High magnetic field-induced grain refinement of undercooled Inconel 718 superalloy

ABSTRACT

The undercooled solidification of the Inconel 718 superalloy under high magnetic field was performed for the first time at high undercoolings (∼ 200 °C). The results show that the high magnetic field can significantly refine the grains of the undercooled alloys, with the average grain size decreasing from 241 ± 92 μm at 0 T to about a third under high magnetic fields (3 T ∼ 9 T). Detailed EBSD analysis provides clear evidence that Icosahedral Short-Range Order (ISRO) enhanced nucleation occurs. The present work opens up a new way for grain refinement of alloys.

GRAPHICAL ABSTRACT

IMPACT STATEMENT

This work shows for the first time that an applied high static magnetic field can induce significant grain refinement in the undercooled IN718 superalloy, and provides clear evidence that ISRO-enhanced nucleation occurs.

KEYWORDS:

• Nickel-based superalloys
• grain refinement
• undercooled solidification
• high magnetic field
• icosahedral short-range order

1. Introduction

Due to their excellent mid-temperature properties and low production cost, nickel-based polycrystalline superalloys, one of which is Inconel 718 (IN718) alloy, are widely used as various components in jet engines, gas turbines, and electric and nuclear power plants [1–3]. Grain refinement is an important method to strengthen the mechanical properties of materials under intermediate temperature and many efforts have been made in the development of fine grain casting processes [3–6]. Among many developed grain refinement strategies, deep undercooling rapid solidification technology is an effective method to refine grains spontaneously [7,8], which has been extensively investigated since it was reported by Walker [9] in pure Ni. Numerous findings have shown that during undercooled solidification, the alloy usually undergoes two grain refinement stages with the increase of undercooling, which occur at lower and higher undercooling [8,10–15]. However, grain refinement caused by undercooling usually faces two issues. First, it is relatively difficult to obtain a high degree of undercooling, which often requires electromagnetic levitation and other technologies, or the increase of undercooling is limited. Second, the size or weight of the samples used within those techniques is in general small in order to produce a greater degree of undercooling.

In recent years, with the development of superconducting technology, superconducting magnets with 10 T or even higher magnetic field intensity have been commercialized. Solidification science under high magnetic field has been developed rapidly. It has been reported that applying a high magnetic field during directional solidification of alloys can cause the columnar-to-equiaxed transition [16], the refinement of microstructure [17], the reduction of segregation [18], and so on. In the case of bulk solidification, the applied high magnetic field can lead to the refinement of primary phase [19], the change of crystal orientation [20] and the improvement of properties [21–23]. It is evident that high magnetic field has become a potential means to control the solidification process of alloys.

Based on this, in the present work, a high magnetic field of up to 10 T was applied for the first time in the undercooled solidification of IN718 superalloy. The effect of high magnetic field on the grain structure of undercooled alloys was investigated. Meanwhile, combined with detailed electron backscattered diffraction (EBSD) analysis, the possible mechanisms of grain refinement of undercooled alloys under high magnetic field were discussed.

2. Experimental method

Figure 1(a) shows a schematic of the experimental facility for undercooling and solidification of alloys under high magnetic field. Details on the apparatus can be found in our previous paper [24]. Cylindrical samples of the IN718 alloy (see its chemical composition in Table S1 in supplementary material) with a diameter of 6 mm and a height of 8 mm were used for the experiments. Each sample placed in a quartz tube with boron trioxide flux was located in the center of the resistance furnace and superconducting magnet (Cryogenic Ltd, 12 T). The undercooled solidification of each sample was carried out under a static magnetic field. Heated to and maintained at 1420 °C for 30 min, each sample was then cooled to 1000 °C at a rate of 10 °C/min. After achieving the required undercooling using this repeated melting/freezing cycle, the quartz tube and sample were quenched in water from 1000 °C. Sets of undercooling experiments were conducted for different magnetic field intensities. The in-situ temperature-time profile was recorded on-line by a two-color pyrometer (ISR50-LO, IMPAC) with a sampling frequency of 50 Hz and an accuracy of ±2 °C. An example of the temperature-time profile obtained by this infrared pyrometer is shown in Figure 1(b). The indicated recalescence peak (T_R) in Figure 1(b) allows to read the undercooling ΔT. In order to investigate the effect of high magnetic field on the grain refinement of undercooled alloys, the undercooling ΔT of the IN718 alloy is indicated in Figure 1(c) being approximately the same for different magnetic fields. The applied magnetic field intensities discussed in this paper were 0 T, 3 T, 6 T and 9 T, respectively, with the corresponding undercoolings of 206 °C, 214 °C, 206 °C and 203 °C, as shown in Figure 1(c). Note that there are two reasons for choosing an undercooling of ∼ 200 °C in the experiment. First, according to previous studies [25,26], when undercooling exceeds 90 °C, the grain size of the IN718 alloy decreases with the increase of undercooling. Second, the maximum undercooling that our experimental facility can achieve is about 230 °C.

Figure 1. (a) Schematic illustration of the experimental facility for undercooling and solidification of alloys under high magnetic field. (b) An example of the temperature-time profile obtained by an infrared pyrometer. The heating rate is 0.62 °C/s, and the cooling rate is 0.16 °C/s. The required undercooling is achieved through repeated melting/freezing cycles. (c) Cooling and recalescence curves of the IN718 alloy showing approximately the same undercooling under different high magnetic fields.

After the experiment, the cross-section at the center of the as-solidified sample, which was a plane perpendicular to the direction of the imposed magnetic field, was taken for further analysis. The cut cross-section (∼2 mm thick) was mounted, ground and polished by standard metallographic methods, and then electro-polished for EBSD characterization. EBSD measurements were performed in a Zeiss Gemini field-emission scanning electron microscope (Sigma 300) equipped with an EBSD detection (Nordlys Nano, Oxford). An accelerating voltage of 20 kV, a camera binning of 4 × 4 and a step size of 0.64 μm ∼ 4 μm were selected for EBSD test. Analysis of the EBSD raw data was performed with the EDAX OIM software. The grain boundaries (GBs) were classified into low-angle grain boundaries (LAGBs) with a misorientation angle between 2° to 15° and high-angle grain boundaries (HAGBs) with a misorientation angle > 15°. In HAGBs, two types of coincidence site lattice (CSL) boundaries [27], also known as twin boundaries (TBs), were identified, namely Σ3 GBs (60° rotation around a <111> direction, with a 5° tolerance) and Σ9 GBs (38.5° rotation around a <110> direction, with a 5° tolerance).

3. Results and discussion

Figure 2 shows EBSD inverse pole figure (IPF) maps with grain boundary overlay of the IN718 alloy solidified at approximately the same undercooling under different high magnetic fields. LAGBs and HAGBs are represented by white and black lines, respectively. It can be seen that the grain structures of the undercooled samples are significantly refined under high magnetic field. The mean grain size (diameter) under a 0 T magnetic field is 241 ± 92 μm, while under 3 T, 6 T and 9 T magnetic fields, the mean grain size is reduced to 89 ± 50 μm, 83 ± 42 μm and 79 ± 32 μm, respectively (see the grain size distribution in Figure S1 in supplementary material). The average length fraction of HAGBs and LAGBs with no applied magnetic field is determined as 53.6% and 46.4%, respectively. While under a high magnetic field (i.e. 3 T, 6 T and 9 T), the average length fraction of LAGBs is reduced and the average length fraction of HAGBs is increased, e.g. HAGBs of 76.0% and LAGBs of 24.0% under a 3 T magnetic field. Also of interest is the proportion of GBs with misorientation angles less than 5°, which is often associated with dendrite remelting and will be further analyzed in the discussion section. It can be observed that the percentage of GBs with misorientation angles less than 5° is determined as high as 34.3% when no magnetic field is applied. Under high magnetic fields, the percentage of GBs with misorientation angles less than 5° is decreased significantly.

Figure 2. EBSD IPF maps with grain boundary overlay of the IN718 alloy solidified at a given undercooling under different high magnetic fields. LAGBs (misorientation angle: 2°∼15°) and HAGBs (misorientation angle: >15°) are represented by white and black lines, respectively. (a) 0 T, (b) 3 T, (c) 6 T, (d) 9 T.

An effort was further conducted to investigate the mechanism of high magnetic field-induced grain refinement in undercooled alloys. First, from the histogram of GB misorientation angle distribution, as shown in Figure 3(a1) ∼ (a4), it can be observed that there is an excess of TBs (i.e. Σ3 GBs and Σ9 GBs) in the undercooled samples with or without an applied magnetic field. And the fraction of Σ3 GBs and Σ9 GBs is very different under various magnetic field intensities. Under a 0 T magnetic field, the average length fraction of Σ3 GBs and Σ9 GBs is determined as 16.9% and 4.6%, respectively. When applying 3 T, 6 T and 9 T magnetic fields, the average length fraction of Σ3 GBs is significantly increased to 29.9%, 29.4% and 26.5%, and the average length fraction of Σ9 GBs is dramatically increased to 16.7%, 13.3% and 11.5%, respectively. In other words, the applied magnetic field increases the proportion of TBs of the undercooled alloys at approximately the same undercooling. Actually, in previous studies, many reports on the undercooling behavior of binary and multicomponent alloys (e.g. Fe-Ni [10], Ni-B [11], Ni-Cu [12], Ni-Fe-Pb [13], Ni-Cu-Co [8], CoCrFeNi high-entropy alloy [15] and Inconel 600 superalloy [14], etc.) have shown that Σ3 TBs occur in undercooled alloys at high undercooling degrees. These twins are coherent twins, with straight GBs and lenticular morphology, which are consistent with the characteristics of annealing twins. Therefore, it has become a consensus that recrystallization is the main mechanism of grain refinement under high undercooling [8]. However, in this study, with or without an applied magnetic field, almost all the TBs are not straight, and the characteristics of twins and grain boundary morphology do not conform to the recrystallization mechanism. This means that there is a new mechanism for grain refinement in undercooled alloys induced by high magnetic field.

Figure 3. (a1)∼(a4) GB misorientation angle distributions and corresponding GB distribution maps for the IN718 alloy solidified at a given undercooling under different high magnetic fields. The red, blue and black lines denote Σ3, Σ9 and random boundaries, respectively. The gray lines represent LAGBs. (b1) ∼ (b4) EBSD IPF maps of selected several nearest neighbor grains with twin orientation relationships. (c1) ∼ (c4) The <110> pole figures corresponding to b1, b2, b3 and b4, respectively, show the orientation relationships between pairs of grains 1-2, 2-3, 3-4, 4-5, 5-1. The red arcs of circles indicate common 111 twin planes. The 5-fold symmetry axis (i.e. common pole) is located at the intersection of the twin planes, highlighted by the black dashed circle. (d1)∼(d4) A schematic icosahedron whose facets have the same number and color as the grains in c1, c2, c3 and c4, respectively. (a1)∼(d1) 0 T, (a2)∼(d2) 3 T, (a3)∼(d3) 6 T, (a4)∼(d4) 9 T.

Another mechanism capable to generate Σ3 TBs during solidification has been reported to be the local ordering of the liquid, commonly known as Icosahedral Short Range Order (ISRO), which has been proven via in-situ X-ray diffraction in levitated liquid alloys [28,29]. The new grains nucleate by an ISRO-dominated nucleation mechanism, and present a Σ3 twin relationship and multiple <110> five-fold symmetries axes [30]. In order to determine whether this mechanism is the cause of the formation of Σ3 TBs in this paper, the orientation relationships of adjacent grains are investigated. Several clusters of nearest-neighbor grains with multiple-twin orientation relationships have been found for those undercooled samples within different magnetic fields. Figure 3(b1) ∼ (b4) respectively show examples of such clusters of twinned grains within the solidified samples of undercooled melts, which were processed in magnetic fields of various intensities. Those figures are extracted from the EBSD IPF maps from Figure 2, and each grain is numbered. The twin orientation relationships between pairs of grains belonging to each cluster are present in <110> pole figures shown in Figures S2 ∼ S5 in supplementary material. The pairs of grains 1-2, 2-3, 3-4, 4-5, 5–1 exhibit twin orientation relationships, and share a common <110> five-fold symmetry axes, highlighted with the black dotted lines in the <110> pole figures where the red arcs of circles indicate common 111 planes, as shown in Figure 3(c1) ∼ (c4). To clearly link these orientation relationships, a perfect icosahedron with 10 visible triangular facets is plotted schematically in Figure 3(d1) ∼ (d4), respectively. The colors and labeled numbers of these facets were chosen to be consistent with the IPF coloring of the grains displayed in Figure 3(b1) ∼ (b4). Obviously, grains 1, 2, 3, 4, 5 are rotated around a common <110> direction (i.e. five-fold symmetry axes). It can thus be concluded that, the ISRO-dominated nucleation mechanism is effective for the nucleation of the IN718 alloy under high undercooling.

Further, the grain refinement of the undercooled alloys induced by high magnetic field may also be attributed to the effect of magnetic field on ISRO of the liquid. Since it is currently difficult to characterize the liquid structure of undercooled IN718 alloy under high magnetic field by neutron diffraction experiments or molecular-dynamics simulations, here we propose a possible mechanism. When there is no applied magnetic field, the undercooled liquid includes a small amount of ISRO, which acts as nucleation sites for the nucleation and growth of new grains (see Figure 5(a)). In this case, only a few TBs are present in the as-solidified sample (see Figure 3(a1)). After applying the magnetic field, the number of ISRO in the undercooled melt is increased, and thus the corresponding nucleation sites are increased, leading to the increase of the fraction of TBs as well as significant grain refinement, as illustrated in Figure 3(a1) ∼ (a4) and schematic Figure 5(b).

In addition, from the EBSD IPF maps with drawn GBs, as shown in Figure 4(a1) and (a2), it can be clearly noticed that some small grains are wrapped inside some large grains (see the red dotted line in Figure 4(a1) and the red arrows in Figure 4(a2)). Figure 4(b1) and (b2) are the corresponding kernel average misorientation (KAM) maps. The KAM maps have been used here to describe the degree of plastic deformation or local strain distribution within grains [31,32]. In the absence of the magnetic field, as displayed in Figure 4(a1), a large number of LAGBs (i.e. ∼ 46.4% shown in Figure 2(a)) or sub-grains exist within grains. This can be explained by the mechanism of dendrite remelting and fragmentation, which is most widely accepted for the formation of LAGBs or sub-grains in undercooled alloys [8,15,33]. During recalescence, due to the release of latent heat, the dendrite arms are remelted and broken. The fragmented dendrite arms are wrapped inside the original grains and have a low-angle misorientation with the original grains. Please note that a few small grains are also observed inside the large grains, with small KAM values and no strain, as shown in the red dotted lines in Figure 4(a1) and (b1), which are likely to be formed by rotation of fragmented dendrite arms under the influence of convection and other factors.

Figure 4. (a1) and (a2) EBSD IPF maps with drawn grain boundaries. The black and white lines denote HAGBs and LAGBs, respectively. (b1) and (b2) Corresponding KAM maps. (a1) and (b1) 0 T, (a2) and (b2) 9 T.

In contrast, at approximately the same undercooling, when a high magnetic field is applied, the proportion of LAGBs inside the grains is decreased (see Figure 2(b) ∼ (d)) and many small grains appear inside the grains. Here, taking the application of a 9 T magnetic field as an example, as illustrated in Figure 4(a2), many small grains can be clearly observed inside the grains. The corresponding KAM map shows that a few small grains with small KAM values and no strain are observed inside the large grains (see the red dotted line in Figure 4(a2) and (b2)), which is the same as that in the absence of the magnetic field. While a large number of small grains have large KAM values and significant strain, as shown in the red arrows in Figure 4(a2) and (b2). The 3 T and 6 T cases are similar to the case of 9 T (see Figures S6 and S7 in supplementary material). Thermoelectric magnetohydrodynamics (TEMHD) [34–39] can well explain the above differences. In the case of undercooled growth, the inherent thermoelectric currents, illustrated in Figure 5(c) [40], generate in the mushy zone of the alloy due to the various Seebeck coefficients of the solid and liquid phases and the inevitable thermal gradients [35]. In this paper, as schematically shown in Figure 5(d), when a high magnetic field is imposed, the growing undercooled dendrite arms are fragmented by the thermoelectric Lorentz force (TELF), and the fragmented dendrite arms are deflected by the TELF and new convection produced by TELF, resulting in the formation of large misorientations and new small grains wrapped inside the grains. At the same time, the remelted dendrite arm fragments also undergo significant deflection under the action of new flow and TELF, leading to the reduction of LAGBs or sub-grains (see Figure 2) and the formation of new grains wrapped inside the grains. It should be noted that the dendrite arms fragmented by TELF have undergone deformation, and the small grains formed by these fragments have obvious strain, while the dendrite arms fragmented by remelting have not experienced plastic deformation, so there is no significant strain within the grains formed by them, which is consistent with the results shown in KAM maps.

Figure 5. Schematic diagram showing the mechanism of high magnetic field-induced grain refinement in an undercooled nickel-based superalloy. (a) Under no magnetic field, (b) and (d) under a high magnetic field, (c) thermoelectric currents around a growing undercooled equiaxed dendrite [40].

4. Conclusion

To conclude, the grain refinement of undercooled IN718 superalloy induced by high magnetic field has been found for the first time. The results show that the high magnetic field can significantly refine the grains of the undercooled alloys, with the average grain size decreasing from 241 ± 92 μm at 0 T to about a third under high magnetic fields (3 T ∼ 9 T). An excess of TBs (i.e. Σ3 GBs and Σ9 GBs) is also observed in undercooled samples and the proportion of TBs is increased dramatically under high magnetic field. Detailed EBSD analysis provides clear evidence that ISRO enhanced nucleation occurs. The applied magnetic field may change the liquid structure and increase the number of ISRO in the undercooled melt, leading to significant grain refinement. In addition, in terms of dendrite growth, the TELF breaks the dendrite arms, and these fragments and remelted dendrite arms become new grains under the influence of TEMHD, which are wrapped inside the large grains, leading to further grain refinement. These results open up a new way for grain refinement of alloys.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was financially supported by the Natural Science Foundation of China (52031012, 51690163) and Science and Technology Innovation Team Plan of Shaan Xi Province (2021TD-17).