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The morphological evolution and precipitation kinetics of γ′ and D0 19 (Co 3 W) phase in Co–Al–W alloys at 900 °C have been studied by applying Phase-field method and experiment in order to understand the transformation process of γ′ phase and D0 19 phase. The growth processes of D0 19 phase and precipitation of γ′ phase under elastic fields were simulated through coupling with thermodynamics and dynamics databases. The simulation results indicate that the misfit δ ≥ 0.53% has a greater impact on γ′ particle morphology in γ/γ′ structure. Co–Al–W alloy with low Al and high W is one of the factors to promote the precipitation of D0 19 phase. Three stages during aging, namely the γ′ phase incubation stage, the γ′ phase fast nucleation and growth stage, and the transformation from γ′ phase to D0 19 phase stage can be observed with the non-constant coarsening rate that varying with the decrease of γ′ phase. The particle size distribution(PSD) during the precipitation of D0 19 phase is more in line with MLSW theory than LSW theory. This simulation results are in good agreement with the experiment results to help analyze microstructure evolution of Co–Al–W alloy. Image 1 • The microstructure evolution of γ′ and D0 19 phase in Co-Al-W was studied through experiment and phase-field methods. • This work provides insights into the effects of composition changes, misfits, and D0 19 phases on microstructure. • This work can be helpful to analyze the precipitation behavior of D0 19 phase and γ/γ′ micstructure Co–Al–W alloys. [ABSTRACT FROM AUTHOR]

Abstract The temporal evolution of a γ(f.c.c.)/γ’ (L1 2) Co-8.8Al-7.3 W superalloy aged at 650 °C (10 min–4096 h) and 750 °C (10 min–256 h) is studied utilizing atom-probe tomography (APT), scanning electron microscopy, and Vickers microhardness testing. The evolution of the phase compositions, γ’ (L1 2) volume fraction, and mean precipitate radius, , are determined. Coarsening rate constants and temporal exponents are calculated for of the γ’ (L1 2)-nanoprecipitates. The temporal exponents are found to be generally close to 1/p = 1/3 as required for diffusion-limited coarsening. Tungsten solid-solubility is significantly reduced in the γ(f.c.c.)-matrix at 650 °C (0.54 ± 0.04 at. %) and 750 °C (1.35 ± 0.06 at. %) when compared with aging at 900 °C (5.5 at. %). The value of of the γ’ (L1 2)-nanoprecipitates increases with increasing aging time corresponding to an increase in the Vickers microhardness; the peak strength was not, however, achieved for the aging times investigated. The morphology of the γ’ (L1 2)-nanoprecipitates begins as spheroids but transitions to cuboids at longer aging times, with final the γ’ (L1 2) volume fractions for aging at 650 °C and 750 °C being ϕ = 53% and 54%, respectively. The effect of quench-rate (either furnace-cooled, air-cooled, oil quenched, or water quenched) from a supersolvus temperature of 1050 °C on the microstructure of the alloy is also investigated. Slow cooling (furnace and air-cooling) is shown to result in a uniform distribution of nanometer sized γ’ (L1 2)-nanoprecipitates, unlike Ni-based superalloys in which the γ’ (L1 2)-nanoprecipitates form in a non-uniform or multimodal distribution. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The Al-Co-W system and its binary sub-systems Al-Co, Al-W and Co-W were critically reviewed. The thermodynamic description of the Al-Co-W system including all three binaries was developed considering thermodynamic and constitutional data of the systems. Results from density functional theory calculations were employed to improve reliability of the descriptions. The Gibbs energy for the thermal vacancy (G Va ) in the BCC_A2 phase is discussed. The revised descriptions obtained for the Al-W and Co-W systems describe the thermodynamic and phase equilibrium data well and are free of undesired artefacts for temperatures below 6000 K. The ordered γ’ phase of the Al-Co-W system is described as a metastable phase in the entire temperature range. The calculated Gibbs energy of the γ’ is only slightly above that of the equilibrium state, which indicates that there is good possibility of stabilizing the γ’ phase with the addition of γ’-stabilizing elements, such as Ti, Ta, Hf, Nb and Ni. [ABSTRACT FROM AUTHOR]

An experimental study of the mechanical and magnetic properties of Co-8 at % Al-X at % W (where Х = 4.6; 6.8; 8.5; 10; 12) polycrystalline alloys has been carried out depending on the tungsten content. It has been shown that an increase in the tungsten content induces the following changes: Young's modulus changes from 240 to 259 GPa and the microhardness HV changes from 290 to 413 MPa. The Curie temperature of the intermetallic γ' phase ( L1) grows from 1025 to 1049 K, and the saturation magnetization of the alloys decreases. The value of the coercive force also increases, from 1 to 500 Oe. The calculations of the specific heat of the γ' ( L1)-γ (dcc) phase transition has shown that the highest amount of heat (304 J/g) is absorbed upon the phase transition in the sample containing the largest amount of tungsten (12.6 at %). The results obtained indicate the diffusion character of the transition, the rate of which is controlled by the diffusion of the slowest component of the alloy (in this case, tungsten). It has been found that the solvus temperature of the γ' phase increases from 1009 to 1044°С with an increase in the tungsten content from 4.6 to 12.6 at %. [ABSTRACT FROM AUTHOR]

The precipitation strengthened Co-based superalloys show the great potential for the next generation superalloys of aerospace engine, their mechanical properties are associated with the morphology stability of L1 2 -γʹ phase under high temperature and loading state. The antiphase boundaries (APBs) formed by different variants of the ordered fcc-L1 2 -γʹ-Co 3 (Al, W) phase, affect creep morphologies and the high-temperature properties of Co-based superalloys. By coupling with the Kim-Kim-Suzuki (KKS) model and crystal plasticity model, the directional rafting and evolution kinetics of the γʹ phase are studied during the creep of Co–10Al–10W (at.%) superalloy. Creep rafting under high temperature and low stress is a process of stress-induced directional coarsening, rafting is affected by the APBs on hindering the directional element diffusion. In the early stage of creep, the increasing number of APBs leads to a lower creep strain. Higher temperature accelerates the rafting and reduces the γʹ area fraction. Furthermore, under tensile stress and compressive stress, the APBs show the hindering effect on both P-type rafting and N-type rafting. This study reveals the effects of APBs on the rafting behaviors and creep properties, the findings are theoretically important for the microstructure optimization of Co-based superalloys. • Directional diffusion of W is hindered by APBs during creep of Co–Al–W superalloy. • APBs inhibit the rafting of γʹ phase under tensile and compressive creep stress. • APBs improve the creep resistance at the early stage of creep. [ABSTRACT FROM AUTHOR]

Low-temperature portion of the polythermal section for the Co-Al-W system in the vicinity of the Co(Al, W) intermetallic composition has been studied experimentally using electron microscopy and hightemperature X-ray diffraction analysis. Low-temperature structural phase transformations and temperature ranges of the existence of phases have been determined. The morphology of Co(Al, W) intermetallic particles was studied as a function of the tungsten content in alloys. [ABSTRACT FROM AUTHOR]

Diffusion coefficients of the fcc Co–Al–W alloys were extracted from extensive diffusion couple experiments performed at 1173–1573 K, which were afterwards assessed to develop their diffusion mobility in the context of phenomenological treatment. We demonstrate that the mobility enables the prediction of diffusion picture of technologically important Co–Al–W alloys. [ABSTRACT FROM AUTHOR]