Microstructure and Mechanical Properties of Oxide Eutectic Fibers
- PDF / 1,757,816 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 5 Downloads / 219 Views
Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
The SEM images were analyzed by processing the digitized micrographs with the public domain NIH Image program (available at http://rsb.info.nih.gov/nih-image/), using custom procedures to calculate parameters characterizing the microstructure, as described in the next section. When we segmented the grayscale images into distinct sapphire and garnet domains by visual inspection, the calculated sapphire volume fraction f , closely matched the value of 0.45 expected from the proportions of the initial powder mixture. To facilitate consistent, objective analysis, further processing was performed with images automatically segmented subject to the constraint that f , equal the expected volume fraction. This procedure gave results agreeing with individual judgement, and was justified by the high homogeneity of the microstructure. The data obtained from such analysis represented averages over tens or hundreds of domains, and the variation from region to region or fiber to fiber was roughly 10%. The atomic force microscope provides higher resolution than SEM, yet lacks a direct means of distinguishing the sapphire and garnet phases. Samples observed in their as-grown condition had a suggestive though indistinct appearance, but we found that samples etched for 30 minutes in Ar atmosphere at 1300 - 1500'C (far below the eutectic temperature of 1830 °C) exhibited clearly discernable domains, which had different morphologies and were separated by grooves where the etching was stronger. Samples prepared in this way from both the outer surface of fibers and from inside the fibers as exposed by fracture were imaged in air with a Digital Instruments Nanoscope III AFM. Analysis of these images also made use of the NIH Image software with custom extensions. RESULTS AND DISCUSSION The fibers obtained in our experiments were generally milky and opaque in appearance, sometimes with slight coloring due to the rare earth ion (e.g. pinkish for Er, yellowish for Dy, brownish for Ho). The fiber diameter could be controlled over the range 200 [tm to 2 mm by adjustment of the heating power and pulling rate. Stable growth was observed from a pulling rate of 0.1 mm/min to 20 mm/min, the diameter varying less than 5%. The maximum length of 500 mm was limited by the apparatus. Powder XRD patterns from crushed fibers showed the presence of only the A120 3 aalumina and R3Al.0O2 garnet crystal forms. The interface between these phases in the fiber was sharp and devoid of amorphous material, as evidenced by the TEM lattice images for A12 0JR3A150 12 shown in [6]. Figure 1 shows backscattered electron images (BEI) of microstructures of various fibers grown at a range of pulling rates. The two phases form interpenetrating, three-dimensional networks, which in cross section display the "Chinese script" microstructure homogeneously over the whole fiber. The conventional concept of lamellar spacing is not clearly defined for this complex pattern, so we characterized the average domain
Data Loading...
