On the applicability of conventional contrast theory to weak beam images of stacking faults

Experimental observations of single (intrinsic) and double (two overlapping single) stacking faults are carried out in wedge crystals of f.c.c. cobalt. A range of values of the deviation parameter, S/sub g/, corresponding to weak beam diffraction conditions is explored and both signs of the parameter (g . R) S/sub g/ are considered. All of the images of stacking faults obtained under these conditions exhibit contrast which varies periodically with increasing crystal thickness. None of the single fault images examined exhibit a detectable intensity asymmetry as the sign of (g . R) S/sub g/ is changed. Although some double fault images behave in the same way, others exhibit an intensity asymmetry, the magnitude of which depends on the value of S/sub g/ used. Experimental images are compared with the predictions of the conventional theory of stacking fault contrast. Good agreement between theory and experiment is obtained for all conditions explored. (author)