High Temperature Mechanical Behavior of MgAl₂O₄-YAG Eutectic Ceramic In Situ Composites by Float Zone Method

Creep behavior

The compressive stress (σ)– strain (ε˙) curves of the MAS-YAG samples are given in Figure 3 as a function of temperature. The creep data were analyzed using power-law relationship:

Figure 3:

The compressive strain rate-stress behavior as a function of temperature for MgAl₂O₄-Y₃Al₅O₁₂ crystal.

Here, A is a constant, k the Boltzmann’s constant, T the absolute temperature, σ the creep stress, n the stress exponent, Q the activation energy of an Arrhenius temperature dependence and R the gas constant [37]. The stress dependence of strain rates at 1,500 °C, 1,600 °C and 1,700 °C is almost similar for the MgAl₂O₄-Y₃Al₅O₁₂ crystal. The creep rate of MAS-YAG eutectic at 1,700 °C exceeds that of MAS-YAG eutectics at 1,600 and 1,500 °C. Under the same stress, the strain rate of MAS-YAG eutectic at 1,700 °C is slightly higher than that of 1,600 °C and 1,500 °C. The results indicate that the creep strength of MAS/YAG ceramics was increased insignificantly by 2.4 %, and 5.2 % when the temperatures were increased from 1,500 °C to 1,600 °C, and from 1,600 °C to 1,700 °C, respectively. Elevated temperatures cause no significant change in the creep resistance. For traditional sintered ceramics, increase of temperature decreases creep resistance. But for directionally solidified MAS/YAG ceramics, specific structure of interface between two phases prevents crack propagation. Because this region enables dislocation motion as well as restricts crack growth.

Tensile properties

The tensile strength-temperature curve is shown in Figure 4. It can be seen that tensile strength decreases gradually with increasing in temperature. MAS-YAG structure maintains its ambient temperature strength up to 1,200 °C. The tensile strength dropped rapidly above 1,200 °C. The strength of directionally solidified MAS-YAG (DSMY) sample is higher than the Al₂O₃-YAG (≈780 MPa) structure [38] for 1,200 °C. The strength of DSMY at 1,400 °C was significantly lower than at 1,200 °C. It is obvious that temperature dependent tensile strength properties are effective on ductility of MAS-YAG crystal. At 1,200 °C, MAS-YAG crystal having higher tensile strength than directionally solidified Al₂O₃-YAG composite shows that MAS-YAG crystal eliminates the stress exposed under high temperature but at the same time it preserves its strength.

Figure 4:

Temperature-dependent tensile strength of MgAl₂O₄-Y₃Al₅O₁₂.

Temperature dependence of Young’s modulus

The Young’s moduli of MAS-YAG sample of 10 mm (width)×5 mm (height)×48 mm (length) were measured by resonant frequency damping analyzer (RFDA). The Young’s modulus of the MAS-YAG sample calculated as [39,40].

where m is the mass of sample, f_f is the resonant frequency, b, d and l are the width, thickness and length of the MAS-YAG structure respectively, and T₁ is a correction factor. Figure 5 shows the temperature dependence of elastic modulus directionally solidified MAS-YAG crystal. The elastic modulus of MAS-YAG decreased as temperature increased. At 1,200 °C the Young’s moduli were measured to be 328±91 GPa for MAS-YAG crystal. Similar results were obtained using a finite element method (FEM) for Al₂O₃-YAG system [40]. The Young’s modulus of MAS-YAG crystal decreases with temperature and remains 298 ± 62 GPa at 1,400 °C.

Figure 5:

Young’s modulus at varying temperatures for directionally solidified eutectic ceramic MgAl₂O₄-Y₃Al₅O₁₂.

Fracture toughness

The fracture toughness of MAS-YAG sample was determined by means of three point bending test using SENB method. The effect of temperature on the fracture toughness of the MAS-YAG crystal is shown in Figure 6. It can be seen that the fracture toughness of MAS-YAG eutectic slightly decreases when the temperature changes from 25 to 1,200 °C in vacuum. The toughness of MAS-YAG was 3.92 MPa m^1/2 at room temperature, 3.84 MPa m^1/2 at 1,200 °C. The fracture toughness of MAS-YAG material is not affected from temperature variation as much as elasticity. Although it has been reported that fracture toughness increases when the temperature of cubic complex crystals increases, MAS-YAG eutectic structure is not been affected in this situation [36]. At traditional materials, the increasing of toughness as much as the increasing of temperature and the decreasing of elasticity was expected. The retention of toughness’s value together with temperature has been to explain why MAS-YAG crystal is not deformed at high temperatures. This situation has been evaluated at the following sections in terms of microstructure.

Figure 6:

The fracture toughness of MgAl₂O₄-Y₃Al₅O₁₂ crystal as a function of temperatures.

Microstructure analysis

Figure 7 shows the scanning electron microscope (SEM) image of the cross-section fracture surface for the directionally solidified eutectic MAS-YAG crystal after creep test at various temperatures. EDS analysis indicated that the bright area was Y₃Al₅O₁₂ and the dark area was MgAl₂O₄. MAS-YAG crystals show no grain growth at three different temperatures. A very limited ductile deformation has been seen at the samples that are exposed to creep test at 1,600 °C and 1,700 °C.

Figure 7:

Scanning electron micrograph of the MgAl₂O₄-Y₃Al₅O₁₂ eutectic after creep deformation at various temperatures. For a directionally solidified MAS-YAG structure: (a) at 1,500 °C, (b) at 1,600 °C, and (c) at 1,700 °C. The bright areas are Y₃Al₅O₁₂ and the dark areas are MgAl₂O₄.

Figure 8 shows a TEM image of the microstructure of the plastically deformed DSMY specimen after the tensile test at 1,500 °C. Dislocation structures have been seen in both phases too. Formed plastic deformation is the result of dislocation motion. A plastic behavior occurring as a result of grain boundary sliding as it happened at polycrystals is out of question. The difference at deformation behaviors in both phases is due to discrepancies of creep resistances of both phases [41]. This discrepancy has obstructed seeing dislocation structure between two-phase boundary. Figure 9 shows SEM images of the fracture surface after tensile testing at 1,200 °C and 1,500 °C. The cracks shown in Figure 9(b) are to decrease tensile strength of overall structure when Figure 4 is considered. In this context, also no formation of cracks in Figure 9(a) is due to this reason. Despite the increase in the temperature, preservation of rigid structure explains why temperature has no impact upon toughness at directionally solidified eutectic MAS-YAG ceramics. As it has also been seen in Figure 8, it is possible that dislocations do not exist in the interface area and it is assumed that this situation provides that crack propagation stays in the structure.

Figure 8:

TEM images showing dislocation structures of MgAl₂O₄-Y₃Al₅O₁₂ phase boundary in the eutectic structure after the tensile test at 1,500 °C.

Figure 9:

SEM images of the fracture surfaces for MgAl₂O₄-Y₃Al₅O₁₂ eutectics after the tensile tests (a) at 1,200 °C, and (b) at 1,500 °C. The bright areas are Y₃Al₅O₁₂ and the dark areas are MgAl₂O₄.