Atomic-scale microstructures of Zr2Al3C4 and Zr3Al3C5 ceramics

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3848 Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851 Fig. 6. Z-contrast STEM image of the intergrown structure of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 : (a) raw image and (b) FFT filtered image. There can be either two or three Zr layers separating the Al 3 C 2 units. Besides the intergrown structure of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 , stacking faults were also identified in these two ternary carbides. Fig. 7(a) shows a low-magnification bright-field image of stacking faults in Zr 3 Al 3 C 5 . Bright and dark fringes were observed. In order to clarify the microstructure of the stacking faults, Z-contrast STEM images were obtained. As displayed in Fig. 7(b), the periodicity of the stacking sequence was not identical. The atomic-scale microstructure of the stacking faults is further illustrated in Fig. 7(c) and (d). The high-resolution Z- contrast image indicated that the stacking faults resulted from the insertion of an additional Zr–C layer. Yu et al. [27] also reported similar stacking faults with the insertion of additional Ti–C layers in the ternary Ti–Si–C system. The above observations demonstrate that the structural difference between Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 is the thickness of (ZrC) m units between the Al 3 C 2 units. In order to illustrate the influence of this difference on the properties of ternary Zr–Al–C carbides, theoretical mechanical properties for Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 are compared. The calculated bulk modulus B, shear modulus G, and anisotropic Young’s modulus E of ZrC, Zr 2 Al 3 C 4 , and Zr 3 Al 3 C 5 [28], together with experimental values for polycrystalline ZrC [29] are presented in Table 1. It is seen that the calculated data for ZrC agree well with the experimental values, suggesting the reliability of the theoretical results. The main difference in mechanical properties of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 lies in the magnitude of the shear modulus. The shear modulus of Zr 3 Al 3 C 5 (166 GPa) is 37% higher than that of Zr 2 Al 3 C 4 (121 GPa). Therefore, the shear modulus of ternary Zr–Al–C carbides strongly depends on the atomic-scale arrangements. It is also seen that Zr 3 Al 3 C 5 properly conserves the excellent mechanical properties of binary carbide ZrC. Consequently, Zr 3 Al 3 C 5 is predicted to be a promising carbide for ultrahigh-temperature applications. On the other hand, the low shear modulus may lead to Zr 2 Al 3 C 4 possessing similar properties to ternary carbides such as Ti 3 AlC 2 [3]. Therefore, it is very important to control the microstructure in the synthesized Zr–Al–C compounds. 3.3. Determination of ZrC inclusions in Zr 3 Al 3 C 5 grains As shown in a Z-contrast image in Fig. 8(a), inclusions of rounded grains have been found. The contrasting white regions showed that the diameter of the circular grains ranged from 20 to 200 nm. Typical nanobeam diffraction (NBD) patterns taken from the inclusions are displayed in Fig. 8(b) and (c). These two NBD patterns were indexed to be [110] and [111] of cubic ZrC, which indicated that the inclusions were ZrC particles. Morgiel et al. [30] have also reported the inclusions of binary carbide TiC in the ternary Ti 3 SiC 2 ceramic. The present results for the microstructure are definitely useful in optimizing the processing and properties of these ternary carbides. Both carbides can be fabricated from elemental Zr, Al, and graphite powders at 1700 °C. Although X-ray diffraction analysis showed that the Zr 3 Al 3 C 5 obtained is predominantly single phase, TEM analysis indicated that minor amounts of ZrC inclusions were present in the Zr 3 Al 3 C 5 grains (Fig. 8(a)–(c)). It has been demonstrated that Zr 3 Al 3 C 5 has higher oxidation resistance than ZrC [14]. To achieve good high-temperature oxidation resistance, further optimization is needed to improve the purity of Zr 3 Al 3 C 5 . High-resolution imaging of the inclusions was conducted. A typical FFT pattern of an HRTEM image of ZrC is shown in Fig. 9(a). The FFT pattern was indexed to be twinned ZrC with the incident beam parallel to the [110] direction. Linear diffraction streaks besides the twinned reflections were observed, suggesting the presence of two-dimensional defects. Insight into the twinned ZrC
Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851 3849 Fig. 7. (a) Low-magnification bright-field image of stacking faults of Zr 3 Al 3 C 5 . (b) Medium-magnification Z-contrast image showing the periodicity of the stacking faults. (c) Raw high-magnification Z-contrast STEM image of a stacking fault resulting from the insertion of an additional ZrC layer. (d) FFT filtered image of (c). The arrows in (b) indicate the stacking faults. Table 1 Calculated bulk modulus B, shear modulus G, and anisotropic Young’s modulus E of ZrC, Zr 2 Al 3 C 4 , and Zr 3 Al 3 C 5 , together with experimental values for polycrystalline ZrC for comparison Method B (GPa) G (GPa) E (GPa) ZrC Calc. [28] 229 170 408 Expt. [29] 223 170 407 Zr 2 Al 3 C 4 Calc. [28] 190 121 E x = 356 E y = 302 Zr 3 Al 3 C 5 Calc. [28] 202 166 E x = 388 E y = 346 microstructure showed that a very thin Al platelet with several atomic layers was present at the ZrC twin boundaries. A typical Z-contrast image with three Al atomic layers at a ZrC twin boundary is displayed in Fig. 9(b) and (c). The spots with higher intensity correspond to Zr and the rest denote Al. The structure of this thin Al platelet was identical to the Al 3 C 2 unit in ternary Zr–Al–C carbides, suggesting that ternary Zr–Al–C thin platelets could form at the ZrC twin boundaries. TEM investigations of bulk ZrC are very limited because of the difficulties in sintering ZrC ceramic [31].
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Atomic-scale microstructures of Zr2Al3C4 and Zr3Al3C5 ceramics

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