Three cells with trigonal, orthorhombic or hexagonal symmetry have been proposed to describe M
7
C
3 carbides. Examinations of heavily faulted chromium, iron, vanadium mixed carbides present in white cast irons and in chromium steels have been performed by means of electron microscopy and diffraction. Electron diffraction patterns of the first-order Laue zone have been obtained with crystals having their c axis nearly parallel to the electron beam. They contain reflections which are typical of the symmetry and streaks which are typical of the faults. This original method of examination has systematically been applied to numerous carbides and comparisons of these experimental patterns with theoretical patterns expected from models which view the three structures of M
7
C
3 carbides as three different superlattices of a hypothetical disordered lattice lead to the following two main results: the structure of these carbides is well described with the orthorhombic cell previously given by Fruchart, Senateur, Bouchard & Michel [C.R. Acad. Sci. (1965), 260, 913]; the planar faults, which are often periodic, are antiphase boundaries or twins with fault planes {10{\bar 1}0} and fault vectors R = a, b or a + b.
First-order hyperbolic form of velocity-stress equations for waves in elastic solids with hexagonal symmetry