Microstructural stability of long-term aged wrought Ni–Fe-based superalloys

Coarsening of γ′ particles

Representative SEM micrographs for the aged samples, at different aging times, are presented in Figure 1. The distribution of γ′ in the alloy, after solution annealing and aging treatment, is provided in Figure 1(a). An average grain size of approximately 97 µm was measured following the heat treatment. Following aging at 750°C, the γ′ precipitates were homogeneously dispersed in the γ matrix. The size of γ′ phase was increased with the increase of aging time, and the γ′ phase morphology was maintained as spherical due to the absence of applied stresses. This implies a strong coherency with the matrix and a low degree of mismatch between the lattice parameters of γ′ and γ phases. Moreover, the higher stability of γ′ shape suggests enhanced precipitation strengthening of γ′, which will contribute to improving the mechanical performance. OPEN IN VIEWER

The variation of γ′ particle size and volume fraction in the long-term aging samples were investigated. The measured and calculated diameter values of γ′ particles from the experiments and simulation results of the JMatPro software are provided in Figure 2(a). The determined average diameter of γ′ particles following 0, 500, 935 or 3000 h was 39, 84, 107 and 135 nm, respectively. The coarsening rate at the initial exposure stage was higher and then gradually decreased with the increase of aging time. It was concluded that the JMatPro results were consistent with the experimental results, which predicted that these particles would coarsen at 750°C. Also, additional micrographs were used for measuring the γ′ fraction. The results in Figure 2(b) show that by increasing aging time, the volume fraction of γ′ is slightly decreased, which is contradictory with the observations of alloy 740 [14]. As the aging time was further increased, the average size of γ′ gradually increased, and the volume fraction of γ′ slightly decreased. It was indicated that the distance between adjacent γ′ particles became larger, and the variation of γ′ particle spacing had much more effect on the volume fraction of γ′ than that of γ′ coarsening. The dissolution of small γ′ particles and combination of large γ′ particles resulted in the coarsening of γ′. The diameter of γ′ particles and their distance from the adjacent γ′ particles have a significant effect on both the cutting and the Orowan mechanisms, which will be discussed below. OPEN IN VIEWER

It had been proposed that coarsening of γ′ in Ni-based superalloys could be described by the Lifshitz– Slyozov–Wagner (LSW) model [15,16], which was related to the theory of diffusion-controlled Ostwald ripening. The power law expression is as follows: (1) where is the average radius of the precipitates at time t, is the mean particle size at the onset of coarsening (at ) and k is the coarsening rate coefficient. As seen in Figure 3, the values are plotted as a function of the aging time t. The plotted curve in the present study follows the relation described by Equation (1). This suggests that γ′ phase is coarsening in this alloy by following the standard kinetics of diffusion-controlled particles’ growth. The value of the correlation coefficient of 0.94896 was obtained from the trendline. OPEN IN VIEWER

The size of γ′ is one of the major factors that affect the degradation of superalloys; thus, this study was focused on the evolution of γ′ particles in the long-term aging. The fact that the size of γ′ phase was increased with the increase of exposure time has also been reported for other Ni–Fe-based superalloys [11]. In these alloys, the morphology was maintained spherical, which is consistent with the results of the present study. However, for the Ni-based superalloy Inconel 740H, the morphology of γ′ particles was altered from spherical to cubic, by increasing the exposure time at 810°C [14]. The shape of γ′ in Inconel 740H changed from sphere to rounded cubic forms with increasing precipitate size, which related to a low surface energy and lattice mismatch. Compared to Ni-based single crystal superalloys, γ′ phase was always nucleated as sphere and subsequently transformed to cube, during the solid state transformation process. The morphological transformation of γ′ was associated with the coherency degree of γ′ and γ matrix. It has been indicated that, with the service temperature reaching approximately 60% of the melting temperature, a reduction in the γ/γ′ lattice mismatch is beneficial for the mechanical properties. The contribution of lower lattice mismatch, which is responsible for creep-rupture life, was estimated in terms of the γ′ stability at medium temperature [17].

Microstructural change of M₂₃C₆ carbides

The evolution of carbides along the grain boundaries is shown in Figure 4. Discrete, fine, granular-type Cr-rich M₂₃C₆ carbides, with approximately 0.1–0.7 µm in diameter, were precipitated at the grain boundaries, as received from the sample with the initial heat treatment. The length and width of M₂₃C₆ carbides increased, by increasing the exposure time, and the volume fraction of the carbides gradually became higher. At the terminal exposure time of 3000 h, small quantity of short rodlike carbides was found along the grain boundaries. OPEN IN VIEWER

In order to describe the coarsening mechanism of M₂₃C₆ carbides adjacent to the grain boundaries, the length and width of carbides were carefully determined. The parameters of the length and width (L³–L₀³) of carbides as a function of aging time are illustrated in Figure 5. The results showed that the coarsening rate of the width of the carbides fits with the LSW model. Also, the coarsening rate of carbides during the first 935 h of aging was higher; however, it gradually decreased at higher aging times. The variation tendency of carbides coarsening is not monotone increasing. In fact, both the solvus temperature of M₂₃C₆ and the aging temperature are critical to the carbides’ growth. The nucleated carbides would grow up with the thermal exposure time. Meanwhile, with increasing the aging time, partial M₂₃C₆-type carbides would dissolve in the matrix due to solute diffusion in the alloy. The dissolving behaviour of M₂₃C₆ was discovered following long-time aging at 750 and 800°C [18]. OPEN IN VIEWER

The volume fraction and morphology of carbides along grain boundaries are important for creep life. The fine distribution of M₂₃C₆ can be considered as beneficial for creep strength. However, the substantial coarsening of M₂₃C₆ carbides decreased the grain boundaries strengthening during the aging process [19]. Furthermore, the increased width of M₂₃C₆ at the grain boundaries caused a decrease in impact toughness values, which significantly promoted embrittlement in the alloy [20]. So the preventing of carbides’ coarsening is essential to the grain boundary strengthening. Wang et al. [21] reported that the nucleation and growth of M₂₃C₆ during long-term aging might be suppressed by P additions.

Variation of microhardness

Figure 6 presents the overall relationship of microhardness with aging time. The microhardness was maintained, following 935 h thermal exposure, and then slightly decreased, following 3000 h of aging. It is well known that precipitation strengthening is usually controlled by the cutting mechanism, which can be subdivided into two models: the weakly coupled dislocations (WCD) model and the strongly coupled dislocations (SCD) model. Furthermore, in the case of coarse precipitates, precipitation strengthening of superalloys can be controlled by the Orowan mechanism. The critical resolved shear stress (CRSS) Δτ would change with the increase of γ′ particles’ size. The equation of the three models can be written as follows [15,22]: (2) (3) (4) where Δτ is the CRSS, γ_APB is the anti-phase boundary energy, b is the Burgers vector of , f is the volume fraction of γ′ phase, r is the radius of γ′ particles, μ is the shear modulus, T is the line tension of the dislocations and ω is a dimensionless constant. As shown in the equations, the variables related to the aging time and temperatures are f and r. In order to calculate Δτ, the γ′ volume value, used at the present study, was 0.265, which was obtained in the as-received sample. The rest of the parameters were as follows: , , , and [22]. The theoretical Δτ as a function of γ′ size was obtained by Figure 7. For a given radius value of γ′ particles, the lowest CRSS is primarily contributing to the strengthening mechanism. By increasing the γ′ particles’ size, the dominated strengthening mechanism of the alloy changed from the WCD to the SCD model and finally to the Orowan mechanism. It was found that the transition from the WCD to SCD model occurred at a critical radius (r_WS) value of approximately 22.5 nm. However, the subsequent transition from the SCD to Orowan model occurred at a higher critical radius (r_SO) value of 33.4 nm. For this theoretical calculation, there was a clear transition of the governing mechanism from SCD cutting to Orowan mechanism, which was different from the results reported in the Inconel 740 alloy [14]. For Inconel 740 alloy, there was a transition of the governing mechanism from WCD cutting to Orowan mechanism, and there was no obvious SCD cutting mechanism with increasing γ′. OPEN IN VIEWER

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Figure 7

Theoretical analysis of critical resolved shear stress vs. different radius of γ′.

In Figure 7, the γ′ volume fraction was considered as a constant value. However, the γ′ volume fraction of the present alloy was slightly decreased during the long-term aging time. It is generally accepted that the dislocations shear particles mechanism or the dislocations by-pass particles mechanism does not refer to the changes of γ′ volume fraction. In order to study the effect of γ′ volume fraction on microhardness, the effect of γ′ volume fraction on the particles strengthening mechanism is demonstrated in Figure 8. The value of r_SO is continuously decreased with the decrease of γ′ volume fraction, whereas the value of r_WS is maintained. This indicates that the transition from the WCD model to SCD model was less affected by the γ′ volume fraction. With the dissolution and combination of smaller γ′ particles, the decrease in volume fraction of γ′ phase may also decrease the strength of the alloy. After exposure for 3000 h, it was clear that the γ′ volume fraction was slightly decreased, which would accelerate the transition from the WCD model to Orowan mechanism. One extreme case is that the SCD cutting mechanism could be eliminated during the coarsening process of γ′. OPEN IN VIEWER

The average γ′ size is higher than that of the r_SO following aging for 500 h, so the dislocations that bypass particles became the main strengthening mechanism and the alloy strength would decrease with the coarsening of γ′. However, although there was a decrease in the volume fraction of γ′ by increasing aging time, the microhardness of the alloys maintained high values. It is commonly recognised that the strength of the superalloys was predominantly related to the precipitation of γ′ particles and the γ matrix, while all the microhardness testing was performed within the grains. In general, the variation of solid solution strengthening of superalloys was minimal during the thermal exposure at a constant temperature [19]. Therefore, although the effect of γ′ on the microhardness might have been slightly decreased, the variation of microhardness implies that the amount of contribution rate of γ′ precipitation strengthening was minimal following aging for 935 h.