in γ′-Ni₃Al-X precipitates, which are associated with the partial bonding characteristics between Ni, Al, and X atoms. Therefore, in the current work, the site occupancy tendencies of transition X metals were revealed via first-principles ab initio calculations at 0 K. Bonding features of Ni-Al, Ni-X, and Al-X pairs were simulated by using the charge density difference (CDD), electron localization function (ELF), and density of states (DOS) methods, respectively. According to simulations, higher atomic size X elements preferably occupy Al sites of γ′-Ni₃Al-X intermetallics and lead to strong covalent-like directional bondings between themselves and their nearest neighbor (NN) Ni atoms along 〈110〉 directions. However, if these larger X metals substituted for Ni sites, the bonding properties would differ by plane due to the nature of the L1₂-type crystal structure of γ′-Ni₃Al-X precipitates. Considering all transition elements, refractory metals (i.e., X = Re, W, Mo, Ta, or Nb) appear as the most effective strength inducers, improving the structural stability of γ′ phase, even if Ni site substitution of X = Re atoms would start to increase structural instability. On the other hand, relatively small alloying X elements having electron configuration similarities with Ni (i.e., X = Co, Cu, Rh, Pd, Ag, Ir, Pt, or Au) are more likely to worsen bonding strengthening. Instead, these transition X metals creating metallic bondings with NN Ni atoms would contribute to ductility and malleability of Ni-based superalloys. Furthermore, depending on the relative atomic size of γ′-former and refractory elements, the phase and site preferences of refractory atoms would alter in multicomponent systems. As a result of the attractive or weak repulsive forces between Re-Re, Re-Mo, and Re-W pairs, the structural stability of the constituent phases would deteriorate and harmful topologically close-packed (TCP) phases would precipitate.

     

On the Precipitation-Strengthening Contribution of the Ta-Containing Co3(Al,W)-Phase to the Creep Properties of γ/γ′ Cobalt-Base Superalloys

Metallurgical and Materials Transactions A - The creep strength of single-crystalline Co-based superalloys was found to be comparable to first-generation Ni-base superalloys. However, considerable...     

Ion-bombardment-induced disordering of γ′Ni3Si

Thin films of γ′Ni₃Si were formed on the surface of Ni-12.7at%Si discs by Ni¹-ion bombardment at ~575°C. The disordering of these films at temperatures below ~400 C was monitored using γ′-dark-field TEM, electron diffraction and infrared emissivitity changes occurring as a result of further Ni¹-ion bombardment.The degree of long range order. S. did not change during subsequent bombardment at temperatures ⩾440C. At temperatures ≤300C. S decreased from its initial value close to 1.0 to essentially zero. The mechanism for this change involved the local formation of disordered zones surrounding uncollapsed collision cascades. The average zone size was ~ 100 Å at 80°C and decreased with temperature to ~ 50 Å at 250°C. The zone production rate was~2 × 10⁻⁸ zones A³s and did not vary greatly with temperature.Disordered zones were not observed in samples bombarded at temperatures ⩾300°C. but “steady” “state” S values between 0 and 1 were noted. Disorder in these samples was uniformly distributed rather than locally deposited. Dark features visible in γ′-dark field images of these samples are shown to result only from the strain contrast of faulted, interstitial dislocation loops.     

Temperature Dependence of the γ/γ′ Interfacial Energy in Binary Ni–Al Alloys

Published data on the coarsening kinetics of γ′ (Ni₃Al) precipitates in binary Ni–Al alloys aged at 12 temperatures ranging from 773 K to 1073 K are analyzed to provide a comprehensive evaluation of the temperature dependence of the γ/γ′ interfacial free energy, σ. The data are analyzed using equations of the trans-interface-diffusion-controlled (TIDC) theory of coarsening, with temporal exponent n = 2.4. The results show that σ decreases with increasing temperature, T. A linear empirical equation is fitted to the data on σ vs T; it extrapolates to σ = 0 in the liquid region of the Ni–Al phase diagram, as it should do. A quantitative temperature-dependent transition radius, r_trans, is calculated; it depends on the product of the interface width and the ratio of the chemical diffusion coefficients in the γ phase and interface regions. Applying the TIDC coarsening equations to calculate σ is justified when the average radius, 〈r〉, satisfies the condition 〈r〉 < r_trans, which is valid for all the data used in the fit. The data on σ vs T are compared with theoretical predictions. The results are discussed in the context of previous work, as well as with values of σ obtained through analyses using the equations of traditional LSW coarsening kinetics, n = 3.

     

A study of the sintering of spherical silver powder—II. The initial stage

Sintering studies between 700 and 800°C in air and in argon were conducted using spherical silver powder. The technique of three-dimensional replication of the pore structure of silver powder samples has proved a precise means for measuring the sintering parameters, neck size and curvature, for calculation of theoretical neck growth and shrinkage rate. A neck growth model gave a close correlation with measured neck size after sintering in air, showing the important contribution of surface and grain boundary diffusion to neck growth. Grain boundary diffusion was found to be the dominant mechanism for densification in the initial stage of sintering in air of spherical silver powder. However, the experimental data obtained after sintering in argon disagreed with both the neck growth model (based on surface and grain boundary diffusion) and the densification model (based on grain boundary diffusion). The important effect of sintering atmosphere is discussed in terms of its interaction with solute impurities in the silver powder.     

A dislocation-based model for variant selection during the γ-to-α′ transformation

A phase transformation model is described for variant selection during the austenite-to-martensite transformation. The model depends entirely on the presence of glide dislocations in the deformed austenite. The direct correlation between the 24 slip systems of the Bishop and Hill (B-H) crystal plasticity model and the 24 〈112〉 rotation axes of the Kurdjumov-Sachs (K-S) orientation relationship is employed. Two selection criteria, based on slip activity and permissible dislocation reactions, govern the variants that are chosen to represent the final transformation texture. The development of the model via analysis of the experimental results of Liu and Bunge is described. The model is applied to the four distinct strain paths: (1) plane strain rolling, (2) axisymmetric extension, (3) axisymmetric compression, and (4) simple shear. Experimental deformation and transformation textures were produced for comparison purposes via appropriate deformation and quenching procedures. In each case, the transformation texture predicted using the dislocation reaction model is in excellent agreement with the experimental findings.     

Concurrent size and shape coarsening of γ′ in AlAg

The coarsening of γ′ precipitates in aluminum-silver has been investigated. Thesplate-shaped precipitates have coherent broad faces, and a hexagonal close packed crystal structure and develop on ∝111∝ planes of the aluminum matrix. During growth, a dispersion of nonequilibrium shaped γ′ precipitates develops. To understand the subsequent size and shape coarsening of these γ′ precipitates, several microstructural parameters have been experimentally determined as a function of aging time through quantitative electron microscopy. The measured thickening rate and nterledge spacing confirm that precipitate thickening takes place by a ledge mechanism and that the overall coarsening process of these precipitates is related to the average interledge spacing which remains relatively constant throughout the coarsening stage. The change in precipitate diameter does not appear to obey any temporal law and most likely accomodates the precipitate thickening and local supersaturation.     

High resolution electron microscopy of the early decomposition stage of AlLi alloys

The early decomposition stages of Al-7.8at.%Li, Al-10.4at.%Li and Al-11.8at.%Li alloys were studied by high resolution electron microscopy. In all the above alloys, it was confirmed that in the as-quenched stage small ordered domains of Ll₂ structure were present surrounded by the disordered matrix. From this microstructure, it was concluded that the alloy was already decomposed as in the as-quenched stage. No clear evidence was found for congruent ordering which was proposed to occur prior to spinodal decomposition. Examination of the sublattices of the individual ordered domains also failed to give convincing evidence for congruent ordering. The present observations suggest that the kinetics of spinodal decomposition of this system is too fast to be detected by TEM. In order to slow down the decomposition kinetics, solution treated samples were quenched to just above the solvus line for δ′ and then to room temperature. By this heat treatment, a solution treated sample with muck weaker order spots could be obtained. However, this microstructure could not be interpreted as congruently ordered either based on the weak Ll₂ spots. As there is a limitation to the quenching speed, it is suggested that ordering and spinodal decomposition progress concomitantly during the observable range of time.     

On the deformation of the 〈100〉 orientated γ′ strengthening phase of the CMSX2 superalloy

Both conventional macroscopic deformation tests and in situ straining experiments were devoted to the plastic behaviour study of the 〈100〉 orientated γ′ strengthening phase of the CMSX2 superalloy. Tensile test experiments, performed at various temperatures, reproduce the anomalous variation of the flow stress. Specimen unloading/reloading experiments show that the flow stress anomaly does not ensue from the exhaustion of the mobile superdislocation population. Stress relaxation experiments suggest that at both low and high temperatures, the superdislocation motion is controlled by a thermally activated mechanism, while in the temperature range around 300°C, the laws of thermal activation fail and instabilities occur on both the stress/strain and stress relaxation curves. In situ straining experiments, performed at 25°C and 320°C, lead to a microscopic scale approach of the deformation. At room temperature, a locking/unlocking mechanism controls the superdislocation motion. At 320°C, in addition to the locking/unlocking mechanism, local pinning points (either of extrinsic or intrinsic nature) affect the superdislocation motion. The in situ experiments permit us to propose various superdislocation core configurations so as to account for the experimental facts described above.     

Modeling of γ/γ′ phase equilibrium in the nickel-aluminum system

A theoretical model is proposed for the determination of phase equilibrium in alloys taking into consideration dissimilar lattice parameters. Volume dependent pair interactions are introduced by means of phenomenological Lennard-Jones potentials and the configurational entropy of the system is treated in the tetrahedron approximation of the cluster variation method. The model is applied to the superalloy relevant nickel-rich, γ/γ′ phase region of the Ni-Al phase diagram. The model predicts reasonable values for the lattice parameters and the enthalpy of formation as a function of composition, and the calculated phase diagram closely approximates the experimental diagram.