Residual Stress Determination in Al₂O₃/SiC (Whisker) Composites By X-Ray Diffraction

Abstract

Residual strains and stresses were determined in both phases of a hot pressed α-Al 2 O 3 composite containing 25 wt % β (cubic) SiC whiskers using conventional x-ray diffraction and profile analysis. Both phases in this composite were randomly oriented as confirmed by back reflection pinhole photographs. The reflections found most useful with Cu Kα radiation were: 511 + 333 for β-SiC at -134° 2θ and 146 for α-Al 2 O 3 at -136° 2θ. The peak shift and broadening observed in these reflections, relative to the starting powders, were largely due to the two phases mutually constraining each other elastically. This was confirmed by the reversal of the peak shift and most of the broadening in the SiC reflections when the Al 2 O 3 matrix was etched away. Using the method of Cohen and Noyan, it was found possible to separate the macrostresses from the microstress components in each phase. The microstresses were largely hydrostatic; of the order of 895 MPa (130 ksi) compressive in the whiskers and 370 MPa (54 ksi) tensile in the matrix. The macrostresses were ~79 MPa (11.5 ksi) tensile.