HREM observations of microstructrues of silicon carbide whiskers

Abstract

As we know, SiC whiskers take an important role in lightweight composite materials such as Al-Li-SiC material and etc. The study on microstructures of SiC whiskers is needed for well understanding the properties of the composite materials reinforced by SiC whiskers. Microstructures such as multi-twins and polytypes of bulk crystal of SiC have been studied in detail. Information of microstructures of SiC whiskers in atomic resolution, however, has not been reported yet.Defects in SiC whiskers made from rice hulls were identified and analyzed using TEM. The whiskers were characterized by a high density of planar faults lying on closed planes perpendicular to the whisker axis. Defects in SiC whiskers here (made by solid reaction method, IMR, Academia Sinica) were different. The basic structure of SiC whiskers here is β-type (f.c.c.) (Fig.1a). There were many twins and some polytypes, too. Fig.1a shows normal structure with many twins formed on the (111) planes perpendicular to the growth direction of the whiskers. There were two kinds of microtwins. Multiple twins, which may occurred on four sets of the {111} twin planes ( Fig.1b gives its diffraction patterns). Normally, only multiple twins on two sets of {111} twin planes, which were parallel to the direction of electron beams (viewed along a [110] axis), could be seen (Fig.1c). Thin lamellar twins of predominant β phase interspersed with stacking faults (Fig.1a). Within the twinned lamellae, microtwins sometimes terminated in the matrix (B in Fig.1a). Fig.1d gives clear image of this kind of termination in another area. The interfacial structures between such twins and the matrix were twinning dislocations and Shockley partials. It will be discussed in detailed elsewhere. It can be seen in Fig. 2 that there was a intrinsic stacking fault interrupted by a intersected twin and closed by two twins. Generally, there were few intrinsic stacking faults but many extrinsic ones as indicated by the arrows in Fig.1a. This is because that the energy of intrinsic stacking fault is higher than that of extrinsic one. Polytypes of 2H (Fig. 3a), 4H (Fig.b), 6H (Fig.c), 9R (Fig.d) and 15R (Fig.e) were observed in SiC whiskers by HREM. It is interesting that Fig.3c was taken from a bar of the whisker with strange [1010] growth direction, because the normal growth axis should be [0001] in α-SiC.