Crystalline Microstructure, Microsegregations, and Mechanical Properties of Inconel 718 Alloy Samples Processed in Electromagnetic Levitation Facility

Abstract

The solidification of Inconel 718 alloy (IN718) from undercooled liquid is studied. The solidification kinetics is evaluated in melted and undercooled droplets processed using the electromagnetic levitation (EML) technique by the temperature–time profiles and solid/liquid (S/L) interface movement during recalescence. The kinetics is monitored in real time by special pyrometrical measurements and high-speed digital camera. It is shown that the growth velocity of $\gamma$-phase (the primary phase in IN718), the final crystalline microstructure (dendritic and grained), and the mechanical properties (microhardness) are strongly dependent on the initial undercooling $\Delta T$ at which the samples started to solidify with the originating $\gamma$-phase. Particularly, with the increase in undercooling, the secondary dendrite arm spacing decreases from 28 μm to 5 μm. At small and intermediate ranges of undercooling, the solidified droplets have a dendritic crystalline microstructure. At higher undercooling values reached in the experiment, $\Delta T>160$ K (namely, for samples solidified with $\Delta T=170$ K and $\Delta T=263$ K), fine crystalline grains are observed instead of the dendritic structure of solidified drops. Such change in the crystalline morphology is qualitatively consistent with the behavior of crystal growth kinetics which exhibits the change from the power law to linear law at $\Delta T\approx 160$ K in the velocity–undercooling relationship (measured by the advancement of the recalescence front in solidifying droplets). Study of the local mechanical properties shows that the microhardness increases with the increase in the ${\gamma }^{″}$-phase within interdendritic spacing. The obtained data are the basis for testing the theoretical and computational of multicomponent alloy samples.

1. Introduction

Ni-based superalloys are materials that exhibit excellent mechanical strength, high temperature resistance, and good corrosion resistance [1,2]. The IN718 alloy, well known for its exceptional mechanical properties, is a nickel-rich alloy that undergoes a sophisticated precipitation hardening process [3,4]. The alloy is characterized by the formation of ordered body-centered tetragonal precipitates, specifically in the form of ${\gamma }^{″}$-phase, which are finely dispersed within the $\gamma$-matrix. This complex microstructural arrangement plays a key role in enhancing the overall strength and performance of the alloy. The ordered ${\gamma }^{″}$-precipitates, primarily composed of $N{i}_{3}Nb$, contribute significantly to the alloy’s strength by impeding dislocation movement and hindering the progress of plastic deformation. As the volume percentage of ${\gamma }^{″}$ increases, a more refined and densely populated precipitate network forms, leading to a corresponding increase in strength [5,6,7].

Previous experimental studies [8] and computations [9,10] show that the microstructure, segregation of chemical elements, and alloy preparation technique have a substantial effect on the mechanical properties of the Inconel samples. The chemical segregation and crystalline microstructure were investigated experimentally and computationally in the positive temperature gradient [9,10] consistent with the welding and additive manufacturing processing for small thermal gradients and cooling rates. For example, investigations of Rahul et al. [10] were limited by cooling rates of 10 (K/s) and thermal gradients of $5\times {10}^2$ (K/m). The structure and chemical segregation in samples crystallized from undercooled state was also studied [9,11,12]. These studies also dealt with the analysis of samples formed under small values of undercooling: grand-potential statement of the problem within the phase field method $apriory$ assumes small differences in chemical potentials [9] and the as-cast samples assume only units of Kelvin in values of undercooling [11]. Therefore, the motivation of the present study is to extend the analysis for samples of IN718 solidified under undercooling from units to several hundred Kelvin.

As a typical process of sluggish and rapid solidification evolving under small and high values of undercooling, the electromagnetic levitation technique (EML method) is used in the present work. This technique is well known to produce novel materials with improved mechanical and electrical properties, namely, due to reaching large values of undercooling at which metastable phases appear [13]. Therefore, a relationship between the microstructure and mechanical properties of IN718 alloy is established using microhardness measurements. Specific features of dendritic patterns are analyzed as the main crystalline microstructure in alloy samples appearing during solidification from undercooled state. The study of the kinetics of dendritic nucleation and growth enables us to predict the formation of grains and defects in the microstructure of alloys. The obtained experimental results are utilized as inputs and specific test values for computational modeling carried out in our theoretical study [14]. As a final note, the experimentally found solidification kinetics is analyzed in comparison with the sharp interface model predictions for the dendrite velocity–undercooling relationship.

2. Materials and Methods

2.1. Electromagnetic Levitation (EML)

Samples of IN718 having the content specified in

Table 1. Initial composition of IN718 [18] in wt.%.

Element	Ni	Cr	Nb	Mo	Ti	Al	C	Fe
Measured concentration	52.78	18.7	4.95	3.04	1.01	0.55	0.046	Balance
Powder concentration	54	18	8	2	0	0	0	Balance

were prepared in the form of 800 mg cubes, and all sides were ground using P1200 SiC abrasive paper before the EML experiments to eliminate the oxides and impurities. During the electromagnetic levitation experiment, the whole solidification process can be observed directly [15,16]. The main setup of the electromagnetic levitation device can be found in [17]. The levitation process in this study was controlled by the power of a high-frequency generator, which delivered a current to a coil that produced an electromagnetic field with the necessary strength to levitate a specific sample with mass and geometric characteristics. In this study, spherical samples of 5 to 7 mm in diameter were placed on a sample holder between the upper and lower coils. An alternating current with a frequency (f) approximately equal to 300 kHz and a power (P) between 0.3 and 10 kW was used to generate the electromagnetic field to counteract gravity and promote sample heating. For the experiments, the vacuum chamber was first evacuated to $6·{10}^{-6}$ mbar and then refilled with high-purity He (6N) to a pressure of 350 mbar in order to limit the evaporation of the sample. When the sample was completely melted, the temperature was continuously increased to 200 K above liquidus temperature $T_L$ to dissolve/evaporate oxides. Sample cooling and subsequent solidification were initiated using cooling gas (He). The temperature of the sample’s top surface was determined by an IR pyrometer (model IGA 140 MB 30L, Fa. LumaSense Technologies) and an accuracy of ±2 K. In addition, a high-speed camera was utilized to examine the recalescence events on the sample’s side surface in situ to quantify the dendritic tip velocity under undercooled conditions. The camera’s highest frame rate is 125,000 fps and its two-dimensional resolution is 100 × 100 μm² (128 × 32 pixels). These melting and solidification cycles were repeated several times to examine tip velocities across a broad range of undercooling. This resolution is adequate to monitor growth velocities ranging from 0.1 m/s to 100 m/s. The dendrite growth velocity V was determined using the formula $V=d/t$, where d is the sample’s diameter and t is the recalescence period. Additional information regarding EML device can be found in Ref. [17].

2.2. Microstructure Investigation

The as-solidified samples were cut along the center cross-section and mounted in conducting mounting resin, then grounded, and polished to 1 μm. To obtain a better surface quality for further characterization the sample metallography was finished using a vibratory polishing machine (Saphir Vibro). The solidification microstructure of the samples was examined by scanning electron microscopy (SEM) in the backscattering mode (Zeiss Evo 40, Carl Zeiss AG, Jena, Germany). Compositions of the bulk samples and phase constituents were analyzed using an energy-dispersive X-ray spectrometer (EDS) with an EDAX PV7715/89-ME detecting unit.

2.3. Mechanical Characterization

The measurement of microhardness was performed with a 200 g load using a Vickers HMV-2000 hardness tester (Shimadzu Corp., Osaka, Japan). The cross-section of the sample was evenly divided into multiple regions, and fifty microhardness tests were randomly conducted within the designated regions under the same conditions to obtain the statistical results on microhardness.

3. Results and Discussion

3.1. Thermal Behavior of the Samples

As illustrated in Figure 1, the present samples show the recalescence during the whole cooling process from 1700 °C to 1000 °C. The thermal event refers to the nucleation of the primary Ni-fcc phase, and with the increasing of undercooling the nucleation interval also becomes shorter. The second recalescence is hypothesized to be the signal for carbide.

As shown in Figure 2, the interface growth velocity exhibits an exponential increase within the range of small and intermediate undercooling and the possible change to the linear law with the higher level of undercooling. Indeed, within the range of small and moderate values of undercooling, the interface velocity increases gradually; however, the velocity essentially rises at the higher undercooling approximately of $\Delta T$ = 160 K. Along with the variation of growth rate, the recalescence front is also changed from mesoscopically rough to smooth spherical/planar front (see transition in recalescence front in Figure 3a–f. Very probably, the cellular/dendritic microstructure dictated by short-range solute diffusion is changed in this case to the large dendrites growth which is governed by long-range thermal transport. Such kinetic crossover is accompanied by the velocity rising existing due to the nonequilibrium trapping of atoms (so/called “solute trapping effect”) in the alloy [16]. The solute trapping decreases the solutal atmosphere around the solid/liquid interface, respectively, decreasing the solute drag at the solid–liquid interface, and, as a result, it leads to the increase in the crystal growth velocity. This behavior is shown by calculations summarized in Appendix A and shown in Figure A1.

3.2. Microstructure Analysis

The SEM investigations showed that the samples that solidified at $\Delta T<160$ K have a dendritic microstructure. Figure 4 demonstrates the microstructure consisting of three microconstituents: periodic dark gray dendrite $\gamma$-matrix, halos of the precipitated ${\gamma }^{″}$-phase (both ${\gamma }^{″}$-and Laves phases in our samples are found; however, one can only mention them as ${\gamma }^{″}$ due to the rough resolution to quantify exactly these phase), and light gray interdendritic region. Consistently with Figure 3, the microstructure becomes finer with the increase in undercooling in samples (see Figure 4a–c).

In addition to the typical dendritic structure, fine equiaxed grains (Figure 4e) were found in the sample solidified at $\Delta T$ = 170 K. At the boundaries among dendritic cells, precipitation phases of different shapes can be seen due to the segregation of the local chemical composition. The varying brightness of the dendritic cells in Figure 4e exists mainly due to the various crystal orientations. The relationship between the structure of undercooling liquid and nucleation is an issue of discussion in [19,20]. The correlative study carried out showed that solidified samples of IN718 show grain sizes on the order of micrometers, and they are very heterogeneous at the microscale. Our metallographic and electronic microscope studies show that the typical size of grains has the order on micrometers in samples of IN718 solidified at the undercooling of 10–80 K. When the undercooling of samples reaches 170 K, the structural region with equiaxed morphology is found. This region can be caused by the high cooling rate near the bottom of the sample [16]. According to the nucleation theory [19,20], the nucleation rate in the alloy melt increases with the rise of undercooling. Therefore, there are more nuclei formed in the highly undercooled alloy melt than in the less undercooled alloy melt. In addition, the strong recalescence under higher undercooling conditions can also remelt and break the dendrite arms due to their instability [21,22]. Finally, the cross-section with these small grains may represent crystalline cells in the direction perpendicular to the primary growth direction.

3.3. Dendrite Arm Spacing Analysis (Linear Intercept Method)

The primary dendrite arm spacing (PDAS) and secondary dendrite arm spacing (SDAS) can be utilized to evaluate the structure of samples solidified at different undercoolings. PDAS has a significant effect on the mechanical properties, but the secondary dendrite arm spacing has a direct effect on the segregation of components, the secondary phase precipitation, and the distribution of microscopic shrinkage defects. Moreover, the SDAS is responsible for the interdiffusion of alloy components and, respectively, for the homogenization of a sample. Using ImageJ software (version 1.53e), the line intercept method was applied to measure the primary and secondary dendrite spacings, following the procedure described in [23]. It can be seen from Figure 4 that the microstructure of samples solidified at different undercooling is composed of well-developed dendritic structures up to the undercooling $\Delta T$ = 80 K. The PDAS and SDAS are characterized by a decreasing trend in microstructure with the increase in undercooling. This is a well-known trend caused by the higher solidification rate and also the increasing thermal gradient [19]. During the rapid solidification process, a great number of crystal nuclei in the melt pool do not have enough time to grow and create grains. Furthermore, based on the theory introduced by Kirkwood [24], the spacing between secondary dendrite arms is proportional to the undercooling (Figure 5): at lower undercooling, one can find the bigger distance within the space of PDAS and SDAS. In the range of 63 K and 80 K, the characteristic space in PDAS and SDAS tends toward confluence. This means that the characteristic length of the dendrites decreases such that it becomes the order of distance between secondary arms. For the exceptional undercooling of 170 K, the grain size is not presented in Figure 5 because small-grained crystals were found in this highly undercooled sample.

3.4. Phase Fraction Using Scanning Electron Microscopy Image Analysis

In this study, phase distribution in IN718 samples was characterized using scanning electron microscopy (SEM) under the backscattered electron (BSE) mode. The software MATLAB 8.0 was utilized for all BSE image processing. MATLAB open-site programs were developed to process grayscale photos with gray intensity values ranging from 0 to 255. Before background correction, the BSE pictures underwent preliminary processing, which includes histogram normalization and noise filtering. Once the background of the BSE image became homogeneous, a global threshold was applied to the entire grayscale image. If a pixel’s gray intensity was below the threshold, it was assigned the value 0 (black), while all other pixels were assigned the value 1 (white). The fraction of different phases was determined based on the grayscale differences among distinct phases. The results shown in Figure 6 indicate that the fraction of the primary $\gamma$-phase (primary dendrite region) presents a trend of increasing in the range of small undercoolings (13 K–63 K), for instance, the fraction reaches its summit at 63 K of 70.2% and shows a gradual decrease up to 68.9% at $\Delta T$ = 80 K. By contrast, fraction of the ${\gamma }^{″}$-phase shows a downward direction in this range of undercooling, in which the phase fraction changes from 9.2% to 6.3% at $\Delta T$ = 13–80 K.

3.5. Mechanical Properties

Several different methods to measure the mechanical properties are known to quantitatively estimate the elasticity, plasticity, hardness, and fragility of alloy samples. In the present work, microhardness by Vickers was measured to obtain averaged values of microhardness over the entire cross-section of the specimen.

Methodically, each sample was tested at room temperature statistically under the same conditions. Our measurements show that the Vickers hardness of the IN718 samples solidified with the undercooling between 13 K and 170 K is in a range from 211 HV 0.2 ± 10 to 244 HV 0.2 ± 17. Within this measurable range of microhardness, the nonlinear dependence on undercooling was found (see Figure 7). The obtained dependence shows a clear trend for microhardness increasing. The dispersion of dendritic structure with $\gamma$-phase increases (see Figure 6); therefore, microhardness increases with undercooling (Figure 7). These measurements are consistent with the downward branch of the dendrite arm spacing at $\Delta T<85$ K, i.e., within the diffusion-limited range of the dendrite growth. The only points at which dispersion decreases and microhardness increases are attributed to the sample solidified $\Delta T=80$ K. In our understanding, around this point, the transition from solute diffusion-limited growth to thermally controlled growth begins. Namely, around this point, the solute trapping starts with the formation of the metastable crystalline structure, which usually leads to improved material properties of alloys [13].

Because the presently measured microhardness gives only averaged values, which give information for every measurement value on several phases, the reason for the appearance of the nonlinearity of Figure 7 might be clarified after nanoindentation.

4. Conclusions

The solidification behavior of droplets and their growth front was investigated using an EML facility with observations recorded by IR pyrometer and a high-speed camera. The solidification kinetics were evaluated within the undercooling range of 13 K to 263 K. This temperature variation corresponds to dendritic growth velocities ranging from $6·{10}^{-3}$ m/s to 16.8 m/s.

In the range of the diffusion-limited growth within which the dispersion of dendrites increases (the dendrite arm spacing decreases), the increase in microhardness was obtained. At the very beginning of the transition to thermally controlled growth, the dispersion decreases (the dendrite arm spacing increases) but the microhardness is still increased. This tendency exists due to the beginning of solute trapping and formation of the metastable phase, exhibiting the improved hardness of samples at higher undercooling.

The transformation of the microstructure in IN718 alloy, shifting from dendrite to grain, and the decreasing of the dendrite arm spacing were discussed with respect to the increasing undercooling. It was found that

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The sharp change in the shape of the recalescence front exists at $\Delta T=170$ K;

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The crystalline microstructure changes at $\Delta T=170$ K;

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The characteristic dendritic arm spacings change by the novel exponential law at $\Delta T>170$ K.

These drastic changes are the result of the transition from solute diffusion-limited to thermally controlled growth predicted by the experimental measurements of the dendrite growth velocity and confirmed by the predictions of the theoretical model. The transition to pure thermally controlled growth is sharp and it occurs at the dendrite growth velocity equal to the speed of solutes diffusion in bulk liquid.