Preparation of ZnAl2O4 and its effect on fatigue resistance of alumina castables

ABSTRACT Refractories subject to loading and unloading repeatedly because of physical effects and thermal stress attack in the service process, which is an important issue shortening the service life of the thermal equipment. Therefore, researches of predicting the service life of refractories under cyclic loading have the guiding significance. The present work investigated the synthesis of ZnAl2O4 and discussed its effects on the alumina castables. The results show that the optimized generation temperature of ZnAl2O4 phase is 1400°C and the hysteresis loop in the curves of cyclic loading reveals a general trend of first sparse and then dense. The cycle times of the specimen with 3 wt.% ZnAl2O4 is 50 times, and the time from starting to loop to fracture is about 4300 s. The fatigue resistance of the specimen with 3 wt.% ZnAl2O4 is superior to ZA-0 and ZA-1.5. The new method of cyclic loading was innovatively adopted in refractories, which provide data and theoretical support for the thermal shock evaluation methods.


Introduction
Carbon peaking and carbon neutrality is an important measure to break the serious problems of resource and environmental constraints faced by the world. Castables with properties approaching those of shaped refractories, have quicker and cheaper installation, and need not sinter. As people increased regard on environment protection, the development of castables has become an inevitable trend in the refractories field. The performance of castables is usually stable [1,2]. Also, it is much quicker and easier to build compared with shaped refractories. Castables are widely used for thermotechnical equipment in iron and steel making industry, such as steel ladle linings, tundish linings, iron runner, and torpedo car linings, et al. Recently, the development of castables has contributed to a rapid increase in the field of iron and steel making, especially in Japan, where the consumption of castables has increased to more than 75% [3].
There have been numerous researches [4][5][6] investigating the influence of MgAl 2 O 4 spinel on castable properties, including the effect on grain size, density, chemical composition, hot strength and spalling resistance and et al. However, research on the effects of ZnAl 2 O 4 gahnite on the alumina castable properties is rather little. ZnAl 2 O 4 , same spinel compounds as MgAl 2 O 4 , belongs to the cubic lattice system. The general structural formula of spinel is AB 2 O 4 . Spinels have excellent physical and chemical characteristics that benefit special by the structure including high hardness, high melting points (1950°C), low thermal expansion coefficient (25-900°C, 7.0 × 10-6/℃), good chemical attack resistance and strong slag resistance [7,8].
Refractories subject to loading and unloading repeatedly because of physical effects and thermal stress shock in the service process, which is coincided with the fluctuation of the process parameters or the production cycles of the furnace [9][10][11]. Fatigue damage and even breaking are prone to occur in refractories under the repeated loading, which will affect the service life of the thermal equipment. Therefore, researches of predicting the service life of refractories under cyclic loading definitely have the guiding significance. The above questions have been analyzed by many researchers. K. Andreev et al. [12] discussed alternative methods to assess thermal shock in refractories. Methods involve repetitive thermal shock tests, cyclic strain controlled fatigue experiments and fracture mechanical experiments of monotonic loading. The results showed that strain tolerance is a property correlating the results of the alternative test methods and those from the service loads. The mechanical behavior of cement concrete refractory, a quasi-brittle material, was studied by F. Thummen et al. [13] under the tensile cyclic and static loading at room temperature. The results suggest that maximum stresses are the predominant influencing factors. In addition, T. Xu et al. [14] performed the damage and fracture tests on sandstone at different upper and lower stress ratios. The results indicated that the hysteresis loop in the damage failure curve of the model presents the characteristics of "sparse-densesparse", and the upper limit stress is one of the important factors affecting the fatigue life of the sandstone. However, the above mentioned works mainly investigated the compressive fatigue behavior of refractories. In the actual application, the working conditions of refractories are poor, which acted pull and pressure forces. It is believed that the bending force in the cyclic loading should be feasible to research the fatigue impairment. And, mechanical cycle tests can simulate several thousand cycles, while, the traditional damage tests for refractories only recycle several times. Therefore, the bending cyclic tests are more functional for studying fatigue degradation.
In the present research, ZnAl 2 O 4 gahnite powders were fabricated from ZnO and Al 2 O 3 , using solid state sintering technology. Furthermore, the effect of ZnAl 2 O 4 graphite content on the mechanical properties and fatigue characteristics of the alumina castables was investigated by cyclic loading method.

Preparation of ZnAl 2 O 4 powders
The raw materials, alumina (Al 2 O 3 , Purity≥99.0%, <74um) and Zinc oxide (ZnO, purity ≥ 99.0%), were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai). The molar ratio of Al 2 O 3 to ZnO is 1:1 according to the chemical formula of ZnAl 2 O 4 . The raw materials ZnO and Al 2 O 3 were mixed thoroughly in a ball mill, then sintered at 1400°C for 4 h. Additionally, to better investigate the formation process of ZnAl 2 O 4 powders, ZnO and Al 2 O 3 , were sintered at 900°C, 1000°C, and 1200°C.

Preparation of high alumina castables
Fused alumina (98% wt% Al 2 O 3 , 8-5, 5-3, 3-1, 1-0 mm, 320 mesh; Henan, China), the prepared ZnAl 2 O 4 powders, activated Al 2 O 3 (99%wt%, CL370), water reducer (p108, Wuhan Shanda Co., LTD) were used as the raw materials. The constituents of high alumina castable samples are shown in Table 1. Calcium aluminate cement was used as the binder. All the raw materials in certain proportion were mixed and cast in metal molds. The dimensions of all the specimens were 40 mm × 40 mm × 160 mm. The green samples were cured at room temperature for 24 h in airtight containers, then dried at 110°C in drying oven for another 24 h before firing. The samples were fired for 3 h at 1500°C in a laboratory electric furnace in air atmosphere.

Characterization and testing
X-ray diffraction (XRD, Aeris, Panaco Co., Netherlands) was used to determine the phase compositions of the specimens. Scanning electronic microscope (SEM, JSM-1T200, Japan Electronics Corporation) was used to observe the microstructures of the specimens. The distribution of powder clusters of ZnAl 2 O 4 was measured by of laser granularity analyzer (Rise-2018, Jinan Rise Science & Technology Co., Ltd.). The three-point bending test and mechanical cyclic test were conducted at ambient temperature with a universal tester (WD-P4105, Jinan Puye Electromechanical Technology Co. Ltd). The loading rate of the three-point bending test is 0.2kN/S. Mechanical Cyclic test was conducted with triangle loading waveform, as shown in Figure 1. σ max is the maximum stress of cyclic load, taking 0.94σ (σ is the rupture strength of the specimen). σ min is the minimum stress of cyclic load, taking 0.2σ. Firstly, the specimens were loaded to the lower limit stress with the constant loading speed of 0.5 mm/min. Then, the mechanical cyclic tests were carried out in triangle loading waveform until the specimens breaking. The present work focuses on the fatigue characteristics of high alumina castables with different contents of ZnAl 2 O 4 powders.

Microstructural characterizations of synthesized powders
The mixture of ZnO and Al 2 O 3 with the molar ratio of 1:1 was placed at 900°C, 1200°C, and 1400°C in the electric furnace for 4 h to identify the reaction at high temperature. As shown in Figure 2, the XRD patterns indicate that the raw materials consist of ZnO and γ-Al 2 O 3 . At 900°C, the characteristic peaks of γ-Al 2 O 3 almost disappear, and the characteristic peaks of ZnAl 2 O 4 phases appear, revealing that ZnAl 2 O 4 was generated at about 900°C. In the stage from 900°C to 1200°C, the possible reactions are as follows: (i) ZnO reacted with γ-Al 2 O 3 ; (ii) γ-Al 2 O 3 transformed into α-Al 2 O 3 , which is more stable; (iii) ZnO further reacted with the formed α-Al 2 O 3 . The possible reactions described above include the following: At 1400°C, the formation of the ZnAl 2 O 4 phase was completed. No impurity phase was detected and no phase transition occurred in the specimen, indicating that the product was of high purity.
The SEM images of ZnO, Al 2 O 3 and ZnAl 2 O 4 powders are displayed in Figure 3. It can be observed the ZnAl 2 O 4 powder products have agglomerate structures as well as the raw material Al 2 O 3 . While ZnO has short rod-like structure. This results conform with that obtained from previous research [15], further demonstrating the diffusion ability of the various atoms as follows: Zn 2+ ＞Al 3+ ＞O 2-. It means that the diffusion rate of Al 2 O 3 is lower than that of ZnO during the reaction. Hence, ZnO diffuses onto the surface of Al 2 O 3 and reacts in situ to generate ZnAl 2 O 4 phase. This explains that the formed ZnAl 2 O 4 has similar morphology and particle size to the original Al 2 O 3 powders. The particle size distribution curve is shown in Figure 4. It can be seen that the average particle size of the synthesized ZnAl 2 O 4 powders is about 80 um.

Mechanical and physical characteristics
The results of physical properties (apparent porosity and bulk density) and mechanical properties (cold modulus of rupture and cold crushing strength) of the high alumina castables containing different content (0, 1.5 wt%, 3 wt%) of ZnAl 2 O 4 , were shown in Figures 5 and 6. It can be seen that the values of apparent porosity decrease firstly, and then increases, while the values of bulk density exhibit an opposite trend with the increasing of the content of ZnAl 2 O 4 . Also, the specimen ZA-1.5 exhibits the highest strength. It may be related to its higher density and lower porosity. Figure 7 shows the stress-strain curves by monotonic loading (Figure 7(a)) and the results of the specimens under cyclic loading (Figure 7(b,d)). As can be seen from the figure, the hysteresis loop in the curves of cyclic loading revealed a general trend of first sparse and then dense. In the early stage of cyclic loading, the specimen will deform elastically, and the spacings of hysteresis loop are relatively large. Due to compacted microfractures the deformation of specimen developed quickly, which indicates that more energies were consumed at each circulation, and the damage quantity within the specimen is large. After a few cycles, the development rate of specimen deformation  decreased gradually, and the hysteresis loop is getting much denser. The specimen deformation step into a new stage of steady growth. The damage quantity at every circulation reduced sharply. Finally, the specimen appeared brittle fracture, when it reached the fatigue limit.

Mechanical cyclic test
It can be seen that the distribution configuration of the fatigue cumulative damages of high-alumina castables with different ZnAl 2 O 4 content are basically the same. ⅰ. the stage of crack arises: While the stress increasing, micro-cracks are originated quickly within the specimen. The main reason is that the stress has made some weak area almost unbearable inside the specimen. In this stage, the specimens have obvious deformation, and the hysteresis loop is sparse in the cyclic loading curves. ⅱ. expand stage: The contact strength of particles varies evenly under the continuous cyclic load. In this stage, the number of microcracks increased slowly, and the tip of micro-crack extended gradually. ⅲ. break stage: The initiation and propagation of micro-cracks lead to the through cracks, which result in the breakage of the specimens [16,17].
Bucking failure of the specimen ZA-1.5 was observed after only seven mechanical cyclic tests in the range of 0.2σ to 0.94σ. The area of hysteresis loop of the specimen ZA-1.5 is significantly smaller than others in the stable cycle stage, because the density of specimen ZA-1.5 is the highest, and the cracks propagate under every cycle. The cycle times of specimen ZA-0 and ZA-3 are both 50 times, but the time from starting to loop to fracture of specimen ZA-0 is about 2750 seconds, while 4300 seconds for specimen ZA-3, as shown in Figure 7. The results clearly indicated that specimen ZA-3 has more extreme resistance to damage than specimen ZA-0. So, the specimen ZA-3 is expected to have longer service life under the same condition. In addition to filling the inter-particle voids, some ZnAl 2 O 4 powders added to ZA-3 may remain free, which cannot maintain the closest packing of particles. This will result in more micro-cracks formed interior of the fired specimen. And these micro-cracks closed and opened under cyclic loading, which can absorb some shock and improve the toughness. Thus, the cycle times increased.
SEM images of the specimens are shown in Figure 8, which reveals the micro-structure of the castables prior to mechanical testing. It can be seen that the densest specimen is ZA-1.5, the micro-cracks of ZA-0 specimen mainly exist in the grain matrix boundary, while the pores of ZA-3 specimen are mainly from matrix. When the micro-      cracks develop into macro-cracks, catastrophic fracture will happen. The typical fractured specimens after cyclic loading test are shown in Figure 9. The low interface strength of ZA-0 results in the cracks propagation along the boundary of Al 2 O 3 grains and matrix. Therefore, there are many whole Al 2 O 3 grains in the fractured surfaces, as red area shown in Figure 9. The specimens of ZA-1.5 and ZA-3 exhibit transgranular fracture. The fatigue resistance of ZA-3 is superior to ZA-1.5, due to the matrix has some pores and micro-cracks, which can relieve stress effectively under cyclic loading.

Conclusion
(1) The optimized formation temperature of ZnAl 2 O 4 phase is 1400°C. The synthesized ZnAl 2 O 4 particles have similar morphology and particle size of Al 2 O 3 particles. The average particle size of ZnAl 2 O 4 powders is about 80um. (2) The cycle times of specimen ZA-0 and ZA-3 are both 50 times, but the time from starting to loop to fracture of specimen ZA-0 is about 2750 seconds, while 4300 seconds for specimen ZA-3. Therefore, the specimen ZA-3 has higher resistance to damage than specimen ZA-0, indicating that the specimen ZA-3 has longer service life under the same condition. (3) The results obtained by cyclic loading are similar to the rules of thermal shock resistance of refractories. That is the thermal stress factor is inversely proportional to critical stress of rupture. Because of high interface strength and high porosity in the matrix, the fatigue resistance of ZA-3 is superior to ZA-0 and ZA-1.5. The relationship between cyclic loading and conventional thermal shock experiment is still in investigation.