Abstract
The coarsening behavior of γʹ precipitate phase at different temperatures and the compressive performance of novel Co-Ni-Al-W superalloy were investigated. Experiment results show that the evolution of the mean radius and volume fraction of the γʹ phase obeys the classical Lifshitz-Slyozov-Wagner model. The coarsening rate of the γʹ phase exhibits a significant dependence on the aging temperature, which increases from 1.30×1
Co-based superalloys possess excellent thermal corrosion resistance, fatigue resistance, and weldability, which are widely suitable for the manufacture of critical hot-end components of aircraft engines and ground gas turbine
The melting point of Co-Al-W-based alloys is higher than that of Ni-based alloys by nearly 100 °C, and the dissolution temperature of the γ′-Co3(Al, W) phase exceeds 1000 °
The novel Co-based superalloy employs γ′ precipitates as its primary coherent strengthening phase. The morphology, size, distribution, and volume fraction of γ′ phase play a crucial role in the strengthening effects, thereby influencing the mechanical properties of the alloys under high-temperature conditions. However, exposure to high-temperature environment may lead to microstructure transformation in the alloy. Therefore, the microstructure stability of the alloy under high-temperature conditions should be optimized to enhance the mechanical properties. Coarsening phenomenon of γ′ phase usually serves as a critical factor influencing the microstructure stability and high-temperature mechanical properties. During high-temperature operation, the γ′ phase undergoes coarsening and growth processes. The faster the coarsening rate, the more significant the increase in the size of γ′ phase, leading to the reduction in the strengthening effec
In this research, the microstructure evolution of Co-Ni-Al-W-based superalloys after aging under different conditions was investigated. The evolution in the morphology, size, and volume fraction of the γ′ phase was studied as a function of the aging process, exploring the influence factors of γ′ phase coarsening. The microstructure stability of the Co-Ni-Al-W-based superalloy at high temperatures was discussed. Furthermore, the compressive properties of the Co-Ni-Al-W-based superalloys at 25–950 °C were studied.
The experiment alloy was melted into ingots using the vacuum induction technique at 1480 °C under the vacuum atmosphere of 0.5 Pa. The cylindrical samples with height of 80 mm and diameter of 15 mm were cast. The composition of the experiment alloy was 41.7Co-27.2Ni-9.0W-6.6Cr-5.7Ta-4.4Ti-3.0Al-2.3Mo-0.02C (wt%), namely Co-Ni-Al-W alloy. The samples were put into quartz tubes filled with pure argon gas to prevent oxidation in the following heat treatment. The heat treatment process included solid solution treatment and aging treatment. The samples were firstly subjected to solid solution treatment at 1230 °C for 24 h and then cooled to room temperature in air. After that, the solid solution-treated samples were aged at 800, 850, and 900 °C for 24, 72, 144, and 216 h and then cooled to room temperature in air.
The morphology, size, and distribution of the γ′ phase in the Co-Ni-Al-W alloy were characterized using Ultra-55 field emission scanning electron microscope (SEM). The samples were mechanically ground and polished before electrochemical etching using the mixed solution of 42vol% H3PO4+34vol% H2SO4+24vol% H2O. The radius of the γ′ precipitate (r) was estimated to be a/2, where a represents the mean edge length of the cubic precipitates. SEM images were analyzed using Image-Pro Plus software to calculate the volume fraction of γʹ precipitates by measuring their total area.
The phase transformation temperatures of the Co-Ni-Al-W alloys were accurately determined by NETZSCH 404 F3 differential scanning calorimetry (DSC) equipment at the heating rate of 10 °C/min. The mass density measurements were conducted according to ASTM B311-08. Cylindrical samples with 10 mm in diameter and 12 mm in height were prepared for compression tests (Gleeble 3800 thermo-simulator system) at ambient and high temperatures (600–950 °C) under strain rate of 1

Fig.1 SEM microstructures of Co-Ni-Al-W alloys after aging at 800 °C for 24 h (a), 72 h (b), 144 h (c), and 216 h (d)

Fig.2 SEM microstructures of Co-Ni-Al-W alloys after aging at 850 °C for 24 h (a), 72 h (b), 144 h (c), and 216 h (d)

Fig.3 SEM microstructures of Co-Ni-Al-W alloys after aging at 900 °C for 24 h (a), 72 h (b), 144 h (c), and 216 h (d)
When the aging temperature further increases to 900 °C, the γʹ phase basically presents the cubic morphology, as shown in

Fig.4 Evolution of average radius (a) and content (b) of γʹ precipitate phases after different aging treatments
Therefore, it is concluded that the volume fraction of γʹ phase is slightly increased with the prolongation of aging time under the same aging temperature. This phenomenon is primarily attributed to the growth of γʹ phase during the continuous aging process, thereby increasing the γʹ phase volume fraction. However, the increase in the aging temperature results in a decrease in the precipitation of γʹ phase, which in turn decreases the volume fraction of the γʹ phase. Additionally, the merging of γʹ phases, namely the formation of large phases through the consumption of small phases, ultimately results in the decrease in the volume fraction of the γʹ phas
Ref.[
(1) |
where rt is the radius of γʹ phase aged for h; r0 is the radius of γʹ phase aged for h; is the time exponent; is the coarsening rate coefficient. The controlling mechanism of the coarsening rate in two models mainly depends on the time exponent n in
To clarify the coarsening kinetics of γʹ phase in the Co-Ni-Al-W alloy, it is necessary to determine the value of time exponent in
(2) |

Fig.5 Relationship of lgr-lgt of γʹ phase under different aging conditions
Temperature/ °C | Temporal exponent, 1/ | Coarsening rate coefficient, /×1 |
---|---|---|
800 | 0.31 | 1.30 |
850 | 0.30 | 3.27 |
900 | 0.31 | 9.56 |
The relationship between the average radius of γʹ phase (3) and aging time () is illustrated in

Fig.6 Relationship of
regulates the coarsening process of γʹ phase.
In LSW model, the coarsening rate of γʹ phase is influenced by various factors, including the diffusion coefficient of solute elements, the interfacial energy between γ and γʹ phases, and aging temperature. The coarsening rate coefficient can be expressed by
(3) |
where is the diffusion coefficient of the solute element, is the γ/γʹ interfacial energy, is the molar volume of the γʹ phase, is the equilibrium solubility of solute element, is the gas constant, and is the absolute temperature. According to the Arrhenius equatio
(4) |
where is a constant and is the activation energy. Therefore, by combining
(5) |
Assuming that T/Cm is constant in

Fig.7 Relationship between lnK and 1/T
Furthermore, the grain growth is influenced by temperature gradient, undercooling, and the addition of rare earth elements. Ref.[

Fig.8 Comparison of solvus temperature (a) and density (b) of Co-Ni-Al-W alloys and Co-based superalloys
The Co-Ni-Al-W alloy has a low density of approximately 8.7 g/cm³, as shown in
To investigate the temperature dependence of compressive yield strength of Co-Ni-Al-W alloy, compression tests were conducted at 25–950 °C. The compressive yield strengths of the Co-Ni-Al-W alloy and other Co-based super-alloy

Fig.9 Compressive yield strengths of Co-Ni-Al-W and Co-based superalloys at different temperatures
The compressive yield strength of the Co-Ni-Al-W alloy can be divided into three stages. In the first stage, the compressive yield strength is decreased with the increase in temperature from 25 °C to 650 °C. In the second stage, the compressive yield strength exhibits a positive temperature dependence when the temperature increases from 650 °C to 850 °C. This phenomenon is similar to that in Ni-based superalloys. Finally, the third stage demonstrates a negative temperature dependence of compressive yield strength above 850 °C. Apart from the first stage, the yield strength decreases from 761 MPa to 714 MPa when the temperature increases from 600 to 650 °C. Subsequently, it gradually increases, reaching a maximum compressive yield strength of 774 MPa at 850 °C. However, with the further increase in temperature to 950 °C, the compressive yield strength decreases to 651 MPa. It is worth noting that the Co-Ni-Al-W alloy exhibits an anomaly behavior in compressive yield strength within the temperature range of 650–850 °C. This behavior can also be observed in superalloys contain γʹ-strengthened Co-T
1) The microstructure of the Co-Ni-Al-W superalloy aged at 800 °C for 24 h primarily consists of γ matrix phase and 69vol% γʹ phase. The Co-Ni-Al-W superalloy has high γʹ solvus temperature of 1221 °C and low density of 8.7 g/cm³.
2) The γʹ phase morphology changes from spherical to cubic in the Co-Ni-Al-W superalloy aged at 800–900 °C. The average radius grows with higher aging temperatures and longer time. The γʹ phase volume fraction is decreased with the increase in temperature but increased with the prolongation of aging time.
3) The coarsening behavior of γʹ phase in the Co-Ni-Al-W superalloy obeys the diffusion-controlled LSW coarsening mode. The γʹ phase coarsening rate coefficients are 1.30×1
4) The activation energy for the Co-Ni-Al-W superalloy is 210 kJ/mol during aging at 800–900 °C, which is related to the W content in the alloy. The diffusion of W element in the alloy affects the coarsening rate of γʹ phase.
5) The compressive yield strength curve of the Co-Ni-Al-W alloy has three variation stages. With the increase in temperature from 650 °C to 850 °C, the compressive yield strength exhibits a positive temperature dependence and reaches the maximum compressive yield strength of 774 MPa at 850 °C.
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