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Tensile Strength and Microstructure of Al2O3-ZrO2 Hypo-Eutectic Fibers Studied

Oxide eutectics offer high-temperature strength retention and creep resistance in oxidizing environments. Al2O3-ZrO2 eutectic strengths have been studied since the 1970's (refs. 1 to 10). Directionally solidified oxide eutectics exhibit improved resistance to slow crack growth and excellent strength retention at high temperatures up to 1400 °C (refs. 1 to 10). Materials studied typically contain Y2O3 to metastably retain the high-temperature cubic and tetragonal polymorphs at room temperature.

Al2O3-ZrO2 is of fundamental interest for creep studies because it combines a creep-resistant material, Al2O3, with a very low creep resistance material, ZrO2. Results on mechanical properties and microstructures of these materials will be used to define compositions for creep testing in future work. Substantial variations from the eutectic alumina to zirconia ratio can be tolerated without a loss in room-temperature strength. The effect of increasing Y2O3 addition on the room-temperature tensile strength of an Al2O3-ZrO2 material containing excess Al2O3 was examined at the NASA Glenn Research Center, where the materials were grown using Glenn's world-class laser growth facilities.

ELASTIC MODULUS FOR 68 mol% Al2O3 MATERIALS WITH INCREASING Y2O3 ADDITIONS ESTIMATED FROM THE SLOPE OF STRESS-STRAIN DATA PLOTS
Y2O3 contents, mol% None 1.1 3.2 5.1 7.6
Elastic modulus, GPa 270 370 320 290 310

During tensile testing, an optical extensometer was used to measure the relative displacement of two flags that define the test specimen gauge length. The elastic modulus was then estimated by curve fitting the slope of the stress-strain plot. Results for the five compositions are shown in the table. The range in moduli, 270 to 370 GPa, is large and can be explained, at least in part, by considering the variations in phase content with composition and growth defects.

Graphs
Left: Weibull probability plot of tensile strength for various compositions: (1) 68 mol% Al2O3 [0 mol% Y2O3], (2) 68 mol% Al2O3 [1.1 mol% Y2O3], (3) 68 mol% Al2O3 [3.2 mol% Y2O3]. Right: Weibull probability plot of tensile strength for various compositions: (1) 68 mol% Al2O3 [3.2 mol% Y2O3], (2) 68 mol% Al2O3 [5.1 mol% Y2O3], (3) 68 mol% Al2O3 [7.6 mol% Y2O3].
Long description

Factors 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, ~0.3 to ~0.2 mm, whereas pores and shrinkage cavities are several micrometers in size in materials with the highest Y2O3 content. The figures give the tensile strength data in the form of single-parameter Weibull probability, Ln(Ln(1/survival probability)), versus Ln(fracture stress). The figure on the left 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 figure on the right). 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.

The Al2O3-ZrO2(Y2O3) materials proved highly tolerant of growth defects, maintaining an average strength of 1 GPa in the presence of 1- to 2-mm pores and large shrinkage cavities, which extend with cracklike morphology along the fiber axis. Critical defects were external facets, intercolony pores, and other stress concentrators contained within the low-toughness Al2O3 phase that is in residual tension.

Future studies are planned to concentrate on strengthening the Al2O3 phase and increasing the amount of crack deflection experienced at the Al2O3-ZrO2 interfaces through the use of dopants to achieve a combination of high strength and an acceptable toughness.

References

  1. Schmid, F.; and Viechnicki, D.: Oriented Eutectic Microstructures in the System Al2O3/ZrO2. J. Mat. Sci., vol. 5, 1970, pp. 470-473.
  2. Cocks, F.H.; Pollock, J.T.A.; and Bailey, J.S.: Preparation of In Situ Composites by Edge-Defined, Film-Fed Growth (EFG) Directional Solidification. GIPXA, 1973, pp. 141-152.
  3. Hulse, C.O.; and Batt, J.A.: The Effect of Eutectic Microstructures on the Mechanical Properties of Ceramic Oxides. United Aircraft Corp. UARL-N910803-10, 1974.
  4. Mazerolles, L.; Michel, D.; and Portier, R.: Interfaces in Oriented Al2O3-ZrO2(Y2O3) Eutectics. J. Am. Ceram. Soc., vol. 69, no. 3, 1986, pp. 252-255.
  5. Borodin, V.A.; Starostin, M. Yu.; and Yalovets, T.N.: Structure and Related Mechanical Properties of Shaped Eutectic Al2O3-ZrO2(Y2O3) Composites. J. Cryst. Growth, vol. 104, 1990, pp. 148-153.
  6. Bates, H.E.: EFG Growth of Alumina-Zirconia Eutectic Fibers. Ceram. Eng. Sci. Proc., July-Aug., 1992, pp. 190-197.
  7. Courtright, E.L.; Haggerty, H.S.; and Sigalovsky, J.: Controlling Microstructures in ZrO2(Y2O3)-Al 2O3 Eutectic Fibers. Ceram. Eng. Sci. Proc., July-Aug., 1993, pp. 671-681.
  8. Farmer, S.C.; Sayir, A.; and Dickerson, P.O.: Mechanical and Microstructural Characterization of Directionally-Solidified Alumina-Zirconia Eutectic Fibers. Proceedings of a Symposium on In Situ Composites: Science and Technology, TMS, Warrendale, PA, 1993, pp. 167-182.
  9. Sayir, A., et al.: High Temperature Mechanical Properties of Al2O3/ZrO2(Y2 O3) Fibers. Mater. Res. Soc. Symp. Proc., vol. 365, MRS, Warrendale, PA, 1995, pp. 21-27.
  10. Pardo, Jose A., et al.: Piezospectroscopic Study of Residual Stresses in Al2O3-ZrO2 Directionally Solidified Eutectics. J. Am. Ceram. Soc., vol. 83, no. 11, 2000, pp. 2745-2752.

Glenn contact: Dr. Serene Farmer, 216-433-3289, Serene.C.Farmer@grc.nasa.gov
CWRU contact: Ali Sayir, 216-433-6254, Ali.Sayir@grc.nasa.gov
Authors: Dr. Serene C. Farmer and Dr. Ali Sayir
Headquarters program office: OAT
Programs/Projects: HOTPC

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