Back to articleFactors expected to affect eutectic strength are lamellar spacing, width of intercolony boundary regions and associated phases, voids and growth defects. The scale of these microstructural features extends over an order of magnitude. Interlamellar spacings are very fine, ~0.3to ~0.2 mm, whereas pores and shrinkage cavities are several micrometers in size in materials with the highest Y2O3 content. The tensile strength data are given in the form of single-parameter Weibull probability, Ln(Ln(1/survival probability), versus Ln(fracture stress). The first figure shows the test results for fibers with 68 mol% Al2O3 and 0 to 3.2 mol% Y2O3. Mean strengths range from 0.74 for no Y2O3 addition to a maximum of 1.3 for the 1.1 mol% Y2O3 addition. Fracture strength of the strongest 1.1 mol% Y2O3 fibers is 1.9 GPa. Strengths decline with additional Y2O3. The differences in the means of the first figure, although small, are significant at the 90-percent confidence level calculated using the student t-test. Strengths remain at ~1 GPa for 3.2 to 7.6 mol% Y2O3 materials (as shown in the second figure). Differences in the means for these compositions were not statistically significant. For compositions with the highest Y2O3 content (5.1 and 7.6 mol% Y2O3), there is an increase in Weibull modulus. The highest Y2O3 fibers contain a population of large pores and shrinkage voids within the wide colony boundaries. The defects result from the solidification conditions. Even with an apparently large flaw population contributing to failure over a narrower range of strengths, the mean strength remains approximately 1 GPa.
Last updated: June 2002
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