RESIDUAL-STRESSES IN ALUMINA SILICON-CARBIDE (WHISKER) COMPOSITES BY X-RAY-DIFFRACTION

Residual strains and stresses were determined in hot‐pressed α‐Al2O3 composites containing 29, 18, and 10 vol%β‐SiC whiskers using X‐ray diffraction. A 29 vol% SiC hot‐pressed specimen containing whiskers coated with a 5‐ to 10‐nm‐thick carbon film was also investigated. Strains were determined from the 511 and 333 reflections from SiC and the 146 reflection from Al2O3 using CuKα radiation and applying both the sin2Ω and generalized least‐squares methods. The total residual stresses were obtained from the least‐squares strains and were further separated into macrostresses and microstresses in each phase using the method of Noyan and Cohen. An error analysis of the strains and stresses was also performed. The average microstresses in the Al2O3 matrix were nearly hydrostatic and decreased with decreasing volume percent of SiC as follows: 397, 222, and 94 MPa for 29, 18, and 10 vol% SiC, respectively. The average microstresses in the whiskers were hydrostatic and were ‐958, ‐1058, and ‐979 MPa for 29, 18, and 10 vol% SiC, respectively. The presence of the carbon film decreased the magnitudes of the microstresses by about 10% and 5%, respectively, for the whiskers and the matrix. The X‐ray penetration depth was large (60 to 100 μm) compared with the whisker dimensions, so that the microstresses were representative of the bulk. The measured strains and microstresses were compared with those predicted by the models of Li and Bradt, Hsueh et al., and Majumdar et al., and with strains measured by neutron diffraction. In general, the agreement with the models and among the models was satisfactory, although the small decrease in the measured SiC microstresses in going from 18 to 10 vol% SiC was not predicted. The measured macrostresses were small (<140 MPa tensile), as expected from the light surface grinding of the surfaces (prepared using MIL standard 1942‐A). Copyright © 1990, Wiley Blackwell. All rights reserved