Atomic-scale microstructures of Zr2Al3C4 and Zr3Al3C5 ceramics

3846 Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851
The symmetry of Zr 3 Al 3 C 5 was also determined using
SAED and CBED. The lattice parameters obtained were
a = 0.33 nm and c = 2.76 nm, which are in good agreement
with previously reported data [9,16]. Zr 3 Al 3 C 5 was also
determined to have a point group 6/mmm and a space
group P6 3 /mmc, which is consistent with that reported by
Mikhalenko et al. [8,9]. The SAED and CBED patterns
of Zr 3 Al 3 C 5 were similar to those of Zr 2 Al 3 C 4 and are
therefore not shown for brevity.
3.2. Microstructure of hexagonal Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5
Fig. 3(a) and (b) display the low-magnification brightfield
TEM images of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 , respectively.
The overall characteristics of the microstructures of the assynthesized
ceramics can be observed. Extensive TEM
observations have shown that the Zr 2 Al 3 C 4 grains had salient
features, i.e. the grains generally had elongated morphologies
ranging from 2 to 20 lm in length and 30 to
500 nm in width in the perpendicular direction. SAED
analysis revealed that the crystallographic direction
[0001] of Zr 2 Al 3 C 4 was perpendicular to the elongated
direction of the Zr 2 Al 3 C 4 grains. Rotations between grains
were found to be along the [0001] axis and along the direction
perpendicular to [0001]. Most of the grain boundaries
were large-angle grain boundaries; and grains with misorientation
of only several degrees were also observed. In contrast,
the Zr 3 Al 3 C 5 grains were less regular as displayed in
Fig. 3(b). In addition, some grains were found to crystallize
in the form of elongated slabs, which were similar to those
in Zr 2 Al 3 C 4 .
It is generally believed that elongated grains with large
aspect ratio would benefit the toughness of ceramics. For
example, elongated Si 3 N 4 grains were effective in toughening
a ceramic by crack deflection and crack bridging
[21,22].SoZr 2 Al 3 C 4 with elongated grains may display better
toughness than Zr 3 Al 3 C 5 . The difference of grain morphologies
between Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 may originate
from the different liquid phase content during the synthesizing
processes. It is possible that the relatively higher
liquid phase during the synthesis of Zr 2 Al 3 C 4 favors the
formation of elongated grains. During the synthesis of
Si 3 N 4 , Perera et al. [22] have shown that sintering additives
strongly influence the grain morphologies of the ceramics
by controlling the liquid phase content. Their work may
support our hypothesis.
Fig. 4 shows a high-resolution TEM (HRTEM) image
of Zr 2 Al 3 C 4 obtained with the incident beam parallel to
the ½1 210Š direction. The image fringes with a periodicity
of 2.24 nm along the [0001] direction can be clearly seen.
The C atoms cannot be resolved in the microscopes
because of its weak diffraction power. The bright spots
can be described as a layered stacking with a sequence of
ABCBACBABC along the [0001] direction. This type of
stacking sequence corresponds to the Zr and Al atoms in
the Zr 2 Al 3 C 4 structure and is consistent with the structure
previously proposed by Parthé and Chabot [10].
Unfortunately, the atomic positions of Zr and Al cannot
be distinguished using conventional HRTEM because this
technique uses phase-contrast imaging.
Z-contrast STEM imaging, which was developed by
Pennycook and co-workers [23,24], can efficiently
distinguish different atoms because this technique uses
high-angle inelastic electrons. This technique removes the
coherent effects of diffraction and leads to strong atomic
number, Z, contrast. The intensity of the obtained image
is proportional to Z 2 , the square of atomic number.
Therefore, Z-contrast STEM images are more directly
Fig. 3. Typical TEM bright-field images of: (a) Zr 2 Al 3 C 4 and (b) Zr 3 Al 3 C 5 . The Zr 2 Al 3 C 4 grains generally have elongated morphologies while the
Zr 3 Al 3 C 5 grains are irregular.