Insight into tuning of ZrO2 distribution and mechanical properties of directionally solidified Al2O3/(5Re0.2)AG/ZrO2 eutectic ceramic composites

doi:10.21203/rs.3.rs-3137798/v1

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Insight into tuning of ZrO2 distribution and mechanical properties of directionally solidified Al2O3/(5Re0.2)AG/ZrO2 eutectic ceramic composites

https://doi.org/10.21203/rs.3.rs-3137798/v1

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A novel Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂((5Re_0.2)AG)/ZrO₂ eutectic high entropy oxide ceramic composites (HEOCs) was prepared by the directional solidification technique. The Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs had a refined microstructure and different crystallographic orientation relationships of < 10–10 > Al₂O₃ || <103>(5Re_0.2)AG || <100 > ZrO₂, {11–20}Al₂O₃ || {100}(5Re_0.2)AG || {100}ZrO₂, {0001} Al₂O₃ || {103}(5Re_0.2)AG || {100}ZrO₂ compared to Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. ZrO₂ in the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs was distributed more uniformly and dispersedly than in Al₂O₃/YAG/ZrO₂ eutectic ceramic composites due to the similar volume strain of Al₂O₃/(5Re_0.2)AG, (5Re_0.2)AG/ZrO₂ and Al₂O₃/ZrO₂. As a result, mechanical performance including hardness, elastic modulus, and fracture toughness had been greatly improved because of the refined microstructure, tailored interfacial structure, and homogeneous distribution of ZrO₂ caused by the introduction of the high entropy (5Re_0.2)AG.

directional solidification

high-entropy oxide ceramic composites

eutectics

orientation relationship

interfacial structure

High-performance aero-engines put forward higher requirements on the temperature-bearing capacity of turbine blade materials. For example, the turbine inlet temperature of an engine with a thrust-to-weight ratio of 10 or more is up to 1600 ^oC-1650 ^oC [1]. Directionally solidified Al₂O₃-based eutectic ceramic composites with high melting point (~ 1800 ^oC), outstanding high-temperature creep and oxidation resistance are considered to be one of the next generation of ultra-high temperature structural materials [2, 3]. Previous studies have shown that if they can be applied as a guide vane or combustion chamber liner, it could significantly improve the thrust-to-weight ratio of an engine and thermal efficiency will also increase by 9% [4]. However, the poor fracture toughness and high-temperature strength limit their application as a high-temperature structural material.

Several classes of Al₂O₃-based directionally solidified ceramic composites have been widely studied, such as binary eutectics Al₂O₃/Y₃Al₅O₁₂ [5, 6], Al₂O₃/Ed₃Al₅O₁₂ [7, 8], Al₂O₃/GdAlO₃ [9–11], Al₂O₃/SmAlO₃ [12, 13] and ternary eutectics Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ [14, 15], Al₂O₃/Ed₃Al₅O₁₂/ZrO₂ [16]. In general, the fracture toughness of ternary eutectics is higher than that of binary eutectics due to the addition of ZrO₂ [17–21]. The introduction of ZrO₂ makes the ternary eutectic layer finer than the binary eutectic layer under the same solidification process, which improves its bending strength [22]. The crack kinked [20, 23], deflected [23] at ZrO₂, or even was prevented by ZrO₂ [24]. ZrO₂ acts as the toughening phase, and its morphology, size and distribution play a decisive role in the toughening effect.

However, according to the morphology of directionally solidified Al₂O₃/YAG/ZrO₂ eutectic ceramic composites reported in literature [15, 20, 25–30], it is found that ZrO₂ was mainly distributed between Al₂O₃ and YAG in the form of fine round rods or lamellae, and a small amount located in Al₂O₃, and a very small amount in YAG. It evidently shows that the morphology and distribution of ZrO₂ are not random, but are governed by certain laws. Our previous work found that the distribution of ZrO₂ was mainly dominated by the minimization of the interfacial strain energy. In directionally solidified Al₂O₃/YAG/ZrO₂ eutectic ceramic composites, the mismatch strain of Al₂O₃-YAG, Al₂O₃-ZrO₂, and YAG-ZrO₂ is -0.9%, -0.1% and 0.2%, respectively [15], which imply that the interfacial strain energy of Al₂O₃-YAG is much higher than that of YAG-ZrO₂ and Al₂O₃-ZrO₂. To minimize the total interfacial strain energy of the eutectic system, ZrO₂ diffuses and distributes between Al₂O₃ and YAG with the largest possible surface area (i.e., rod or lamellar) to reduce the interface area formed by YAG and Al₂O₃ phase, which has the largest interfacial strain energy. This heterogeneous distribution of ZrO₂ weakens its strengthening and toughening effect. It is expected that ZrO₂ can be homogeneously distributed in Al₂O₃, YAG, and between the two, to achieve the best strengthening and toughening effect. Therefore, the authors envision if the distribution of ZrO₂ could be modified by tuning the interfacial mismatch.

The solid solution will lead to lattice distortion, which becomes more severe as the number of solid solution elements increases [31]. Lattice distortion is bound to cause changes in the d-spacing. Therefore, the solid solution seems to be an effective way to tailor the interfacial mismatch. Recently, high-entropy ceramic composites, which are commonly referred to as a solid solution formed by four or more ceramic components, have shown unexpected properties such as high hardness, superior elastic modulus as well as high fracture toughness [32]. For example, the fracture toughness of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)(C_0.5N_0.5) is 8.4 MPa·m^1/2, much higher than that of binary ceramic composites [33]. The fracture toughness of (Zr_0.25Nb_0.25Ti_0.25V_0.25)C is up to 4.7 ± 0.5 MPa·m^1/2, larger than that of the individual component [32]. The above studies suggest that in addition to raising the performance of high-entropy ceramic composites, the solid solution may also tune the interfacial structure formed by the high-entropy with other phases through lattice distortion, thereby further optimizing the mechanical properties.

In the present work, the YAG phase in Al₂O₃/YAG/ZrO₂ eutectic ceramic composites is replaced by an oxide solid solution (Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂, and directionally solidified Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂((5Re_0.2)AG)/ZrO₂ ternary eutectic high entropy oxide ceramic composites (HEOCs) is prepared with an optical floating zone furnace. The as-prepared eutectic HEOCs are composed of Al₂O₃, the (5Re_0.2)AG, and ZrO₂ single-crystal phases. The effect of the lattice distortion of (5Re_0.2)AG on the distribution of ZrO₂ and interfacial structure is reported for the first time. Mechanical properties including nano/Vickers hardness, elastic modulus, and fracture toughness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs are studied and compared with those of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. These results may cast light for optimizing the mechanical properties of Al₂O₃-based eutectic ceramic composites.

2.1 Preparation of samples and direction solidification

Commercially available ceramic powders of Re₂O₃ (Re = Y, Lu, Ho, Er and Yb, 99.99% purity), ZrO₂ (99.99% purity), and Al₂O₃ (99.95% purity) were used to prepare the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs. Al₂O₃, Re₂O₃ and ZrO₂ powders were mixed with a molar ratio of 65:16:19 according to the eutectic point in the ternary phase diagram [34, 35]. Re₂O₃ powders were mixed in equal molar fractions. Then the prepared powder was wet ball milled with alcohol for 12 h to ensure homogeneity. The slurry was dried and sieved through a 120-mesh net to obtain the requested powders. Precursors were prepared by holding at 45 MPa for 5 minutes in a stainless-steel mold with a dimension of 10 × 10 × 100 mm³ and then sintering without pressure in a muffle furnace at 1823 K for 2.5 h.

The directional solidification process was carried out in an optical floating zone (OFZ) furnace with a slight overpressure (~ 1.1 bars) in an argon atmosphere. During the preparation process, precursors were melted using four 3 kW xenon arc lamps with a withdrawal rate of 20 mm/h. The prepared crystals measured up to 120 mm in length and about 10 mm in diameter.

2.2 Microstructure characterization

The as-prepared Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOC bars and Al₂O₃/YAG/ZrO₂ eutectic bars were sliced by wire-cutting equipment. Samples were well grounded with 600#, 1000# diamond discs, and SiC paper up to 2000#. They were further polished using 2.5 µm diamond paste. TEM samples were prepared using a dual-beam focused ion beam (FIB, Thermo Scientific Scios 2, USA).

The phase composition was analyzed employing an X-ray diffractometer (LabX XRD-6000, Japan). The microstructure was characterized using scanning electron microscopy (SEM, JSM-6700 F, JEOL, Japan). Orientation relationships were determined with electron-back scattered diffraction (EBSD, NordlysNano, Oxfordshire, UK). A 300 kV transmission electron microscopy (TEM, FEI Talos F300C, U) equipped with an energy dispersive X-ray spectroscopy (EDS) analysis unit was used for high-resolution transmission electron microscopy (HRTEM) observation and EDS elemental analysis.

2.3 Mechanical properties testing

The Vickers hardness was obtained using a Vickers Indenter (HVS-10AT, China) when loaded at 2.94, 4.9, 9.8, 19.6, 29.4, and 49 N. The holding time is 15 s. In addition, the fracture toughness (K_IC) was measured on the basis of the indentation method using the following equation[36]:

K_IC = 0.016 × (E/H_V)^1/2p/c^3/2 (1)

where E is Young's modulus, H_V is the Vickers hardness, c is the half-length indentation diagonal and p is the indentation load. Nanoindentation tests were performed on well-polished sample surfaces employing a Nano-Indenter G200 (KLA, USA) system equipped with a diamond Berkovich indenter with a 20 nm tip radius to evaluate their nanohardness and elastic modulus. The maximum load for nanoindentation mapping was 29 mN and the indentation spacing was 10 µm.

3.1 Microstructure of the Al₂O₃/(5Re_0.2)AG/ZrO₂ ternary eutectic HEOCs

The composition of the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs was investigated by the XRD technique and compared with Al₂O₃/YAG/ZrO₂ eutectic ceramic composites, as depicted in Fig. 1. In comparison with standard PDF cards, the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs consist of three phases: Al₂O₃, the (5Re_0.2)AG and cubic ZrO₂ (c-ZrO₂). Taking a careful observation of the magnified regions b and c, it is found that neither the (1 0 4) peak position of Al₂O₃ nor the (2 0 0) peak position of ZrO₂ shifts, while the (4 2 0) peak position of the (5Re_0.2)AG shifts towards the high angle in contrast to Al₂O₃/Y₃A₅O₁₂/ZrO₂ eutectic ceramic composites. The shift is more pronounced at the high-angle peak position, which means a reduction of d-spacing of the (5Re_0.2)AG phase according to the Bragg equation.

The transverse microstructures of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs and Al₂O₃/YAG/ZrO₂ eutectic ceramic composites are displayed in Fig. 2. Both of the eutectics have a so-called Chinese-script microstructure. The dark area is Al₂O₃ phase, the grey area is the (5Re_0.2)AG or YAG phase and the white area is ZrO₂ phase. Comparing Figs. 2a and c at the same magnification, it is found that the microstructure size of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is finer and more diffuse under the same solidification process. It is about 1/2 to 1/3 of that of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. Especially for ZrO₂ phase, comparing Figs. 2b and d, the size of ZrO₂ tissue in Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is about 1/4 to 1/3 of that in Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. In addition, the morphology and distribution of ZrO₂ in the two eutectics are also different. In Al₂O₃/YAG/ZrO₂ eutectic ceramic composites, ZrO₂ is mainly distributed at the interface between Al₂O₃ and YAG in the form of thin laminate and round rod-like shape, as reported previously [15]. However, in the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs, ZrO₂ is rather well-distributed both in the Al₂O₃, (5Re_0.2)AG and their interface with a tiny round rod-like shape.

The microstructure of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs was further observed by TEM, as shown in Fig. 3a. It can be seen that the three phases are well-boned by interfaces, which are clean and smooth, with no intermediate phase or amorphous phase. According to the different electron diffraction patterns, the white is Al₂O₃, the light gray is the (5Re_0.2)AG, and the dark gray is c-ZrO₂. XRD results signify that there is a change in the d-spacing of the (5Re_0.2)AG compared to the YAG. To verify the XRD result, the d-spacings of both (0 2 0) planes of two were carefully measured based on the electron diffraction patterns, as marked in HRTEM maps (Figs. 3b and c). The d-spacing of (5Re_0.2)AG (0 2 0) plane is 0.5894 nm, smaller than that of the YAG (0 2 0) plane (0.6004 nm). To check our measurement results, the d-spacing of (5Re_0.2)AG (3 0–1) was measured and compared with that of (3 0 1) calculated by measured (0 2 0). The two values were found to be consistent. The results indicate that the (5Re_0.2)AG lattice is negatively distorted compared with YAG, which agrees well with the XRD results.

Scanning transmission electron microscopy (STEM) is employed for the investigation of element distribution of the as-grown Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs, as presented in Fig. 4. According to the EDS line scan results shown in Fig. 4b, five cations (i.e., Y³⁺, Er³⁺, Ho³⁺, Lu³⁺ and Yb³⁺) have almost the same composition of ~ 20% as expected. Some of the Y, Yb, Er, Ho, Lu and Al elements are solved into the ZrO₂ phase, which play a role in stabilizing its cubic crystal structure. Figures 4c-g show a highly homogenous distribution of Y, Er, Yb, Ho, and Lu elements in the (5Re_0.2)AG phase, indicating that the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs are well fabricated.

3.2 Orientation relationships the interfaces

The orientation relationships of the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs are examined by EBSD, and the result is presented in Fig. 5. Figures 5b-d show the preferred growth direction of the three phases, respectively, < 10–10 > for Al₂O₃ (the two colors mean the same direction of < 10–10> [17]), < 103 > for the (5Re_0.2)AG, and < 100 > for ZrO₂. At first glance, Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs has the same preferred growth directions as Al₂O₃/YAG/ZrO₂ eutectic ceramic composites reported in the literature [37]. However, further analysis of the inverse polar figure (lower left corner in Fig. 5c) demonstrates that the preferred growth direction of (5Re_0.2)AG is < 103>, exactly. Therefore, the preferred growth directions are:

< 10–10 > Al₂O₃ || <103> (5Re_0.2)AG || <100 > ZrO₂.

Figure 6 exhibits the pole figures of Al₂O₃, the (5Re_0.2)AG, and ZrO₂ shown in Fig. 5. It can be obtained that the orientation relationships of the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs are:

{11–20} Al₂O₃ || {100} (5Re_0.2)AG || {100} ZrO₂, and

{0001} Al₂O₃ || {103} (5Re_0.2)AG || {100} ZrO₂, as circled by the red circles and black circles in Figs. 6a, b and c, respectively. In general, the orientation relationships of the three eutectic phases are summarized as follows:

< 10–10 > Al₂O₃ || <103> (5Re_0.2)AG || <100 > ZrO₂ (white circles),

{11–20} Al₂O₃ || {100} (5Re_0.2)AG || {100} ZrO₂ (black circles),

{0001} Al₂O₃ || {103} (5Re_0.2)AG || {100} ZrO₂ (red circles).

Notably, the orientation relationships of directionally solidified Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs are different from that of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites [37].

To further investigate the orientation relationship interfacial structure of the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs, thin-foil samples were prepared for the HRTEM tests. Figures 7a, d and g show the HRTEM images of the three interfaces marked by the red squares in Fig. 3. Interface I is Al₂O₃/ZrO₂, interface II is ZrO₂/(5Re_0.2)AG and interface III is Al₂O₃/(5Re_0.2)AG. It can be seen that all three interfaces are clean and well-oriented. From the corresponding selected-area diffraction patterns of the interfaces, depicted in Figs. 7b, e and h, the orientation relationship of the three interfaces are:

Interface I: [1-100] (11–20) Al₂O₃ || [100] (020) ZrO₂,

Interface II: [100] (020) ZrO₂ || [103] (040) (5Re_0.2)AG,

Interface III: [1-100] (11–20) Al₂O₃ || [103] (040) (5Re_0.2)AG,

which are the same as that obtained from the EBSD experiments. These well-oriented interfaces are semi-coherent, as shown in Figs. 7c, f and i. The lattice misfit is accommodated by a series of periodic misfit dislocations. The periods of the misfit dislocation in sequence are 1/13 for Al₂O₃/ZrO₂, 1/7 for ZrO₂/(5Re_0.2)AG and 1/5 for Al₂O₃/(5Re_0.2)AG.

3.3 Mechanical properties

To compare and study the influence of (5Re_0.2)AG instead of YAG, mechanical properties including nanohardness, Vickers hardness, elastic modulus, and fracture toughness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs and Al₂O₃/YAG/ZrO₂ eutectic ceramic composites prepared by the same process were studied at the same time.

Figure 8 shows the Vickers hardness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs and Al₂O₃/YAG/ZrO₂ eutectic ceramic composites under different applied loads. It can be found that the microhardness of both samples decreases with the increasing load. This may be due to the porosity in the ceramic composites. However, the Vickers hardness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is higher than that of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. At a load of 9.8 N, the Vickers hardness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is about 19.7 ± 0.4 GPa and that of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites is about 16.9 ± 0.57 GPa.

The nanoindentation method was used to test the nanohardness and elastic modulus of the two samples, as summarized in Table I. The nanohardness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is 33.17 ± 0.52 GPa, higher than 21.66 ± 0.43 GPa of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. The elastic modulus is 436.1 ± 6.7 GPa, while that of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites is 357.8 ± 8.9 GPa. It is worth mentioning that the elastic modulus is not only higher than that of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites but also higher than that of their component. The fracture toughness is estimated based on the indentation method with a load of 49 N. The fracture toughness is calculated to be 8.55 ± 0.36 MPa/m^1/2, almost 2.5 times that of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites (3.54 ± 0.34 MPa/m^1/2).

Table I. The measured nanohardness, elastic modulus, and fracture toughness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs and Al₂O₃/YAG/ZrO₂ eutectic ceramic composites prepared with the same solidification process.

Samples

Nanohardness (GPa)

Elastic modulus (GPa)

Fracture toughness

(K_IC, MPa/m^1/2)

Al₂O₃/(5Re_0.2)AG/ZrO₂

33.17 ± 0.52

436.1 ± 6.7

8.55 ± 0.36

Al₂O₃/YAG/ZrO₂

21.66 ± 0.43

357.8 ± 8.9

3.54 ± 0.34

4.1 Distribution of ZrO₂

Figure 9 illustrates the combined orientation relationships between Al₂O₃, the (5Re_0.2)AG and ZrO₂ obtained from the above experimental results, together with that of Al₂O₃/YAG/ZrO₂ of our previous work [15] for comparison. The difference was marked with a pink dashed rectangle. It can be seen qualitatively that the mismatch between (0003)Al₂O₃ and (30 − 1)(5Re_0.2)AG in Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is significantly smaller than that between (0003)Al₂O₃ and (004)YAG in Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. That is to say, the mismatch difference between Al₂O₃/ZrO₂ and (5Re_0.2)AG/ZrO₂ in Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is small, but that between Al₂O₃/ZrO₂ and YAG/ZrO₂ in Al₂O₃/YAG/ZrO₂ eutectic ceramic composites is relatively large. Therefore, ZrO₂ in Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs can be distributed in both Al₂O₃, (5Re_0.2)AG, and between them, making ZrO₂ homogenously dispersed without segregation.

According to the measured d-spacing in Figs. 3b and c, the lattice constant of the (5Re_0.2)AG phase is calculated to be 1.1788 nm, smaller than 1.2008 nm of the YAG phase. It can be inferred that the lattice distortion leads to a change in the crystal orientation relationship, as clearly illustrated in Fig. 10. Comparing Figs. 10a and b, the preferred growth direction of (5Re_0.2)AG changes to be [103] and the parallel plane changes to be {-301}. However, the orientation relationship of Al₂O₃ and ZrO₂ remains unchanged.

To quantitatively investigate the effect of the change of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs orientation relationship on the interfacial strain, the 3D lattice matching of the interfaces is investigated by the near-coincident site lattice (NCSL) theory. Al₂O₃ is a rhombohedral structure with a = 0.47544 nm and c = 1.29725 nm. The (5Re_0.2)AG is a cubic structure, with a = 1.1788 nm. And ZrO₂ is a cubic structure with a = 0.51523 nm. All lattice constants are derived from the average of multiple measurements of the d-spacings in HRTEM. In the NCSL analysis, a typically used tolerance factor of 0.05 nm is applied to define 'near-coincident sites' [38]. The projections of the Al₂O₃, ZrO₂ and (5Re_0.2)AG lattices along the parallel axes of [10–10] Al₂O₃ || [100] ZrO₂, [103] (5Re_0.2)AG || [100] ZrO₂ and [10–10] Al₂O₃ || [103] (5Re_0.2)AG, are displayed in Figs. 11a, b and c, respectively. The so-called smallest NCSL models along the parallel axes are formed by vectors, as listed in Table II.

Table II. The smallest NCSL model of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs formed by vectors along the parallel axes.

[10–10] Al₂O₃ \|\| [100] ZrO₂	[103] (5Re_0.2)AG \|\| [100] ZrO₂	[10–10] Al₂O₃ \|\| [103] (5Re_0.2)AG
u₁ = 4[001]ZrO₂ ≈ 5[10–10]Al₂O₃	u₁'=5[001]ZrO₂ ≈ 7[103](5Re_0.2)AG	u₁"=11[103](5Re_0.2)AG ≈ 10[10–10]Al₂O₃
u₂ = 5[100]ZrO₂ ≈ 6[0003]Al₂O₃	u₂'=5[100]ZrO₂ ≈ 7[30 − 1](5Re_0.2)AG	u₂"=7[30 − 1](5Re_0.2)AG ≈ 6[0003]Al₂O₃
u₃ = 6[010]ZrO₂ ≈ 13[1-210]Al₂O₃	u₃'=7[010]ZrO₂ ≈ 12[040](5Re_0.2)AG	u₃"=4[040](5Re_0.2)AG ≈ 5[1-210] Al₂O₃

The volumetric strain between the NCSL models referred to each lattice can be calculated by [40]

$$\begin{array}{c} {\epsilon }_{{v}_{{Al}_{2}{O}_{3}-Zr{O}_{2}}}= \frac{{V}_{NCSL-{Al}_{2}{O}_{3}}-{V}_{NCSL-Zr{O}_{2}}}{{(V}_{NCSL-{Al}_{2}{O}_{3}}+{V}_{NCSL-Zr{O}_{2}})/2}\#\left(2\right)\end{array}$$

$$\begin{array}{c}{\epsilon }_{{v}_{{Al}_{2}{O}_{3}-\left(5{Re}_{0.2}\right)AG}}= \frac{{V}_{NCSL-{Al}_{2}{O}_{3}}-{V}_{NCSL-\left(5{Re}_{0.2}\right)AG}}{{(V}_{NCSL-{Al}_{2}{O}_{3}}+{V}_{NCSL-\left(5{Re}_{0.2}\right)AG})/2}\#\left(3\right)\end{array}$$

$$\begin{array}{c}{\epsilon }_{{v}_{{ZrO}_{2}-\left(5{Re}_{0.2}\right)AG}}= \frac{{{V}_{NCSL-Zr{O}_{2}}-V}_{NCSL-\left(5{Re}_{0.2}\right)AG}}{{(V}_{NCSL-\left(5{Re}_{0.2}\right)AG}+{V}_{NCSL-Zr{O}_{2}})/2}\#\left(4\right)\end{array}$$

where ${V}_{NCSL-{Al}_{2}{O}_{3}}$, ${V}_{NCSL-Zr{O}_{2}}$and ${V}_{NCSL-\left(5{Re}_{0.2}\right)AG}$ are volumes of the NCSL unit cells of the Al₂O₃, ZrO₂ and (5Re_0.2)AG lattices, respectively. The volumetric strain is computed to be 0.576% for ${\epsilon }_{{v}_{{Al}_{2}{O}_{3}-Zr{O}_{2}}}$, 0.668% for ${\epsilon }_{{v}_{{Al}_{2}{O}_{3}-\left(5{Re}_{0.2}\right)AG}}$ and − 0.604% for ${\epsilon }_{{v}_{{ZrO}_{2}-\left(5{Re}_{0.2}\right)AG}}$. Calculations indicate that the volume strains of the three phases are almost identical, implying the almost same interfacial strain energy.

For comparison, the smallest NCSL model of the Al₂O₃/YAG/ZrO₂ eutectic ceramic composites along the parallel axes is also formed by vectors, as detailed in Table III. Moreover, the volume strain is calculated to be 0.576% for ${\epsilon }_{{v}_{{Al}_{2}{O}_{3}-Zr{O}_{2}}}$, -2.3% for ${\epsilon }_{{v}_{{Al}_{2}{O}_{3}-YAG}}$, and 0.295% for ${\epsilon }_{{v}_{{ZrO}_{2}-YAG}}$. Apparently, the value of ${\epsilon }_{{v}_{{Al}_{2}{O}_{3}-YAG}}$ is an order of magnitude greater than the other two. The results verified the results previously reported in the literature [15]. According to the principle of minimizing the interfacial strain energy of the system, ZrO₂ tends to distribute between Al₂O₃ and YAG, so as to suppress the formation of the Al₂O₃/YAG interface having high interfacial strain. However, the volume strain differences of the three interfaces in Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs are very small, making ZrO₂ phase uniformly distributed in Al₂O₃, the (5Re_0.2)AG, and between them.

Table III. The smallest NCSL model of Al₂O₃/YAG/ZrO₂ eutectic ceramic composites formed by vectors along the parallel axes.

[10–10] Al₂O₃ \|\| [100] ZrO₂	[100] YAG \|\| [100] ZrO₂	[10–10] Al₂O₃ \|\| [100] YAG
u₁ = 4[001]ZrO₂ ≈ 5[10–10]Al₂O₃	u₁'=7[001]ZrO₂ ≈ 12[004]YAG	u₁"=4[004]YAG ≈ 3[10–10]Al₂O₃
u₂ = 5[100]ZrO₂ ≈ 6[0003]Al₂O₃	u₂'=7[100]ZrO₂ ≈ 12[400]YAG	u₂"=3[400]YAG ≈ 2[0003]Al₂O₃
u₃ = 12[020]ZrO₂ ≈ 13[1-210]Al₂O₃	u₃'=7[020]ZrO₂ ≈ 6[040]YAG	u₃"=4[040]YAG ≈ 5[1-210] Al₂O₃

4.2 Effect of microstructure on mechanical performance

It is obvious that the mechanical properties of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs are better than Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. The reasons are generally attributed to (i) the substitution of YAG with (5Re_0.2)AG, (ii) tailored interfacial structure, and (iii) the homogeneous distribution of ZrO₂.

As shown in Fig. 2, the eutectic microstructure of directionally solidified Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs is much finer compared to Al₂O₃/YAG/ZrO₂ eutectic ceramic composites prepared with the same preparation process. This can be attributed to the 'hysteresis diffusion' effect of high-entropy materials [39]. During the liquid-solid transition, the atoms of each group diffuse corporately from a chaotic state to an ordered state. There are more atoms in the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs, leading to a relatively low diffusion rate and thus inhibiting the growth of the lamellae. Therefore, compared with Al₂O₃/YAG/ZrO₂, the interfacial area per unit area is more in Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs. The increase of planar defects will undoubtedly increase its hardness. Furthermore, as demonstrated by the XRD and TEM results, due to the reduced lattice constant (r₀) of (5Re_0.2)AG compared to YAG, the lattice energy of (5Re_0.2)AG becomes higher according to the Born–Landé equation [40]:

$$\begin{array}{c}{E}_{n}=-\frac{{N}_{A}M{z}^{+}{z}^{-}{e}^{2}}{4\pi {\epsilon }_{0}{r}_{0}}\left(1-\frac{1}{n}\right) \#\left(5\right)\end{array}$$

where N_A is Avogadro constant, M is Madelung constant, z^+/− is cationic/anion charge, ε₀ is vacuum permittivity (a constant), n is Born index (= 5 ~ 12), and r₀ is the distance between the nearest two ions. Therefore, the higher lattice bonding energy leads to higher hardness and elastic modulus of (5Re_0.2)AG. This is another pronounced reason for the improved hardness and elastic modulus of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs. In addition, since the introduction of the (5Re_0.2)AG, the interfacial structures are tailored, making ZrO₂ distribute finer and more dispersed. Fine ZrO₂ is distributed in both Al₂O₃ and (5Re_0.2)AG, as well as between Al₂O₃ and (5Re_0.2)AG, improving the fracture toughness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs.

In summary, Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs were successfully prepared by the directional solidification method. The ceramic was composed of Al₂O₃, the (5Re_0.2)AG and ZrO₂ phases. There was a lattice distortion in the (5Re_0.2)AG phase. The orientation relationship of the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs was determined to be [1-100] Al₂O₃ || [100] ZrO₂ || [103] (5Re_0.2)AG, (11–20) Al₂O₃ || (020) ZrO₂ || (040) (5Re_0.2)AG, different from that of the Al₂O₃/YAG/ZrO₂ eutectic ceramic composites. According to the NCSL model, the volume strain of Al₂O₃/ZrO₂, Al₂O₃/(5Re_0.2)AG and ZrO₂/(5Re_0.2)AG phases was 0.576%, 0.668% and − 0.604%, respectively. The values were comparable, unlike the Al₂O₃/YAG/ZrO₂ eutectic ceramic composites where the volume strain of Al₂O₃/YAG was an order of magnitude larger than the other two. As a result, ZrO₂ was mostly diffusely and uniformly distributed in Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs in the form of fine round rods. The hardness, elastic modulus, and fracture toughness of Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs were significantly improved. This may be contributed to the solid solution (5Re_0.2)AG, which tailored the orientation relationships of the Al₂O₃/(5Re_0.2)AG/ZrO₂ eutectic HEOCs, redistributed ZrO₂, refined the microstructure.

Data availability

The data that support the findings of this study are available from the corresponding author, Doc. Xu Wang, [email protected], upon reasonable request.

Author contribution

Xu Wang supervised the project. Yujie Zhong and Xu Wang designed the experiments. Zhe Li performed the experiments. All authors discussed experiments and results. Yujie Zhong wrote the draft. Yujie Zhong and Xu Wang revised the manuscript. All authors have given approval for the final version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (grant numbers 52171046, 51804252), the Shaanxi Provincial Key Research and Development Program, China (No. 2021GY-133), and the Postgraduate Innovation and Practical Ability Training Program of Xi'an Shiyou University (No. YCS21212142).

Conflict of interest The authors declare no competing interests.

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Insight into tuning of ZrO2 distribution and mechanical properties of directionally solidified Al2O3/(5Re0.2)AG/ZrO2 eutectic ceramic composites

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Abstract

Figures

1. Introduction

2. Experimental section

2.1 Preparation of samples and direction solidification

2.2 Microstructure characterization

2.3 Mechanical properties testing

3. Results

3.1 Microstructure of the Al₂O₃/(5Re_0.2)AG/ZrO₂ ternary eutectic HEOCs

3.2 Orientation relationships the interfaces

3.3 Mechanical properties

4. Discussion

4.1 Distribution of ZrO₂

4.2 Effect of microstructure on mechanical performance

5. Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

Insight into tuning of ZrO2 distribution and mechanical properties of directionally solidified Al2O3/(5Re0.2)AG/ZrO2 eutectic ceramic composites

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental section

2.1 Preparation of samples and direction solidification

2.2 Microstructure characterization

2.3 Mechanical properties testing

3. Results

3.1 Microstructure of the Al2O3/(5Re0.2)AG/ZrO2 ternary eutectic HEOCs

3.2 Orientation relationships the interfaces

3.3 Mechanical properties

4. Discussion

4.1 Distribution of ZrO2

4.2 Effect of microstructure on mechanical performance

5. Conclusion

Declarations

References

Additional Declarations

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3.1 Microstructure of the Al₂O₃/(5Re_0.2)AG/ZrO₂ ternary eutectic HEOCs

4.1 Distribution of ZrO₂