Atomic-scale microstructures of Zr2Al3C4 and Zr3Al3C5 ceramics

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