Influence of whisker volume fraction on the creep behavior of alumina composites reinforced with silicon carbide
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Terence G. Langdon Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, California 90089-1453 (Received 21 July 1994; accepted 4 August 1995)
Four-point bending creep tests were conducted in air on two alumina matrix composites reinforced with 18 and 30 vol% of silicon carbide whiskers, respectively. In both materials, the SiC whiskers tended to form agglomerates. In the temperature range from 1673 to 1823 K, the stress exponents, n, were ~3.9 and —6.3 and the activation energies for creep, Q, were ~690-740 and —930-1010 kJ mor 1 for the composites containing 18 and 30 vol% of SiC, respectively. It is shown that the higher value of n in the composite with 30 vol % of SiC whiskers may be lowered to —3 by incorporating a threshold stress. The creep strength of both composites was enhanced by comparison with a similar composite containing 9.3 vol % of SiC whiskers, but there was only a very minor improvement in creep strength when the volume fraction of whiskers was increased from 18 to 30 vol %.
I. INTRODUCTION An earlier report described the creep properties of an alumina matrix composite containing 9.3 vol % of silicon carbide whiskers.1 The experimental data showed that the composite behaved differently from the matrix alumina, with a larger stress exponent, an activation energy for creep close to that for oxygen diffusion in the alumina lattice, and with microstructural evidence for dislocation activity within the grains. It is reasonable to assume that whiskers in a composite strengthen the matrix by sharing the load, so that an increase in the density of whiskers may be anticipated to give a stronger material. However, higher volume fractions of whiskers may affect their distribution in the matrix and thereby influence the deformation and fracture processes occurring during creep deformation at elevated temperatures. Although there have been several reports of the creep behavior of Al 2 O 3 -SiC composites,2"10 only two investigations6'9 have conducted detailed tests on materials obtained from the same source with two or more whisker contents, and the results from these two studies are contradictory. In the work of Lin and Becher9 where creep experiments were conducted in air, higher creep rates were observed in two composites with 30 and 50 vol % of SiC whiskers than in a composite with 20 vol % of whiskers, and the creep resistance was further increased when testing at 1473 K with a material containing only 10 vol% of SiC whiskers. By contrast, de Arellano-Lopez et al.6 tested the majority J. Mater. Res., Vol. 10, No. 11, Nov 1995 http://journals.cambridge.org
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of their samples in argon and reported that the creep resistance of the composites was enhanced by increasing the whisker fraction from 5 to 15 wt. % (equivalent to —6-18 vol %), but there was no additional improvement when the whisker fraction was further increased to 25 wt. % (equivalent to —30 vol %). In practice, it is difficult to reach any conclusions by comparing
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