High-temperature strength and deformation of γ/γ′ two-phase Co–Al–W-base alloys

The high-temperature strength and deformation behavior of γ/γ′ two-phase Co–Al–W-base alloys have been studied with polycrystalline and single-crystal materials. The ternary, quaternary and higher-order alloys containing Ta, Cr and/or Re exhibit flow stress anomalies above 873 K due to slip of pairs of 1/2〈1 1 0〉 superpartial dislocations on {0 0 1} planes, in addition to {1 1 1} planes, in the γ′ precipitates. Compression tests on the single-crystal specimens reveal a true anomalous peak temperature of 1073 K for both ternary and Ta-containing quaternary alloys. Above the peak, the ternary alloy exhibits a rapid decrease in strength with temperature, as 1/2〈1 1 0〉 dislocations bypass the γ′ precipitates without significant shearing. Conversely, the Ta-containing quaternary alloy sustains strength to higher temperatures due to the activation of 1/3〈1 1 2〉 partial dislocation slip that introduces a high density of stacking faults in the γ′ precipitates.     

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.     

Thermodynamic stability of Mg–Y–Zn ternary alloys through first-principles

To clarify the thermodynamic stability of a Mg-based long-period stacking ordered (LPSO) structure, we systematically study the energetic preference for alloys on multiple stacking sequences with different compositions for random mixing of constituent elements, Mg, Y, and Zn, based on special quasirandom structure (SQS). Through calculation of the formation free energy of SQSs, it was found that the Mg–Y–Zn alloy exhibits phase separation into Mg- and Y–Zn- rich phases, which is consistent with previous theoretical studies. The bulk modulus of SQSs for various compositions, stacking sequences, and atomic configurations is approximately 35 GPa, i.e., it does not show significant dependence on the atomic arrangements, which therefore means that there are not significant differences among the effects of phonon on the stability of each structure in the LPSO structure. Introducing a stacking fault into hcp stacking sequence results in the acquisition of a “negative” energy, which indicates the profound relationship between the introduction of stacking faults and the formation of an LPSO structure.     

Atomistic modeling of the γ and γ′-phases of the Ni–Al system

A new embedded-atom potential has been developed for Ni₃Al by fitting to experimental and first-principles data. The potential describes lattice properties of Ni₃Al, point defects, planar faults, as well as the γ and γ^′ fields on the Ni–Al phase diagram. The potential is applied to calculate the energies of coherent Ni/Ni₃Al interphase boundaries with three different crystallographic orientations. Depending on the orientation, the interface energy varies between 12 and 46 mJ/m². Coherent γ/γ^′ interfaces existing at high temperatures are shown to be more diffuse and are likely to have a lower energy than Ni/Ni₃Al interfaces.     

Strengthening of an L12-ordered γ′-intermetallic by disordered γ-particles. Part II: Measurements of the CRSS and TEM observations of dislocation processes in Ni69Co9Al18Ti4

Strengthening of single crystals of an L1₂-long-range ordered γ′-phase by coherent disordered spherical f.c.c. γ-precipitates has been experimentally investigated. The overall composition of the material is (at.%): Ni 69, Co 9, Al 18, Ti 4. The critical resolved shear stress (CRSS) has been measured at 253 K for homogeneous, i.e. γ-free specimens and for specimens strengthened by three different γ-precipitate dispersions. The experimental results for the CRSS are in good agreement with those obtained by computer simulations (Part I) and are satisfactorily represented by Friedel's relation. The relevant configurations of dislocations have been studied by transmission electron microscopy.     

High-Temperature Structural Stabilities of Ni-Based SingleCrystal Superalloys Ni–Co–Cr–Mo–W–Al–Ti–Ta with Varying Co Contents

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glueatom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like(■), r-forming Cr-like(■) and c-forming Cr-like(■), the optimal formula for single-crystal superalloys is established [Al–Ni_(12)](Al_1■~_(0:5) ■_(1:5)). The Co substitutions for Ni at the shell sites are conducted on the basis of the first-generation single-crystal superalloy AM3, formulated as [Al–■_(12)Co_x](Al_1Ti_(0.25)Ta_(0.25)Cr_1W_(0.25)Mo_(0.25)), with x = 1.5, 1.75, 2 and 2.5(the corresponding weight percents of Co are 9.43, 11.0, 12.57 and 15.71, respectively). The900 ℃ long-term aging follows the Lifshitz–Slyozov–Wagner theory(LSW theory), and the Co content does not have noticeable influence on the coarsening rate of c0. The microstructure and creep behavior of the four(001) single-crystal alloys are investigated. The creep rupture lifetime is reduced as Co increases. The alloy with the lowest Co(9.43 Co) shows the longest lifetime of about 350 h at 1050 ℃/120 MPa, and all the samples show N-type rafting after creep tests.     

Strengthening of an L12-ordered γ′-intermetallic by disordered γ-particles. Part I: Computer simulations

Strengthening of an L1₂-long-range ordered γ′-intermetallic by coherent disordered spherical γ-particles has been investigated by computer simulations and by measuring the critical resolved shear stress (CRSS) of Ni₆₉Co₉Al₁₈Ti₄. In the present Part I the simulations and their results are communicated, in Part II experimental data taken for Ni₆₉Co₉Al₁₈Ti₄ will be reported. The simulations yielded numerical values for the CRSS τ_p. The average radii r and the volume fractions f of the γ-particles have been varied widely: 1 nm≤r≤19 nm and 0.04≤f≤0.10. The numerical results τ_p (r≥7 nm, f) can be well represented by Friedel's relation—provided the dissociation of the dislocations into partial dislocations is allowed for in the calculation of the dislocation line tension.     

Thermodynamic measurements and assessment of the Al–Sc system

A study of the binary Al-Sc phase diagram has been performed by means of thermodynamic calculations and experimental measurements. The enthalpy of formation of all intermetallic compounds has been determined and a cursory examination of the phase equilibria carried out, for compositions greater than 40 at% Sc. Two new invariant reactions have been identified in the Sc-rich part of the diagram: L ↔ (βSc)+Sc₂Al at 1185°C and (βSc) ↔ Sc₂Al+(αSc) at 970°C. A coherent set of Gibbs energy expressions for all the phases in the system has been generated by a least square optimisation procedure using all the experimental data available. The overall agreement is satisfactory but some uncertainties still persist, especially concerning the ScAl phase, owing to experimental difficulties.     

Structural mechanism of scale for 12Cr–W–Mo–Co steels with enhanced oxidation resistance

The phase constituent, morphologies, layer structures of the scale of 12Cr–W–Mo–Co heat resistant steel (HRS) formed in dry air and air with 10% vapour were systematically investigated. The interface between the scale and ferritic/martensitic matrix of this HRS was also studied. For the scale formed in air, single particle- and sheet-shaped oxide layer, which are composed of (Fe, Co, Cr)₂O₃, were formed. The scale combines with steel matrix via coherent or semi-coherent structure. For the scale formed in air with 10% vapour, the oxides take the shape of particulate. Layering phenomenon has been observed, i.e. the external layer is composed of (Fe, Co)₂O₃/(Fe, Co)₃O₄, the internal layer with spinel (Fe, Co, Cr)₃O₄ and the transition layer with Cr rich and Cr poor regions. The interface between the transition layer and the matrix is tight and steady, but the region linking the internal scale and transition layer is shaky and brittle. Both the oxidation processes in air and air with 10% vapour are considered to be controlled by diffusion mechanism.     

Thermodynamic properties and phase stability of nanocrystalline metals

The fundamental thermodynamic functions of enthalpy, entropy, and Gibbs free energy, as functions of the excess free volume at interfaces, temperature, and grain size, have been derived for single-phase metal nanocrystals. The model was applied to predict the thermal features of nano-grain boundaries and the characteristics of phase transformation in nanocrystalline metals, such as the transformation temperature and the critical grain size for phase transformation at a given temperature. The model predictions have been verified by experimental studies on the β-Co ←→α-Co phase transformation in nanocrystalline Co prepared by ball milling.     

In situ straining in the transmission electron microscope of an L12-ordered γ′-intermetallic strengthened by disordered γ-particles

Single crystal samples of quaternary alloy Ni₆₉Co₉Al₁₈Ti₄ were in situ strained in a transmission electron microscope. The alloy consists of an L1₂-long-range ordered γ′matrix strengthened by particles of the disordered γ-phase. During in situ straining the interaction of the dislocations with the γ-particles was investigated under stress. Microscopic features such as the dissociation width of the superpartial dislocations, bending of the dislocations between the γ-particles and the pinning of the dislocations at the departure side of the γ-particles are in excellent agreement with computer simulations performed recently.     

Thermodynamic modeling of Al–Ce–Mg phase equilibria coupled with key experiments

The ternary Al–Ce–Mg phase diagram was calculated using the Calphad method and investigated with selected key experiments. Arc melted alloys were annealed at 400 °C for 500 h and the phases were analyzed using quantitative X-ray powder diffraction (XRD). Differential thermal analysis (DTA) was also performed on an alloy with a composition near the ternary phase Al₁₃CeMg₆ (τ). Temperatures above 1000 °C could be attained due to a special sealing of the sample under argon by welding in a tantalum crucible to avoid evaporation and oxidation. Only with this procedure could reproducible and reliable DTA signals be obtained. The present experimental investigation and the consistent thermodynamic calculation show that the “ternary phase” Ce(Mg,Al)₂, seemingly isolated in the ternary at 400 °C, can be rationalized as a single solid solution phase between the binary end members if a larger temperature range and a solid state miscibility gap is considered. It is demonstrated that previously reported low values of ternary liquidus temperatures must be related to other phase equilibria. The actually found ternary liquidus temperatures are much higher and widely governed by the high melting compound Ce(Al,Mg)₂ and also by Al₁₁Ce₃ with primary solidification fields stretching far into the ternary system.     

Morphology of L10-TiAl(Ag) precipitation in Ag-modified L12–Al3Ti

Transmission electron microscopy (TEM) examinations of Ag-modified Al₃Ti with an L1₂-ordered structure have revealed the precipitation of L1₀–TiAl upon aging after quenching from higher temperatures. TEM observations revealed that plate-like L1₀–TiAl precipitates lie on {001} planes of (Al,Ag)₃Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. A quantitative X-ray microanalysis for determining the chemical compositions of precipitate and matrix has been done by using an analytical electron microscope. The L1₂ phase field in the Ti–Al–Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.