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Phase evolution and microstructure analysis of Al2O3–ZrO2(Y2O3) eutectic powders with ultra-fine nanostructure
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Phase evolution and microstructure analysis of Al2O3–ZrO2(Y2O3) eutectic powders with ultra-fine nanostructure Wanjun Yu, Yongting Zheng∗, Jiayu Pan, Yongdong Yu National Key Laboratory of Science and Technology on Advanced Composites in Special Environments and Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin, 150080, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Rapid solidification Al2O3–ZrO2(Y2O3) Eutectic powders ZrO2 polymorphs Nanostructure
It is difficult to prepare high-quality eutectic particles with regular morphologies and fine microstructure by current technologies. In this work, the Al2O3–ZrO2(Y2O3) eutectic powders with ultra-fine nanostructure is fabricated by a novel combustion synthesis combined with spray cooling (CS-SC) method. The phase structure and microstructure of eutectic particles with different Y2O3 content are discussed in details. The doping of Y2O3 inhibits the transformation of cubic-tetragonal and tetragonal-monoclinic during the solidification process, which gives rise to a rich variety of ZrO2 polymorphs. Moreover, the microstructure of eutectic particles papered by CS-SC is independent of Y2O3 content, which are predominantly manipulated by cooling rate. The evolution of eutectic structure with particle size is also detailly analyzed. The cooling rate declines with the of particle size, which leads to increase of growth rate and consequently resulting in coarse structure. But all for this, the mean phase spacings of all particles remain nanoscale. The CS-SC has much higher growth rate and degree of supercooling than other solidification techniques. This work provides a basis for preparing Al2O3–ZrO2(Y2O3) eutectic ceramics concurrent with large dimension and ultra-fine structure via sintering method.
1. Introduction Al2O3-based eutectic ceramics have attracted scientific and technological attention in diverse applications due to the impressive comprehensive properties, such as superior antioxidation, good thermal shock resistance, excellent creep resistance and outstanding strength at ultra-high temperatures even up to the melting point [1–7]. Generally, the properties of eutectic ceramics are critically dependent on their microstructure characteristics. The fine microstructure and minor defect size are favorable for the improvement of strength [4,7–10]. Thereby, research on manipulating eutectic structure has been very active in the last few decades and still keeps gaining in popularity. Enhancing the growth rate by tuning cooling rate is an effective strategy to refine the microstructure of eutectic materials [11–15]. The characteristic microstructural dimension (grain size, eutectic spacing and defect size) decreased obviously with the increase of growth rate. Based on the principle, several rapid solidification technologies, such as edge-defined film-fed growth (EFG) [16], laser heated pedestal growth (LHPG) [17], laser floating zone (LFZ) [4,18–20], micro-pulling-down (μ-PD) [21,22] and so on, have been used to successfully synthesized Al2O3–ZrO2 eutectic ceramics. The obtained materials have fine and uniform eutectic structure due to the high cooling rate. Nevertheless,
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the high residual stress induced by the large thermal gradient will generate amounts of cracks, which limits the size of eutectic samples. Recently, there have been a few reports on the eutectic ceramics concurrent with large dimension and fine structure obtained by sintering method [23–28]. The basic process of sintering method can be summarized as follows. Firstly, the eutectic materials with fine structure are fabricated by the rapid solidification technologies. After subsequent crushing, the pulverized eutectic particles were densified by sintering technologies, which forms large-volume eutectic bulk materials. The fine and uniform eutectic structure can be well reserved during the sinter process. Herein, the eutectic bulks papered by sintering method have more advantages in size, microstructure and performance than those of eutectic bulks papered by conventional technologies. However, most reports of sintering method were focused on sintering densification and sintering characteristic. Less effort was made on the preparation of high-quality eutectic particles. At present, it is difficult to produce eutectic particles with regular morphologies and fine microstructure by current technologies, which limit further applications of sintering method. In our previous work [29], supra-nanostructure Al2O3–ZrO2 binary eutectic powders was successfully fabricated by combustion synthesis combined with spray cooling (CS-SC) method. The interphase spacing
Corresponding author. E-mail address: [email protected] (Y. Zheng).
https://doi.org/10.1016/j.ceramint.2019.08.046 Received 4 July 2019; Received in revised form 1 August 2019; Accepted 5 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Wanjun Yu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.046
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Fig. 1. The morphologies of the Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content: (a) 0Y; (b) 1Y; (c) 2Y; (d) 3Y; (e) 4.5 Y; (f) 8Y.
of Al2O3–ZrO2 eutectic powders reaches supra nanoscale under huge cooling rate. In addition, the relevant research works reveal that the Al2O3–ZrO2 system could be compatible with a larger amount of Y2O3 [2,7,30]. The additive Y2O3 phase could effectively regulate the phase composition and tailor microstructure of Al2O3–ZrO2 ceramic. Various ZrO2 polymorphs with different content of Y2O3 phase, such as monoclinic, tetragonal and cubic, were retained in eutectic ceramics. The presence of the ZrO2 polymorphs enriches the residual stress states and morphologies, which further affect the mechanical properties of Al2O3–ZrO2 eutectic ceramics. In this paper, Al2O3–ZrO2(Y2O3) ternary eutectic powders with ultra-fine nanostructure were successfully prepared by CS-SC method. The huge cooling rate greatly refines the eutectic structure. The average interphase spacing reaches a few tens of nanometers. The phase structure and microstructure of eutectic particles with different Y2O3 content were also discussed in details.
into micron droplets under the effect of resistance. Finally, molten droplets rapidly solidified into Al2O3–ZrO2(Y2O3) eutectic powders. 2.2. Simulation of solidification process The rapid solidification process of molten drops is simulated by finite element method. For simplicity, the simulation is based on the hypothesizes: (1) the high temperature drops has uniform temperature distribution. (2) the thermal conduction could be negligible comparing to the heat convection and the thermal radiation. (3) Due to the low content of Y2O3, the effect of Y2O3 on the solidification process is not taken into account. (4) The thermo-physical properties of composite system are obtained by rule of mixtures, and these values are considered to be temperature-independent. In the simulation, the initial temperature and the cooling air temperature are 3500 K and 298 K, respectively. In addition, the forced convection heat transfer coefficient was adopted 90 W/(m2·K). More details of finite element simulation have been described in our previous work [29].
2. Experimental procedure 2.1. Preparation of Al2O3–ZrO2(Y2O3) eutectic powders
2.3. Characterization Commercial available Zr(NO3)4 and Al powders were used as oxidant and reductant, respectively. The combustion synthesis reaction for this experiment can be described as Eq (1): 20Al+3Zr(NO3)4 → 10Al2O3+3ZrO2+6N2
The phase composition of Al2O3–ZrO2(Y2O3) eutectic powders was investigated by X-ray Diffraction (XRD, D/MAX-RB, Rigaku, Japan) with the Cu-Kα radiation. A universal laser instrument (Mastersizer 2000) was used to measure particle size of powders. The cross-sectional microstructures of eutectic powders were observed by backscatter electron (BSE, JSM-6390, Japan). The morphologies of eutectic powders were characterized by scanning electron microscopy (SEM, JSM6390, Japan).
(1)
Due to the difficulty in measuring ultra-high system temperature (> 2950 K) by experimental methods, the temperature was approximately estimated through calculating the enthalpy changes of the reaction. In addition, an appropriate amount of Al2O3 and ZrO2 ceramic powders were added to adjust to the system temperature and the component ratio. The experimental pressure can be controlled through a pressure control valve. The original powers were mixed with the eutectic composition of Al2O3–ZrO2 (37 mol % ZrO2 and 63 mol% Al2O3). Moreover, Y2O3 with 6 different content (the mole ratios of Y2O3:ZrO2: 0%, 1%, 2%, 3%, 4.5% and 8%, respectively) were also added to the Al2O3–ZrO2 system. According to the Y2O3 content, the specimens were named by 0Y, 1Y, 2Y, 3Y, 4.5Y and 8Y, respectively. After thoroughly dried, the raw materials were homogenously mixed by mechanical ball milling. Then, the mixture of Al2O3, ZrO2 and Y2O3 were poured into SHS reactor. The direct current was used to ignite reaction. The Al–Zr(NO3)4 reaction released large amounts of heat to rapidly melt raw powders. On the other hand, the N2 generated a high pressure. The molten product burst out from the reactor nozzle and sprayed to the air. The melt crumbles
3. Results and discussion 3.1. Morphologies and phase evolution Fig. 1 shows the particle morphologies of Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content. It can be found that all composite powders share similar morphologies with spherical and uniform particles, acquire a high sintering activity. However, a small amount of the broken shells (red arrows in Fig. 1) and wrapped morphologies (blue arrows in Fig. 1) are also observed in the powders. The sprayed drops have high speed and larger acceleration, which impact on the pre-solidified powders and cooling air, resulting in the formation of the broken shells and wrapped morphologies. The particle size distribution of Al2O3–ZrO2(Y2O3) eutectic powders is shown in 2
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Fig. 2. The particle size distribution of the Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content: (a) 0Y; (b) 1Y; (c) 2Y; (d) 3Y; (e) 4.5 Y; (f) 8Y. Table 1 The surface area mean particle size of Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content. Sample code Average particle size (μm)
0Y 9.61
1Y 8.40
2Y 9.35
3Y 9.60
4.5Y 8.55
8Y 9.63
Fig. 2. The Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content possess uniform particle size, and most particle sizes in each sample are concentrated in the range of 6–20 μm. As summarized in Table 1, all samples display similar particle size, whose the surface area mean particle size lies within the narrow interval from 8.40 μm to 9.63 μm. The result indicates that the effect of Y2O3 phase on particle size is insignificant. Fig. 3 represents XRD patterns of Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content. The measured result reveals that all samples are composed of α-Al2O3 and ZrO2, which proves that the combustion reaction (Eq (1)) carries out as expected and raw materials turned into Al2O3 and ZrO2. It is noteworthy that the diffraction peaks intensity of Al2O3 are very weak, suggesting that part of Al2O3 dissolve in the ZrO2 lattice. In addition, the diffraction peaks of Y2O3 and YAG phase are not identified in XRD patterns of all samples, indicating the solubility of Y2O3 within the ZrO2 phase. These results prove that extreme non-equilibrium solidification condition is favorable for the formation of solid solution. In order to elucidate the phase evolution of ZrO2, the magnified XRD spectrums within specific ranges are demonstrated in Fig. 4. In the case of 0Y sample, the diffraction peaks of ~34.6°, ~50° and ~59.4° indicate a tetragonal phase for ZrO2 (Fig. 4(b, c, d)). Combining Fig. 4(a), it can be deduced that the 0Y eutectic powders have t-ZrO2 and m-ZrO2. Even without addition of Y2O3, the t-ZrO2 phases are primary phases in the 0Y sample because of huge cooling rate. Subsequently, the peak intensity of m-ZrO2 decreases gradually with the increase of Y2O3 content (Fig. 4(a)), which proves that the doping of Y2O3 further inhibits the transformation of ZrO2 to monoclinic phase. When the Y2O3 content rises to 4.5 mol%, the diffraction peaks of m-ZrO2 disappear entirely. From Fig. 4(b, c, d), it is can be found that the diffraction peaks of ~34.6°, ~50° and ~59.4° also decreases with the
Fig. 3. XRD spectrum of the synthesized Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content.
increase of Y2O3 content, which corresponds to the reduction of t-ZrO2 phase. In considering the crystal symmetry evolution, the 1Y, 2Y and 3Y samples should consist of three structures: cubic, tetragonal and monoclinic; while 4.5Y sample is a mixture of two symmetries: cubic and tetragonal. Moreover, when Y2O3 content is 4.5 mol%, the extremely weak peaks of ~34.6°, ~50° and ~59.4° suggest a predominance of the cubic phase. With further increase of Y2O3, full peak merging is observed at 34.8°, 50.1° and 59.6°. It is, therefore, reasonable to believe that the c-ZrO2 is only phase in 8Y sample. These variations reveal that the additive Y2O3 phase also hinders phase transformation of cubic-tetragonal during the solidification process. Based on the above analysis, it is can be concluded that the phase evolution mechanism is related to the transformation of cubic-tetragonal and tetragonal-monoclinic. During the rapid solidification 3
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Fig. 4. Magnified XRD spectrums within specific ranges: (a) 26o-29°; (b) 34 -37°; (c) 48o-53°; (d) 57o-62.°.
Fig. 5. Cross-sectional microstructure of Al2O3–ZrO2(Y2O3) eutectic powders containing different contents of Y2O3 with particle sizes of ~14 μm: (a) 0Y-12 [29]; (b) 1Y-14; (c) 2Y-12; (d) 3Y-14; (e) 4.5Y-15; (f) 8Y-14; (a1) [29]-(f1) are the corresponding magnified images of (a)–(f), respectively.
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Fig. 6. Cross-sectional microstructure of Al2O3–ZrO2(Y2O3) eutectic powders containing different contents of Y2O3 with particle sizes of ~22 μm: (a) 0Y-21 [29]; (b) 1Y-24; (c) 2Y-23; (d) 3Y-21; (e) 4.5Y-21; (f) 8Y-23; (a1) [29]-(f1) are the corresponding magnified images of (a)–(f), respectively.
process, the additive Y2O3 phase soluble in ZrO2 lattice. Furthermore, the solubility of Y2O3 could change the crystal structure of ZrO2 and cause lattice distortion [2,7,31], which effectively inhibit the transformation of cubic-tetragonal and tetragonal-monoclinic. With the increase of Y2O3 content, more metastable ZrO2 phase are stabilized at room temperature, consequently resulting in a rich variety of ZrO2 polymorphs. Therefore, the phase structure evolution of ZrO2 could be controlled by tuning the Y2O3 content.
lamellae (white regions). No enrichment of Al2O3 phase and divorced eutectic is found. The lamellar ZrO2 embedded in Al2O3 matrix in different orientations because of the unidirectional solidification process. It should be cautiously pointed out that the crystalline clusters are also found in the BSE images of Al2O3–ZrO2(Y2O3) particles, especially in the eutectic particles with large size (Figs. (6)). The formation of crystalline clusters can be attributed to homogeneous nucleation: (1) the huge cooling rate supplied by CS-SC generated an higher degree of supercooling exceeds critical degree of supercooling, which is favorable for the homogeneous nucleation; (2) during the spray process, the melt product is broken into micron droplets, which decreases the probability of containing impurities, consequently resulting in inhibition of heterogeneous nucleation. In addition, the microstructure characteristic of eutectic particles with similar sizes has no obvious change with variation of Y2O3 content, as shown in Fig. (5) and Fig. (6). According to quantificational calculation, when particle sizes approximate to ~14 μm, the mean phase spacings of Al2O3–ZrO2(Y2O3) eutectic particles with different Y2O3 content lie within a narrow interval from 62.1 to 64.5 μm (Fig. 5). Simultaneously, the same phenomenon is also observed in eutectic powders when particle sizes approximate to ~22 μm (Fig. 6). These results prove that the microstructure of eutectic particles papered by CS-SC is independent of Y2O3 content, which is predominantly manipulated by cooling rate.
3.2. Analysis of cross-sectional microstructure 3.2.1. Effect of Y2O3 content It can be found that the eutectic powders have different particle size even with same Y2O3 content according to the results in Fig. 2. However, the small particles usually have higher cooling rate than that of large particles during the solidification process, which generated the variation of eutectic microstructure. In order to better investigate the effect of Y2O3 on the microstructure of eutectic powders, two series of Al2O3–ZrO2(Y2O3) eutectic particles with similar size are demonstrated in Fig. (5) and Fig. (6) via the BSE imaging. The selected particles are named by combining the initial powders name and particle size. All particles have a fine and homogeneous nano eutectic structure, as shown in Fig. (5) and Fig. (6), which is composed of Al2O3 matrix (dark regions) and disordered interpenetrating network of ultra-fine ZrO2 5
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Fig. 7. Cross-sectional microstructure of 3Y sample with different particle size: (a) 3Y-8; (b) 3Y-14; (c) 3Y-21; (d) 3Y-27; (e) 3Y-33; (f) 3Y-39; (a1)-(f1) are the corresponding magnified images of (a)–(f), respectively.
Fig. 8. (a) The cooling rate-particle size curve of molten drops; (b) the temperature-time curve of molten drops with a size of 8 μm.
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to 98.2 nm.
Table 2 The mean phase spacing of 3Y particles with different size. Samples
3Y-8
3Y-14
3Y-21
3Y-27
3Y-33
3Y-39
Particle size (μm) Mean phase spacing (nm)
8 55.5
14 63.7
21 71.0
27 78.9
33 84.8
39 98.2
3.3. Solidification feature of CS-SC Based on the above analysis, the microstructure of eutectic particles is independent of Y2O3 content, the mean particle sizes of all samples range from 8.40 μm to 9.63 μm. Therefore, 3Y-8 particle is selected as a representative to derive insights into solidification behavior. The thermal gradient and the growth rate are quantitatively estimated by Eq. (2) and Eq. (3) according to the Jackson-Hunt model [33], which are 9.56 K and 3.57 mm/s, respectively. In addition, the mean phase spacing, growth rate and the degree of supercooling of 3Y-8 particle and correlative comparisons are listed in Table 3. The growth rate supplied by CS-SC is much higher than that of other solidification techniques as contrasted with previously reports [7,13,18,33,35,36]. Furthermore, the degree of supercooling of 9.56 K obtained in this paper has more advantage than the values of 2.7 K and 2 K reported by Wang [33] and Song [34], respectively. As expected, the 3Y-8 particle presents an ultra-fine and uniform eutectic structure, and the corresponding interphase spacing reaches a low value of 55.5 nm, which exhibits unparalleled nanoscale over all other techniques.
Table 3 Mean phase spacing (λa), growth rate (V) and the degree of supercooling (ΔT) of 3Y-8 particle and correlative comparisons. Material
Eutectic structure
λa (nm)
V (mm/s)
ΔT (K)
Fabricated method
3Y-8 Al2O3–ZrO2 Al2O3–ZrO2 Al2O3–ZrO2
lamellar lamellar lamellar rod
~55.5 ~200 ~150 ~200
3.57 0.275 ~0.28 0.2
9.56 2.7 – –
Al2O3–ZrO2(Y2O3) Al2O3–ZrO2 Al2O3–ZrO2(Y2O3)
lamellar lamellar lamellar
~430 130 1100–1800
0.208 0.25 0.001
– – –
This work SPS [33] μ-PD [13] Laser 3D Printing [35] LFZ [18] ED [36] MG [7]
SPS: spark plasma sintering, μ-PD: micro-pulling down, LFZ: laser float zone, ED: eutectic drops, MG: melt grown.
ΔT =
3.2.2. Effect of particle size The evolution of eutectic structure with particle size is also conducted via BSE analyses. Fig. 7 shows the cross-sectional microstructure of 3Y sample with different particle size. For simplicity, the 3Y sample with particle size of 8, 14, 21, 27, 33 and 39 μm are labeled as 3Y-8, 3Y14, 3Y-21, 3Y-27, 3Y-33 and 3Y-39. Apparently, the eutectic particles with different size exhibit different microstructure because of the variations of cooling rate (Fig. 7). The solidification process is simulated by ANSYS method based on microstructure evolution. Fig. 8(a) depicts the cooling rate-particle size curve of molten drops. The cooling rate reaches 1.9 × 105 K/s when the particle size is as low as 3.94 μm. Moreover, the cooling rate declines from 1.9 × 105 K/s to 2.3 × 104 K/ s with the increase of particle size increases from 3.94 μm to 42 μm. The result indicates that the huge cooling rate can be obtained by the heat convection and the thermal radiation. In particular, Fig. 8(a) shows the temperature of molten drops with a size of 8 μm varies with time. The curve has higher slopes, which corresponds to the huge cooling rate. In addition, the temperature platform appears at ~1860 °C, which is mainly due to latent heat of fusion released by eutectic reaction. The leading precipitated phases result in a surplus of the other component in the nearby melt during the solidification process. Subsequently, the second phase generates and forms a molten symbiotic nano-eutectic structure. According to the theory of metal solidification [32], the temperature gradient plays decisive influence on the growth rate, and the growth rate of the solid-liquid interface has obviously positive correlation with the cooling rate. In addition, the degree of supercooling also has positive effects on the growth rate of the solid-liquid interface. Therefore, it can be concluded that both of the degree of supercooling and growth rate change with the increase of cooling rate, which eventually could effectively manipulate the eutectic structure. Combining the simulation results and the cross-sectional microstructure (Fig. 7), the evolution is summarized as follows. The molten drops with ultra-small size have an ultra-high cooling rate, leading to a huge growth rate, consequently resulting in short diffusion time. Therefore, it is easier to form ultra-fine nano eutectic structure during the solidification process. As summarized in Table 2, the mean interphase spacing of 3Y-8 is 55.5 nm. With the increase of particle size, the growth rate becomes slower caused by the decrease of the cooling rate. Nano eutectic structure grows up because of easy solute diffusion, which leads to coarse structure (Fig. 7). But all for this, the mean phase spacings of all particles remain nanoscale, which lie within the range from 55.5 nm
V=
2K γ λα
(2)
2K γ K c λa2
(3)
Where ΔT is the degree of supercooling; Kr and Kc are Jackson-Hunt constants; V is the growth rate; λa is the average interphase spacing. 4. Conclusions The Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content were synthesized successfully by a novel route combustion synthesis combined with spray cooling method. All the samples share perfect morphologies with spherical and uniform particles, whose mean size lies within the narrow interval from 8.40 μm to 9.63 μm. The additional Y2O3 phase effectivity hinders the phase transformation of cubic-tetragonal and tetragonal-monoclinic during the solidification process, which gives rise to a rich variety of ZrO2 polymorphs in eutectic powders. However, the effect of Y2O3 phase on the microstructure is insignificant comparing to the cooling rate. All particles have a fine and homogeneous nano eutectic structure, which is composed of an Al2O3 matrix and a disordered interpenetrating network of ultra-fine ZrO2 lamellae. And the crystalline clusters are formed because of homogeneous nucleation. On the other hand, due to the variations of cooling rate with size, the eutectic particles even with same Y2O3 content have different microstructure. When the particle size is 8 μm, the mean interphase spacing of 3Y-8 is as low as 55.5 nm. With the increase of particle size, the growth rate becomes slower with the decrease of the cooling rate, leading to coarse structure. But all for this, the mean phase spacings range from 55.5 nm to 98.2 nm, which still remain in nanoscale (< 100 nm). Compared with other solidification techniques, the CS-SC has much higher growth rate and degree of supercooling, which is favorable for the formation of ultra-fine eutectic structure. Acknowledgments This project was financially supported by the National Natural Science Foundation of China (Grant No. 91016014 and Grant No. 51872062). References [1] Y. Waku, N. Nakagawa, T. Wakamoto, A ductile ceramic eutectic composite with high strength at 1,873 K, Nature 389 (1997) 49–52.
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448–456. [20] R.I. Merino, I. Francisco, J.I. Peña, Ionic conductivity in directionally solidified Al2O3–ZrO2(3%mol Y2O3) near eutectic composites, Solid State Ion. 178 (2007) 239–247. [21] J.H. Lee, A. Yoshikawa, H. Kaiden, K. Lebbou, T. Fukuda, D.H. Yoon, Y. Waku, Microstructure of Y2O3 doped Al2O3/ZrO2 eutectic fibers grown by the micro–pulling down method, J. Cryst. Growth 231 (2001) 179–185. [22] J.H. Lee, A. Yoshikawa, T. Fukuda, Y. Waku, Growth and characterization of Al2O3/ Y3Al5O12/ZrO2 ternary eutectic fibers, J. Cryst. Growth 231 (2001) 115–120. [23] T.I. Mah, T.A. Parthasarathy, R.J. Kerans, Processing, microstructure, and strength of alumina–YAG Eutectic Polycrystals, J. Am. Ceram. Soc. 83 (2000) 2088–2090. [24] D. Liu, Y. Gao, J. Liu, F. Liu, K. Li, H. Su, Y. Wang, L. An, Preparation of Al2O3–Y3Al5O12–ZrO2 eutectic ceramic by flash sintering, Scr. Mater. 114 (2016) 108–111. [25] Y. Harada, N. Uekawa, T. Kojima, K. Kakegawa, Fabrication of dense material having homogeneous GdAlO3–Al2O3 eutectic–like microstructure with off–eutectic composition by consolidation of the amorphous, J. Eur. Ceram. Soc. 29 (2009) 2419–2422. [26] M. Omori, T. Isobe, T. Hirai, Consolidation of eutectic powder of Al2O3–GdAlO3, J. Am. Ceram. Soc. 83 (2000) 2878–2880. [27] B. Yao, H. Su, J. Zhang, Q. Ren, W. Ma, L. Liu, H. Fu, Sintering densification and microstructure formation of bulk Al2O3/YAG eutectic ceramics by hot pressing based on fine eutectic structure, Mater. Des. 92 (2016) 213–222. [28] T. Isobea, M. Omoria, S. Uchidab, T. Sato, T. Hirai, Consolidation of Al2O3–Y3Al5O12(YAG) eutectic powder prepared from induction–melted solid and strength at high temperature, J. Eur. Ceram. Soc. 22 (2002) 2621–2625. [29] W. Yu, Y. Zheng, Y. Yu, X. Su, Microstructural evolution of supra–nanostructure Al2O3/ZrO2 eutectic powders by combustion synthesis–spray cooling, J. Am. Ceram. Soc. 00 (2019) 1–10 https://doi.org/10.1111/jace.16646. [30] L.S. Fu, Z. Wang, X.S. Fu, G.Q. Chen, W.L. Zhou, Microstructure and mechanical properties of Y2O3–doped melt–grown Al2O3–ZrO2 eutectic ceramic, J. Mater. Sci. Eng. 703 (2017) 372–379. [31] N.R. Harlan, R.I. Merino, J.I. Peña, A. Larrea, V.M. Orera, C. González, P. Poza, J. LLorca, Phase distribution and residual stresses in melt-grown Al2O3–ZrO2(Y2O3) eutectics, J. Am. Ceram. Soc. 85 (2002) 2025–2032. [32] J. Lipton, W. Kurz, R. Trivedi, Rapid dendrite growth in undercooled alloys, Acta Metall. 35 (1987) 957–964. [33] Z. Wang, J. Ouyang, Y. Ma, Y. Wang, Z. Liu, Insights into microstructural formation of pulse plasma semisolid to liquid processing of Al2O3–ZrO2 eutectic ceramics, J. Am. Ceram. Soc. 101 (2018) 3773–3779. [34] K. Song, J. Zhang, X. Jia, H. Su, L. Liu, H. Fu, Longitudinal cross–section microstructure of growth striation in Al2O3/Y3Al5O12/ZrO2 directionally solidified eutectic ceramic prepared by laser floating zone, J. Eur. Ceram. Soc. 33 (2013) 1123–1128. [35] Z. Liu, K. Song, B. Gao, T. Tian, H. Yang, X. Lin, W. Huang, Microstructure and mechanical properties of Al2O3/ZrO2 directionally solidified eutectic ceramic prepared by laser 3D printing, J. Mater. Sci. Technol. 32 (2016) 320–325. [36] J.M. Calderon–Moreno, M. Yoshimura, Stabilization of zirconia lamellae in rapidly solidified alumina–zirconia eutectic composites, J. Eur. Ceram. Soc. 25 (2005) 1369–1372.
[2] J. Llorca, J.Y. Pastor, P. Pedro, J.I. Peña, F. Isabel, L. Angel, V.M. Orera, Influence of the Y2O3 content and temperature on the mechanical properties of melt‐grown Al2O3–ZrO2 eutectics, J. Am. Ceram. Soc. 87 (2004) 633–639. [3] Y. Zheng, H. Li, T. Zhou, J. Zhao, P. Yang, Microstructure and mechanical properties of Al2O3/ZrO2 eutectic ceramic composites prepared by explosion synthesis, J. Alloy. Comp. 551 (2013) 475–480. [4] W. Ma, J. Zhang, H. Su, Q. Ren, B. Yao, L. Liu, H. Fu, Microstructure transformation from irregular eutectic to complex regular eutectic in directionally solidified Al2O3/ GdAlO3/ZrO2 ceramics by laser floating zone melting, J. Eur. Ceram. Soc. 36 (2016) 1447–1454. [5] W. Yu, Y. Zheng, Y. Yu, F. Lin, X. Su, P. Yang, The reaction mechanism analysis and mechanical properties of large–size Al2O3/ZrO2 eutectic ceramics prepared by a novel combustion synthesis, Ceram. Int. 44 (2018) 12987–12995. [6] S. Yan, D. Wu, G. Ma, F. Niu, R. Kang, D. Guo, Formation mechanism and process optimization of nano Al2O3-ZrO2 eutectic ceramic via laser engineered net shaping (LENS), Ceram. Int. 43 (2017) 14742–14747. [7] L. Fu, X. Fu, G. Chen, W. Han, W. Zhou, Tailoring the morphology in Y2O3 doped melt–grown Al2O3/ZrO2 eutectic ceramic, Scr. Mater. 129 (2017) 20–24. [8] J. Kishino, A. Nishiyama, T. Sakuma, Mechanical properties of sinter–forged Al2O3–ZrO2 ceramics, J. Mater. Sci. 31 (1996) 4991–4995. [9] J. Vleugels, Z.X. Yuan, O.V.D. Biest, Mechanical properties of Y2O3/Al2O3–coated Y–TZP ceramics, J. Eur. Ceram. Soc. 22 (2002) 873–881. [10] I.S. Medeiros, E.R.M. Andreeta, A.C. Hernandes, Al2O3/GdAlO3 eutectic fibers of high modulus of rupture produced by the laser heated pedestal growth technique, J. Mater. Sci. 42 (2007) 3874–3877. [11] J. Pan, Y. Zheng, Y. Zheng, W. Ye, W. Yu, Solidification mechanism and microstructure evolution of Al2O3–ZrO2 ceramic coating prepared by combustion synthesis and thermal explosion spraying, Ceram. Int. 43 (2017) 4037–4041. [12] E.L. Courtright, J.S. Haggerty, J. Sigalovsky, Controlling microstructures in Al2O3–ZrO2(Y2O3) eutectic fibers, Ceram. Eng. Sci. Proc. 14 (1993) 671–681. [13] J.H. Lee, A. Yoshikawa, S.D. Durbin, D.H. Yoon, T. Fukuda, Y. Waku, Microstructure of Al2O3/ZrO2 eutectic fibers grown by the micro–pulling down method, J. Cryst. Growth 222 (2001) 791–796. [14] M.C. Mesa, S. Serrano–Zabaleta, P.B. Oliete, A. Larrea, Microstructural stability and orientation relationships of directionally solidified Al2O3–Er3Al5O12–ZrO2 eutectic ceramics up to 1600°C, J. Eur. Ceram. Soc. 34 (2014) 2071–2080. [15] V.M. Orera, R.I. Merino, J.A. Pardo, A. Larrea, J.I. Peña, C. González, P. Poza, J.Y. Pastor, J. LLorca, Microstructure and physical properties of some oxide eutectic composites processed by directional solidification, Acta Mater. 48 (2000) 4683–4689. [16] J.L. Lorca, V.M. Orera, Directionally solidified eutectic ceramic oxides, Prog. Mater. Sci. 51 (2006) 711–809. [17] A. Laidoune, K. Lebbou, D. Bahloul, Microstructure of the yttria doped Al2O3–ZrO2 eutectic fibers grown by the laser heated pedestal growth (LHPG) method, J. Cryst. Growth 380 (2013) 224–227. [18] J. Ramírez–Rico, J. Martínez–Fernandez, J.I. Peña, D. Singh, J. Routbort, Residual stresses in Al2O3–ZrO2 (3 mol.% Y2O3) directionally solidified eutectic ceramics as a function of temperature, Mater. Sci. Eng. A 541 (2012) 61–66. [19] H. Su, J. Zhang, C. Cui, L. Liu, H. Fu, Rapid solidification of Al2O3/Y3Al5O12/ZrO2 eutectic in situ composites by laser zone remelting, J. Cryst. Growth 307 (2007)
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Phase evolution and microstructure analysis of Al2O3–ZrO2(Y2O3) eutectic powders with ultra-fine nanostructure Wanjun Yu, Yongting Zheng∗, Jiayu Pan, Yongdong Yu National Key Laboratory of Science and Technology on Advanced Composites in Special Environments and Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin, 150080, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Rapid solidification Al2O3–ZrO2(Y2O3) Eutectic powders ZrO2 polymorphs Nanostructure
It is difficult to prepare high-quality eutectic particles with regular morphologies and fine microstructure by current technologies. In this work, the Al2O3–ZrO2(Y2O3) eutectic powders with ultra-fine nanostructure is fabricated by a novel combustion synthesis combined with spray cooling (CS-SC) method. The phase structure and microstructure of eutectic particles with different Y2O3 content are discussed in details. The doping of Y2O3 inhibits the transformation of cubic-tetragonal and tetragonal-monoclinic during the solidification process, which gives rise to a rich variety of ZrO2 polymorphs. Moreover, the microstructure of eutectic particles papered by CS-SC is independent of Y2O3 content, which are predominantly manipulated by cooling rate. The evolution of eutectic structure with particle size is also detailly analyzed. The cooling rate declines with the of particle size, which leads to increase of growth rate and consequently resulting in coarse structure. But all for this, the mean phase spacings of all particles remain nanoscale. The CS-SC has much higher growth rate and degree of supercooling than other solidification techniques. This work provides a basis for preparing Al2O3–ZrO2(Y2O3) eutectic ceramics concurrent with large dimension and ultra-fine structure via sintering method.
1. Introduction Al2O3-based eutectic ceramics have attracted scientific and technological attention in diverse applications due to the impressive comprehensive properties, such as superior antioxidation, good thermal shock resistance, excellent creep resistance and outstanding strength at ultra-high temperatures even up to the melting point [1–7]. Generally, the properties of eutectic ceramics are critically dependent on their microstructure characteristics. The fine microstructure and minor defect size are favorable for the improvement of strength [4,7–10]. Thereby, research on manipulating eutectic structure has been very active in the last few decades and still keeps gaining in popularity. Enhancing the growth rate by tuning cooling rate is an effective strategy to refine the microstructure of eutectic materials [11–15]. The characteristic microstructural dimension (grain size, eutectic spacing and defect size) decreased obviously with the increase of growth rate. Based on the principle, several rapid solidification technologies, such as edge-defined film-fed growth (EFG) [16], laser heated pedestal growth (LHPG) [17], laser floating zone (LFZ) [4,18–20], micro-pulling-down (μ-PD) [21,22] and so on, have been used to successfully synthesized Al2O3–ZrO2 eutectic ceramics. The obtained materials have fine and uniform eutectic structure due to the high cooling rate. Nevertheless,
∗
the high residual stress induced by the large thermal gradient will generate amounts of cracks, which limits the size of eutectic samples. Recently, there have been a few reports on the eutectic ceramics concurrent with large dimension and fine structure obtained by sintering method [23–28]. The basic process of sintering method can be summarized as follows. Firstly, the eutectic materials with fine structure are fabricated by the rapid solidification technologies. After subsequent crushing, the pulverized eutectic particles were densified by sintering technologies, which forms large-volume eutectic bulk materials. The fine and uniform eutectic structure can be well reserved during the sinter process. Herein, the eutectic bulks papered by sintering method have more advantages in size, microstructure and performance than those of eutectic bulks papered by conventional technologies. However, most reports of sintering method were focused on sintering densification and sintering characteristic. Less effort was made on the preparation of high-quality eutectic particles. At present, it is difficult to produce eutectic particles with regular morphologies and fine microstructure by current technologies, which limit further applications of sintering method. In our previous work [29], supra-nanostructure Al2O3–ZrO2 binary eutectic powders was successfully fabricated by combustion synthesis combined with spray cooling (CS-SC) method. The interphase spacing
Corresponding author. E-mail address: [email protected] (Y. Zheng).
https://doi.org/10.1016/j.ceramint.2019.08.046 Received 4 July 2019; Received in revised form 1 August 2019; Accepted 5 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Wanjun Yu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.046
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Fig. 1. The morphologies of the Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content: (a) 0Y; (b) 1Y; (c) 2Y; (d) 3Y; (e) 4.5 Y; (f) 8Y.
of Al2O3–ZrO2 eutectic powders reaches supra nanoscale under huge cooling rate. In addition, the relevant research works reveal that the Al2O3–ZrO2 system could be compatible with a larger amount of Y2O3 [2,7,30]. The additive Y2O3 phase could effectively regulate the phase composition and tailor microstructure of Al2O3–ZrO2 ceramic. Various ZrO2 polymorphs with different content of Y2O3 phase, such as monoclinic, tetragonal and cubic, were retained in eutectic ceramics. The presence of the ZrO2 polymorphs enriches the residual stress states and morphologies, which further affect the mechanical properties of Al2O3–ZrO2 eutectic ceramics. In this paper, Al2O3–ZrO2(Y2O3) ternary eutectic powders with ultra-fine nanostructure were successfully prepared by CS-SC method. The huge cooling rate greatly refines the eutectic structure. The average interphase spacing reaches a few tens of nanometers. The phase structure and microstructure of eutectic particles with different Y2O3 content were also discussed in details.
into micron droplets under the effect of resistance. Finally, molten droplets rapidly solidified into Al2O3–ZrO2(Y2O3) eutectic powders. 2.2. Simulation of solidification process The rapid solidification process of molten drops is simulated by finite element method. For simplicity, the simulation is based on the hypothesizes: (1) the high temperature drops has uniform temperature distribution. (2) the thermal conduction could be negligible comparing to the heat convection and the thermal radiation. (3) Due to the low content of Y2O3, the effect of Y2O3 on the solidification process is not taken into account. (4) The thermo-physical properties of composite system are obtained by rule of mixtures, and these values are considered to be temperature-independent. In the simulation, the initial temperature and the cooling air temperature are 3500 K and 298 K, respectively. In addition, the forced convection heat transfer coefficient was adopted 90 W/(m2·K). More details of finite element simulation have been described in our previous work [29].
2. Experimental procedure 2.1. Preparation of Al2O3–ZrO2(Y2O3) eutectic powders
2.3. Characterization Commercial available Zr(NO3)4 and Al powders were used as oxidant and reductant, respectively. The combustion synthesis reaction for this experiment can be described as Eq (1): 20Al+3Zr(NO3)4 → 10Al2O3+3ZrO2+6N2
The phase composition of Al2O3–ZrO2(Y2O3) eutectic powders was investigated by X-ray Diffraction (XRD, D/MAX-RB, Rigaku, Japan) with the Cu-Kα radiation. A universal laser instrument (Mastersizer 2000) was used to measure particle size of powders. The cross-sectional microstructures of eutectic powders were observed by backscatter electron (BSE, JSM-6390, Japan). The morphologies of eutectic powders were characterized by scanning electron microscopy (SEM, JSM6390, Japan).
(1)
Due to the difficulty in measuring ultra-high system temperature (> 2950 K) by experimental methods, the temperature was approximately estimated through calculating the enthalpy changes of the reaction. In addition, an appropriate amount of Al2O3 and ZrO2 ceramic powders were added to adjust to the system temperature and the component ratio. The experimental pressure can be controlled through a pressure control valve. The original powers were mixed with the eutectic composition of Al2O3–ZrO2 (37 mol % ZrO2 and 63 mol% Al2O3). Moreover, Y2O3 with 6 different content (the mole ratios of Y2O3:ZrO2: 0%, 1%, 2%, 3%, 4.5% and 8%, respectively) were also added to the Al2O3–ZrO2 system. According to the Y2O3 content, the specimens were named by 0Y, 1Y, 2Y, 3Y, 4.5Y and 8Y, respectively. After thoroughly dried, the raw materials were homogenously mixed by mechanical ball milling. Then, the mixture of Al2O3, ZrO2 and Y2O3 were poured into SHS reactor. The direct current was used to ignite reaction. The Al–Zr(NO3)4 reaction released large amounts of heat to rapidly melt raw powders. On the other hand, the N2 generated a high pressure. The molten product burst out from the reactor nozzle and sprayed to the air. The melt crumbles
3. Results and discussion 3.1. Morphologies and phase evolution Fig. 1 shows the particle morphologies of Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content. It can be found that all composite powders share similar morphologies with spherical and uniform particles, acquire a high sintering activity. However, a small amount of the broken shells (red arrows in Fig. 1) and wrapped morphologies (blue arrows in Fig. 1) are also observed in the powders. The sprayed drops have high speed and larger acceleration, which impact on the pre-solidified powders and cooling air, resulting in the formation of the broken shells and wrapped morphologies. The particle size distribution of Al2O3–ZrO2(Y2O3) eutectic powders is shown in 2
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Fig. 2. The particle size distribution of the Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content: (a) 0Y; (b) 1Y; (c) 2Y; (d) 3Y; (e) 4.5 Y; (f) 8Y. Table 1 The surface area mean particle size of Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content. Sample code Average particle size (μm)
0Y 9.61
1Y 8.40
2Y 9.35
3Y 9.60
4.5Y 8.55
8Y 9.63
Fig. 2. The Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content possess uniform particle size, and most particle sizes in each sample are concentrated in the range of 6–20 μm. As summarized in Table 1, all samples display similar particle size, whose the surface area mean particle size lies within the narrow interval from 8.40 μm to 9.63 μm. The result indicates that the effect of Y2O3 phase on particle size is insignificant. Fig. 3 represents XRD patterns of Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content. The measured result reveals that all samples are composed of α-Al2O3 and ZrO2, which proves that the combustion reaction (Eq (1)) carries out as expected and raw materials turned into Al2O3 and ZrO2. It is noteworthy that the diffraction peaks intensity of Al2O3 are very weak, suggesting that part of Al2O3 dissolve in the ZrO2 lattice. In addition, the diffraction peaks of Y2O3 and YAG phase are not identified in XRD patterns of all samples, indicating the solubility of Y2O3 within the ZrO2 phase. These results prove that extreme non-equilibrium solidification condition is favorable for the formation of solid solution. In order to elucidate the phase evolution of ZrO2, the magnified XRD spectrums within specific ranges are demonstrated in Fig. 4. In the case of 0Y sample, the diffraction peaks of ~34.6°, ~50° and ~59.4° indicate a tetragonal phase for ZrO2 (Fig. 4(b, c, d)). Combining Fig. 4(a), it can be deduced that the 0Y eutectic powders have t-ZrO2 and m-ZrO2. Even without addition of Y2O3, the t-ZrO2 phases are primary phases in the 0Y sample because of huge cooling rate. Subsequently, the peak intensity of m-ZrO2 decreases gradually with the increase of Y2O3 content (Fig. 4(a)), which proves that the doping of Y2O3 further inhibits the transformation of ZrO2 to monoclinic phase. When the Y2O3 content rises to 4.5 mol%, the diffraction peaks of m-ZrO2 disappear entirely. From Fig. 4(b, c, d), it is can be found that the diffraction peaks of ~34.6°, ~50° and ~59.4° also decreases with the
Fig. 3. XRD spectrum of the synthesized Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content.
increase of Y2O3 content, which corresponds to the reduction of t-ZrO2 phase. In considering the crystal symmetry evolution, the 1Y, 2Y and 3Y samples should consist of three structures: cubic, tetragonal and monoclinic; while 4.5Y sample is a mixture of two symmetries: cubic and tetragonal. Moreover, when Y2O3 content is 4.5 mol%, the extremely weak peaks of ~34.6°, ~50° and ~59.4° suggest a predominance of the cubic phase. With further increase of Y2O3, full peak merging is observed at 34.8°, 50.1° and 59.6°. It is, therefore, reasonable to believe that the c-ZrO2 is only phase in 8Y sample. These variations reveal that the additive Y2O3 phase also hinders phase transformation of cubic-tetragonal during the solidification process. Based on the above analysis, it is can be concluded that the phase evolution mechanism is related to the transformation of cubic-tetragonal and tetragonal-monoclinic. During the rapid solidification 3
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Fig. 4. Magnified XRD spectrums within specific ranges: (a) 26o-29°; (b) 34 -37°; (c) 48o-53°; (d) 57o-62.°.
Fig. 5. Cross-sectional microstructure of Al2O3–ZrO2(Y2O3) eutectic powders containing different contents of Y2O3 with particle sizes of ~14 μm: (a) 0Y-12 [29]; (b) 1Y-14; (c) 2Y-12; (d) 3Y-14; (e) 4.5Y-15; (f) 8Y-14; (a1) [29]-(f1) are the corresponding magnified images of (a)–(f), respectively.
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Fig. 6. Cross-sectional microstructure of Al2O3–ZrO2(Y2O3) eutectic powders containing different contents of Y2O3 with particle sizes of ~22 μm: (a) 0Y-21 [29]; (b) 1Y-24; (c) 2Y-23; (d) 3Y-21; (e) 4.5Y-21; (f) 8Y-23; (a1) [29]-(f1) are the corresponding magnified images of (a)–(f), respectively.
process, the additive Y2O3 phase soluble in ZrO2 lattice. Furthermore, the solubility of Y2O3 could change the crystal structure of ZrO2 and cause lattice distortion [2,7,31], which effectively inhibit the transformation of cubic-tetragonal and tetragonal-monoclinic. With the increase of Y2O3 content, more metastable ZrO2 phase are stabilized at room temperature, consequently resulting in a rich variety of ZrO2 polymorphs. Therefore, the phase structure evolution of ZrO2 could be controlled by tuning the Y2O3 content.
lamellae (white regions). No enrichment of Al2O3 phase and divorced eutectic is found. The lamellar ZrO2 embedded in Al2O3 matrix in different orientations because of the unidirectional solidification process. It should be cautiously pointed out that the crystalline clusters are also found in the BSE images of Al2O3–ZrO2(Y2O3) particles, especially in the eutectic particles with large size (Figs. (6)). The formation of crystalline clusters can be attributed to homogeneous nucleation: (1) the huge cooling rate supplied by CS-SC generated an higher degree of supercooling exceeds critical degree of supercooling, which is favorable for the homogeneous nucleation; (2) during the spray process, the melt product is broken into micron droplets, which decreases the probability of containing impurities, consequently resulting in inhibition of heterogeneous nucleation. In addition, the microstructure characteristic of eutectic particles with similar sizes has no obvious change with variation of Y2O3 content, as shown in Fig. (5) and Fig. (6). According to quantificational calculation, when particle sizes approximate to ~14 μm, the mean phase spacings of Al2O3–ZrO2(Y2O3) eutectic particles with different Y2O3 content lie within a narrow interval from 62.1 to 64.5 μm (Fig. 5). Simultaneously, the same phenomenon is also observed in eutectic powders when particle sizes approximate to ~22 μm (Fig. 6). These results prove that the microstructure of eutectic particles papered by CS-SC is independent of Y2O3 content, which is predominantly manipulated by cooling rate.
3.2. Analysis of cross-sectional microstructure 3.2.1. Effect of Y2O3 content It can be found that the eutectic powders have different particle size even with same Y2O3 content according to the results in Fig. 2. However, the small particles usually have higher cooling rate than that of large particles during the solidification process, which generated the variation of eutectic microstructure. In order to better investigate the effect of Y2O3 on the microstructure of eutectic powders, two series of Al2O3–ZrO2(Y2O3) eutectic particles with similar size are demonstrated in Fig. (5) and Fig. (6) via the BSE imaging. The selected particles are named by combining the initial powders name and particle size. All particles have a fine and homogeneous nano eutectic structure, as shown in Fig. (5) and Fig. (6), which is composed of Al2O3 matrix (dark regions) and disordered interpenetrating network of ultra-fine ZrO2 5
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Fig. 7. Cross-sectional microstructure of 3Y sample with different particle size: (a) 3Y-8; (b) 3Y-14; (c) 3Y-21; (d) 3Y-27; (e) 3Y-33; (f) 3Y-39; (a1)-(f1) are the corresponding magnified images of (a)–(f), respectively.
Fig. 8. (a) The cooling rate-particle size curve of molten drops; (b) the temperature-time curve of molten drops with a size of 8 μm.
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to 98.2 nm.
Table 2 The mean phase spacing of 3Y particles with different size. Samples
3Y-8
3Y-14
3Y-21
3Y-27
3Y-33
3Y-39
Particle size (μm) Mean phase spacing (nm)
8 55.5
14 63.7
21 71.0
27 78.9
33 84.8
39 98.2
3.3. Solidification feature of CS-SC Based on the above analysis, the microstructure of eutectic particles is independent of Y2O3 content, the mean particle sizes of all samples range from 8.40 μm to 9.63 μm. Therefore, 3Y-8 particle is selected as a representative to derive insights into solidification behavior. The thermal gradient and the growth rate are quantitatively estimated by Eq. (2) and Eq. (3) according to the Jackson-Hunt model [33], which are 9.56 K and 3.57 mm/s, respectively. In addition, the mean phase spacing, growth rate and the degree of supercooling of 3Y-8 particle and correlative comparisons are listed in Table 3. The growth rate supplied by CS-SC is much higher than that of other solidification techniques as contrasted with previously reports [7,13,18,33,35,36]. Furthermore, the degree of supercooling of 9.56 K obtained in this paper has more advantage than the values of 2.7 K and 2 K reported by Wang [33] and Song [34], respectively. As expected, the 3Y-8 particle presents an ultra-fine and uniform eutectic structure, and the corresponding interphase spacing reaches a low value of 55.5 nm, which exhibits unparalleled nanoscale over all other techniques.
Table 3 Mean phase spacing (λa), growth rate (V) and the degree of supercooling (ΔT) of 3Y-8 particle and correlative comparisons. Material
Eutectic structure
λa (nm)
V (mm/s)
ΔT (K)
Fabricated method
3Y-8 Al2O3–ZrO2 Al2O3–ZrO2 Al2O3–ZrO2
lamellar lamellar lamellar rod
~55.5 ~200 ~150 ~200
3.57 0.275 ~0.28 0.2
9.56 2.7 – –
Al2O3–ZrO2(Y2O3) Al2O3–ZrO2 Al2O3–ZrO2(Y2O3)
lamellar lamellar lamellar
~430 130 1100–1800
0.208 0.25 0.001
– – –
This work SPS [33] μ-PD [13] Laser 3D Printing [35] LFZ [18] ED [36] MG [7]
SPS: spark plasma sintering, μ-PD: micro-pulling down, LFZ: laser float zone, ED: eutectic drops, MG: melt grown.
ΔT =
3.2.2. Effect of particle size The evolution of eutectic structure with particle size is also conducted via BSE analyses. Fig. 7 shows the cross-sectional microstructure of 3Y sample with different particle size. For simplicity, the 3Y sample with particle size of 8, 14, 21, 27, 33 and 39 μm are labeled as 3Y-8, 3Y14, 3Y-21, 3Y-27, 3Y-33 and 3Y-39. Apparently, the eutectic particles with different size exhibit different microstructure because of the variations of cooling rate (Fig. 7). The solidification process is simulated by ANSYS method based on microstructure evolution. Fig. 8(a) depicts the cooling rate-particle size curve of molten drops. The cooling rate reaches 1.9 × 105 K/s when the particle size is as low as 3.94 μm. Moreover, the cooling rate declines from 1.9 × 105 K/s to 2.3 × 104 K/ s with the increase of particle size increases from 3.94 μm to 42 μm. The result indicates that the huge cooling rate can be obtained by the heat convection and the thermal radiation. In particular, Fig. 8(a) shows the temperature of molten drops with a size of 8 μm varies with time. The curve has higher slopes, which corresponds to the huge cooling rate. In addition, the temperature platform appears at ~1860 °C, which is mainly due to latent heat of fusion released by eutectic reaction. The leading precipitated phases result in a surplus of the other component in the nearby melt during the solidification process. Subsequently, the second phase generates and forms a molten symbiotic nano-eutectic structure. According to the theory of metal solidification [32], the temperature gradient plays decisive influence on the growth rate, and the growth rate of the solid-liquid interface has obviously positive correlation with the cooling rate. In addition, the degree of supercooling also has positive effects on the growth rate of the solid-liquid interface. Therefore, it can be concluded that both of the degree of supercooling and growth rate change with the increase of cooling rate, which eventually could effectively manipulate the eutectic structure. Combining the simulation results and the cross-sectional microstructure (Fig. 7), the evolution is summarized as follows. The molten drops with ultra-small size have an ultra-high cooling rate, leading to a huge growth rate, consequently resulting in short diffusion time. Therefore, it is easier to form ultra-fine nano eutectic structure during the solidification process. As summarized in Table 2, the mean interphase spacing of 3Y-8 is 55.5 nm. With the increase of particle size, the growth rate becomes slower caused by the decrease of the cooling rate. Nano eutectic structure grows up because of easy solute diffusion, which leads to coarse structure (Fig. 7). But all for this, the mean phase spacings of all particles remain nanoscale, which lie within the range from 55.5 nm
V=
2K γ λα
(2)
2K γ K c λa2
(3)
Where ΔT is the degree of supercooling; Kr and Kc are Jackson-Hunt constants; V is the growth rate; λa is the average interphase spacing. 4. Conclusions The Al2O3–ZrO2(Y2O3) eutectic powders with different Y2O3 content were synthesized successfully by a novel route combustion synthesis combined with spray cooling method. All the samples share perfect morphologies with spherical and uniform particles, whose mean size lies within the narrow interval from 8.40 μm to 9.63 μm. The additional Y2O3 phase effectivity hinders the phase transformation of cubic-tetragonal and tetragonal-monoclinic during the solidification process, which gives rise to a rich variety of ZrO2 polymorphs in eutectic powders. However, the effect of Y2O3 phase on the microstructure is insignificant comparing to the cooling rate. All particles have a fine and homogeneous nano eutectic structure, which is composed of an Al2O3 matrix and a disordered interpenetrating network of ultra-fine ZrO2 lamellae. And the crystalline clusters are formed because of homogeneous nucleation. On the other hand, due to the variations of cooling rate with size, the eutectic particles even with same Y2O3 content have different microstructure. When the particle size is 8 μm, the mean interphase spacing of 3Y-8 is as low as 55.5 nm. With the increase of particle size, the growth rate becomes slower with the decrease of the cooling rate, leading to coarse structure. But all for this, the mean phase spacings range from 55.5 nm to 98.2 nm, which still remain in nanoscale (< 100 nm). Compared with other solidification techniques, the CS-SC has much higher growth rate and degree of supercooling, which is favorable for the formation of ultra-fine eutectic structure. Acknowledgments This project was financially supported by the National Natural Science Foundation of China (Grant No. 91016014 and Grant No. 51872062). References [1] Y. Waku, N. Nakagawa, T. Wakamoto, A ductile ceramic eutectic composite with high strength at 1,873 K, Nature 389 (1997) 49–52.
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Ceramics International xxx (xxxx) xxx–xxx
W. Yu, et al.
448–456. [20] R.I. Merino, I. Francisco, J.I. Peña, Ionic conductivity in directionally solidified Al2O3–ZrO2(3%mol Y2O3) near eutectic composites, Solid State Ion. 178 (2007) 239–247. [21] J.H. Lee, A. Yoshikawa, H. Kaiden, K. Lebbou, T. Fukuda, D.H. Yoon, Y. Waku, Microstructure of Y2O3 doped Al2O3/ZrO2 eutectic fibers grown by the micro–pulling down method, J. Cryst. Growth 231 (2001) 179–185. [22] J.H. Lee, A. Yoshikawa, T. Fukuda, Y. Waku, Growth and characterization of Al2O3/ Y3Al5O12/ZrO2 ternary eutectic fibers, J. Cryst. Growth 231 (2001) 115–120. [23] T.I. Mah, T.A. Parthasarathy, R.J. Kerans, Processing, microstructure, and strength of alumina–YAG Eutectic Polycrystals, J. Am. Ceram. Soc. 83 (2000) 2088–2090. [24] D. Liu, Y. Gao, J. Liu, F. Liu, K. Li, H. Su, Y. Wang, L. An, Preparation of Al2O3–Y3Al5O12–ZrO2 eutectic ceramic by flash sintering, Scr. Mater. 114 (2016) 108–111. [25] Y. Harada, N. Uekawa, T. Kojima, K. Kakegawa, Fabrication of dense material having homogeneous GdAlO3–Al2O3 eutectic–like microstructure with off–eutectic composition by consolidation of the amorphous, J. Eur. Ceram. Soc. 29 (2009) 2419–2422. [26] M. Omori, T. Isobe, T. Hirai, Consolidation of eutectic powder of Al2O3–GdAlO3, J. Am. Ceram. Soc. 83 (2000) 2878–2880. [27] B. Yao, H. Su, J. Zhang, Q. Ren, W. Ma, L. Liu, H. Fu, Sintering densification and microstructure formation of bulk Al2O3/YAG eutectic ceramics by hot pressing based on fine eutectic structure, Mater. Des. 92 (2016) 213–222. [28] T. Isobea, M. Omoria, S. Uchidab, T. Sato, T. Hirai, Consolidation of Al2O3–Y3Al5O12(YAG) eutectic powder prepared from induction–melted solid and strength at high temperature, J. Eur. Ceram. Soc. 22 (2002) 2621–2625. [29] W. Yu, Y. Zheng, Y. Yu, X. Su, Microstructural evolution of supra–nanostructure Al2O3/ZrO2 eutectic powders by combustion synthesis–spray cooling, J. Am. Ceram. Soc. 00 (2019) 1–10 https://doi.org/10.1111/jace.16646. [30] L.S. Fu, Z. Wang, X.S. Fu, G.Q. Chen, W.L. Zhou, Microstructure and mechanical properties of Y2O3–doped melt–grown Al2O3–ZrO2 eutectic ceramic, J. Mater. Sci. Eng. 703 (2017) 372–379. [31] N.R. Harlan, R.I. Merino, J.I. Peña, A. Larrea, V.M. Orera, C. González, P. Poza, J. LLorca, Phase distribution and residual stresses in melt-grown Al2O3–ZrO2(Y2O3) eutectics, J. Am. Ceram. Soc. 85 (2002) 2025–2032. [32] J. Lipton, W. Kurz, R. Trivedi, Rapid dendrite growth in undercooled alloys, Acta Metall. 35 (1987) 957–964. [33] Z. Wang, J. Ouyang, Y. Ma, Y. Wang, Z. Liu, Insights into microstructural formation of pulse plasma semisolid to liquid processing of Al2O3–ZrO2 eutectic ceramics, J. Am. Ceram. Soc. 101 (2018) 3773–3779. [34] K. Song, J. Zhang, X. Jia, H. Su, L. Liu, H. Fu, Longitudinal cross–section microstructure of growth striation in Al2O3/Y3Al5O12/ZrO2 directionally solidified eutectic ceramic prepared by laser floating zone, J. Eur. Ceram. Soc. 33 (2013) 1123–1128. [35] Z. Liu, K. Song, B. Gao, T. Tian, H. Yang, X. Lin, W. Huang, Microstructure and mechanical properties of Al2O3/ZrO2 directionally solidified eutectic ceramic prepared by laser 3D printing, J. Mater. Sci. Technol. 32 (2016) 320–325. [36] J.M. Calderon–Moreno, M. Yoshimura, Stabilization of zirconia lamellae in rapidly solidified alumina–zirconia eutectic composites, J. Eur. Ceram. Soc. 25 (2005) 1369–1372.
[2] J. Llorca, J.Y. Pastor, P. Pedro, J.I. Peña, F. Isabel, L. Angel, V.M. Orera, Influence of the Y2O3 content and temperature on the mechanical properties of melt‐grown Al2O3–ZrO2 eutectics, J. Am. Ceram. Soc. 87 (2004) 633–639. [3] Y. Zheng, H. Li, T. Zhou, J. Zhao, P. Yang, Microstructure and mechanical properties of Al2O3/ZrO2 eutectic ceramic composites prepared by explosion synthesis, J. Alloy. Comp. 551 (2013) 475–480. [4] W. Ma, J. Zhang, H. Su, Q. Ren, B. Yao, L. Liu, H. Fu, Microstructure transformation from irregular eutectic to complex regular eutectic in directionally solidified Al2O3/ GdAlO3/ZrO2 ceramics by laser floating zone melting, J. Eur. Ceram. Soc. 36 (2016) 1447–1454. [5] W. Yu, Y. Zheng, Y. Yu, F. Lin, X. Su, P. Yang, The reaction mechanism analysis and mechanical properties of large–size Al2O3/ZrO2 eutectic ceramics prepared by a novel combustion synthesis, Ceram. Int. 44 (2018) 12987–12995. [6] S. Yan, D. Wu, G. Ma, F. Niu, R. Kang, D. Guo, Formation mechanism and process optimization of nano Al2O3-ZrO2 eutectic ceramic via laser engineered net shaping (LENS), Ceram. Int. 43 (2017) 14742–14747. [7] L. Fu, X. Fu, G. Chen, W. Han, W. Zhou, Tailoring the morphology in Y2O3 doped melt–grown Al2O3/ZrO2 eutectic ceramic, Scr. Mater. 129 (2017) 20–24. [8] J. Kishino, A. Nishiyama, T. Sakuma, Mechanical properties of sinter–forged Al2O3–ZrO2 ceramics, J. Mater. Sci. 31 (1996) 4991–4995. [9] J. Vleugels, Z.X. Yuan, O.V.D. Biest, Mechanical properties of Y2O3/Al2O3–coated Y–TZP ceramics, J. Eur. Ceram. Soc. 22 (2002) 873–881. [10] I.S. Medeiros, E.R.M. Andreeta, A.C. Hernandes, Al2O3/GdAlO3 eutectic fibers of high modulus of rupture produced by the laser heated pedestal growth technique, J. Mater. Sci. 42 (2007) 3874–3877. [11] J. Pan, Y. Zheng, Y. Zheng, W. Ye, W. Yu, Solidification mechanism and microstructure evolution of Al2O3–ZrO2 ceramic coating prepared by combustion synthesis and thermal explosion spraying, Ceram. Int. 43 (2017) 4037–4041. [12] E.L. Courtright, J.S. Haggerty, J. Sigalovsky, Controlling microstructures in Al2O3–ZrO2(Y2O3) eutectic fibers, Ceram. Eng. Sci. Proc. 14 (1993) 671–681. [13] J.H. Lee, A. Yoshikawa, S.D. Durbin, D.H. Yoon, T. Fukuda, Y. Waku, Microstructure of Al2O3/ZrO2 eutectic fibers grown by the micro–pulling down method, J. Cryst. Growth 222 (2001) 791–796. [14] M.C. Mesa, S. Serrano–Zabaleta, P.B. Oliete, A. Larrea, Microstructural stability and orientation relationships of directionally solidified Al2O3–Er3Al5O12–ZrO2 eutectic ceramics up to 1600°C, J. Eur. Ceram. Soc. 34 (2014) 2071–2080. [15] V.M. Orera, R.I. Merino, J.A. Pardo, A. Larrea, J.I. Peña, C. González, P. Poza, J.Y. Pastor, J. LLorca, Microstructure and physical properties of some oxide eutectic composites processed by directional solidification, Acta Mater. 48 (2000) 4683–4689. [16] J.L. Lorca, V.M. Orera, Directionally solidified eutectic ceramic oxides, Prog. Mater. Sci. 51 (2006) 711–809. [17] A. Laidoune, K. Lebbou, D. Bahloul, Microstructure of the yttria doped Al2O3–ZrO2 eutectic fibers grown by the laser heated pedestal growth (LHPG) method, J. Cryst. Growth 380 (2013) 224–227. [18] J. Ramírez–Rico, J. Martínez–Fernandez, J.I. Peña, D. Singh, J. Routbort, Residual stresses in Al2O3–ZrO2 (3 mol.% Y2O3) directionally solidified eutectic ceramics as a function of temperature, Mater. Sci. Eng. A 541 (2012) 61–66. [19] H. Su, J. Zhang, C. Cui, L. Liu, H. Fu, Rapid solidification of Al2O3/Y3Al5O12/ZrO2 eutectic in situ composites by laser zone remelting, J. Cryst. Growth 307 (2007)
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