Alumina-based crucibles with enhanced thermal shock resistance via reinforcement of mullite fibers

: Crucible is the indispensable vessel in smelting of superalloy, and the service life of crucibles is quite important to the development of superalloy. In order to extend the service life of crucible for superalloy smelting, alumina-based crucibles are prepared by slip casting, and mullite fibers are used as reinforcement phase. The effect of mullite fibers on the microstructure and property of the crucibles was studied. The appropriate content of mullite fibers are beneficial to the comprehensive performance of the crucibles. The reinforcement mechanism of mullite fibers includes crack deflection, fiber extraction, and fiber bridging, through which fibers hinder the initiation and propagation of cracks in ceramics. For the crucible samples with 3 wt% of mullite fibers, the values of flexure strength are 22.47 MPa and 19.14 MPa at room temperature and high-temperature, respectively. After thermal shock for 5 times, the residual room-temperature flexural strength is 16.65 MPa and the strength retention rate is 74.1%. It is confirmed that, the 3 wt% of mullite fibers improves the thermal shock resistance of the crucible, which is beneficial for extending the service life of the crucible for superalloy melting.


Introduction
Superalloy [1][2][3] is a class of metal material that has excellent performance at high temperature.Superalloys generally display superior high-temperature strength, resistance to oxidation, and resistance to thermal corrosion, as well as excellent fatigue performance, fracture toughness and other comprehensive properties.Superalloys are widely employed in the aviation jet engines and various industrial gas turbines [4,5].The preparation of superalloy has a significant impact on their properties.In order to obtain superalloy meeting the usage conditions along with excellent performance, it is necessary to pay attention to the melting and preparation process of the alloys.At present, the main method for preparing most superalloys is vacuum induction melting [6,7].Crucibles [8][9][10] are employed as the container for vacuum induction melting, and their performance and quality are very important for the preparation of superalloys.
The crucibles are faced with high temperatures of 1550℃ during melting of superalloy; therefore, high temperature resistance is demanded.Meanwhile, under the environments of high temperature and vacuum during melting of superalloys, the crucible and superalloy are faced with great challenges in temperature and pressure, and various physical and chemical interactions occur between the crucible and alloy melts, which inevitably leads to corrosion of the crucible material and contamination of the melt.Therefore, good chemical stability [11][12][13] is required for the crucible.Moreover, the crucible needs to withstand extremely high temperatures during the melting process and cools down quickly during the casting process.The rapid periodical heating and cooling effects easily result in stress of expansion and contraction within the crucible, which leads to significant decrease in strength, surface peeling and even fracture.Therefore, it is required that the crucible have excellent thermal shock resistance [14][15][16][17][18].A crucible with excellent thermal shock resistance is beneficial to avoid pollution caused by the detachment of crucible materials, improve the performance of high-temperature alloys, and greatly reduce production costs.Therefore, investigation on alumina crucibles with good thermal shock resistance for melting and extending the working life of the crucibles is of great significance for the development of high-temperature alloy melting.
MgO, CaO and Al2O3 are common crucible materials for melting of superalloy, which have high melting point, chemical stability with superalloy melts, and resistance to high temperatures above 1550℃ [19,20].However, under high temperature and vacuum conditions, MgO crucibles generally supply oxygen to the alloy, while CaO crucibles are prone to hydration.In contrast, Al2O3 ceramics have good oxidation resistance, chemical stability, high temperature resistance, and hightemperature strength; moreover, they are abundant in resources and low in price, making them the most widely used oxide ceramics [21][22][23][24][25][26].At present, commonly used alumina crucibles have poor thermal shock resistance, the surface of the crucible often cracks and falls after once usage, making the continue use impossible.The short service life increases the production cost of high-temperature alloys.
To improve the mechanical property and thermal shock resistance of ceramics, various investigations are performed, among which ceramics reinforced by distribution of fibers or whiskers are widely reported [27][28][29][30], displaying superior properties compared to the matrix material.Mullite fiber, known as a superior refractory material, has superior resistance to thermal shock, strong resistance to creep deformation, and high load softening temperature [31][32][33], which can have significant effects on the high-temperature properties of silica-based ceramic cores.Mullite is a very important composition in Al2O3-SiO2 system and the most stable binary solid solution compound.It has good thermal shock resistance, high temperature creep resistance, stable chemical properties, low coefficient of thermal expansion and thermal conductivity, and is a high-quality hightemperature structural refractory.The introduction of mullite fibers in ceramics is expected to increase the fracture energy, enhance the loss tolerance, strengthen the matrix.However, thermal shock resistance of crucibles enhanced by mullite fibers is seldom reported.
In attempt to improve the high-temperature resistance and thermal shock resistance of crucibles, alumina-based crucibles are prepared by slip casting, and mullite fibers are used as reinforcement phase.The effects of mullite fiber content on the mechanical property and thermal shock resistance are explored, and the mechanisms are analyzed.

Raw materials
The major raw materials are consisted of two types of Al2O3 powders with different particle sizes, and SiO2 powders with different particle sizes as the auxiliary materials.Mullite fibers are employed as reinforcement phase with mass percent of 0~4%, whose average diameter is about 8 μm, and the average length is above 100 μm.The dimensional features and specific information of the powders and mullite fibers are listed in Table 1.Ammonia solution was used as pH regulator, and the dispersant is the mixture of triammonium citrate and gum arabic with volume ratio of 1:1.

Sample preparation
The raw materials were mixed in deionized water, and the slurry was prepared with solid loading of 66 vol%.In order to avoid agglomeration during the mixing process, water and dispersant were mixed first, after which fine Al2O3 was added, and other raw materials were added at last.After ball milling for 2 h using a planetary miller, homogeneous slurry with different content of mullite fibers was obtained.The prepared suspension were poured into a gypsum mold with inner size of 6 mm × 6 mm × 60 mm.After the slurry solidified in the gypsum mold, demoulding was carried out.Then the green bodies are dried at room temperature, and then placed in a drying oven at 60 ℃.The dried bodies were sintered at 1300 ℃ for 2 h to obtain ceramic samples.

Testing and characterization
Archimedes' method according to ASTM C373 was conducted to determine the bulk density and porosity of the sintered ceramics.Scanning electron microscope (SEM, JSM-5600LV, JEOL, Japan) was employed to analyze the microstructure of the fracture surface.X-ray diffraction analysis (XRD,D/Max-2500v/pc, Rigaku, Japan) with Cu Kα radiation was performed to characterize the phase composition.The length of samples before and after sintering were measured using a vernier scale, to determine the linear shrinkage of samples.The bending strength were tested by universal testing machine (HMOR16-300LX, China) at room temperature and 1540℃.The thermal expansion coefficient of the sintered sample was measured using a thermal expansion instrument (DIL402 PC, NETZSCH, German).
The thermal shock resistance of sintered samples is measured by rapid cooling in air.To evaluate the thermal shock resistance of the material, the sample was heated at 1300℃ for 0.5 h before air cooling and quenching.After cooling in air for 10 min, the sample was further heated at 1300℃ for 0.5 h.The sample underwent repeated cycles of rapid heating and cooling, and the residual strength of the sample after air quenching was measured at different cycles.

Physical property
The effect of mullite fiber content on the linear shrinkage, apparent porosity, and bulk density of the crucible samples sintered at 1300 ℃ is shown in Figure 1.After fibers are added, the linear shrinkage shows a decreasing trend, which decreases from 1.06% to 0.89% as 1 wt% of mullite fibers are doped.The decreasing trend of linear shrinkage is attributed to the large aspect ratio of mullite fibers, as the fibers form network structures and hinder the structural densification as well as volume shrinkage of ceramics.Due to the hindering of densification by mullite fibers, the content of pores in ceramic samples increases and leading to higher apparent porosity, whose value increases from 18.7% to 24.8% as 1 wt% of mullite fibers are doped.On the contrary, the bulk density decreases from 2.37 g/cm 3 to 2.33 g/cm 3 with the increasing content of fiber from 0 to 1 wt%, attributed to the increase of porosity.As shown in Figure 2, the addition of fibers has a significant impact on the thermal expansion coefficient of ceramics.The thermal expansion coefficient of the crucible samples increases significantly from 2.77×10 −6 to 6.96×10 −6 with the increasing content of mullite fibers from 0 to 1 wt%, but the coefficient basically remains unchanged ranging from 6.80×10 −6 to 6.96×10 −6 when the fiber content continues to increase to 4 wt%.Thermal expansion is a phenomenon based on the change of average distance between atoms in the lattice structure during the variation of temperature.Due to the different thermal expansion coefficients between mullite fibers and the matrix, thermal stresses are generated between the different phases during heating, and lead to additional expansion.Therefore, the doping of mullite fibers obviously increases the thermal expansion coefficient of crucible samples.
Figure 2: The influence of fiber content on thermal expansion coefficient.

Flexural strength
Flexural strengths of the crucible sample with different content of mullite fibers are measured at room temperature and high temperature, as shown in Fig. 3.The effect of mullite fiber content on the flexural strength is ascribed to two aspects.On one hand, the volume fraction of pores increases with the doping of mullite fibers, which is not conducive to the mechanical property of ceramics, considering that pores easily turn to the main location for defect formation, where the cracks and other defects first form, leading to decreased strength of ceramics with higher porosity.On the other hand, the fibers can reinforce the flexural strength by fiber extraction, fiber deflection, and fiber bridging.As shown in Fig. 3, with the increase of fiber content, the room temperature and high temperature strength showed a trend of first increasing and then decreasing, suggesting that the deteriorate caused by pores is level-pegging with the reinforcement of fibers.As the weight percent of mullite fibers is 3 wt%, its room flexural strength showed the highest values of 22.47 MPa and 19.14 MPa at temperature and high temperature, respectively.SEM images are taken to analyze the fracture morphology of samples with different fiber content, as shown in Fig 5 .As the content of mullite fibers is 1 wt%, a small amount of fibers can be observed in the matrix.Due to the low fiber content, fiber extraction and other phenomena can only be detected in local areas.When the content of mullite fibers is 4 wt%, due to the high content of mullite fibers, they aggregate and distributed unevenly in the matrix, which is agreed with the decreased mechanical property.According to the SEM image, it is suggested that crack deflection occurs in the presence of fibers, which reduces the stress field intensity in the material and leads to enhanced flexural strength.Fiber pulling out is also detected in the microstructure, which requires energy consumption, relaxes the stress at the crack tip, reduces the speed of crack propagation, and plays a role in strengthening and toughening.In addition, for fibers with a specific orientation distribution, bridging will occur, connecting the two sides of the crack and applying a certain amount of compressive stress on the surface of the crack.The presence of compressive stress will offset the tensile stress at the crack tip, closing the crack and hindering its further expansion, thus beneficial to the reinforcement effect.The occurrence of these three phenomena contributes to hindering the initiation and propagation of cracks, thereby improving the performance of the crucible, indicating that the addition of an appropriate amount of fibers can play a role of reinforcement.

Thermal shock resistance
In order to evaluate the thermal shock resistance of the crucible, the crucible samples with 3 wt% of mullite fibers are heated at 1300 ℃ for 0.5 h and then air cooled and quenched for 10 min.The residual room-temperature flexural strength and the strength retention rate compare to the initial strength of the crucible samples were measured after the sample underwent rapid heat and cooling cycles for 1, 2, 3 and 5 times, are shown in Fig. 6.After once thermal shock, the room temperature flexural strength is 17.28 MPa, and the strength retention rate is 76.9%.After thermal shock for 5 times, the flexural strength of the sample decreases to 16.65 MPa, and the strength retention rate is 74.1%.Liu [34] tested the thermal shock resistance of alumina ceramics using the residual strength of the sample at a certain temperature difference.When the temperature difference is 700 ℃, the residual strength of the sample is about 45% of the initial strength.In Zhang's work [35], cordierite ceramics were heated to 900 ℃, and suffered water-cooled quenching to explore its thermal shock resistance.When the temperature difference is ΔT=450 ℃, the residual strength decreases to 1/3 of the initial strength.Ma [36] studied the thermal shock resistance of Al2O3-SiC-C refractory material by heating the sample to 1100 ℃ and water-cooled for 5 times, and the strength retention rate was less than 52%.In comparison, the high strength retention rate in this work indicates that the sample has good thermal shock stability, which can improve the service life of the crucible to withstand the temperature difference between high-temperature alloy melting and casting.

Conclusions
In order to extend the service life for alumina-based crucible for superalloy smelting, aluminabased crucibles are prepared by slip casting, and mullite fibers are used as reinforcement phase.The effect of mullite fibers on the microstructure and property of the crucibles was studied.The appropriate content of mullite fibers are beneficial to the comprehensive performance of the crucibles.The reinforcement mechanism of mullite fibers includes crack deflection, fiber extraction, and fiber bridging, through which fibers hinder the initiation and propagation of cracks in ceramics.For the crucible samples with 3 wt% of mullite fibers, the values of flexure strength are 22.47 MPa and 19.14 MPa at room temperature and high-temperature, respectively.After thermal shock for 5 times, the residual room-temperature flexural strength is 16.65 MPa and the strength retention rate is 74.1%.It is confirmed that, the 3 wt% of mullite fibers improves the thermal shock resistance of the crucible, which is beneficial for extending the service life of the crucible for superalloy melting.

Figure 1 :
Figure 1: The influence of fiber content on line shrinkage, apparent porosity and bulk density of crucible samples.

Figure 3 :
Figure 3: The influence of fiber content on flexural strength at room temperature and high temperature.

Figure 4 :
Figure 4: XRD patterns of crucible samples with different contents of mullite fibers

Figure 6 :
Figure 6: Residual strength and strength retention rate of samples with 3 wt% of mullite fibers.

Table 1 :
Information of the raw materials