Observations of non-periodic and periodic defect structures in M₇C₃ carbides

Abstract

A structural analysis of M₇C₃ (M=Fe, Cr, etc.) metal carbides is presented, using X-ray and electron diffraction as well as electron microscopy imaging techniques. It is shown that an orthorhombic superstructure occurs in three orientation variants referred to an underlying hexagonal lattice, which are separated by 1011-type twin planes and antiphase boundaries. The distribution of these defects can occur periodically, quasi-periodically or at random. They are characterized through analysis of the diffraction geometry. The structure is considered as being ‘interface modulated’, ‘polysynthetically twinned’ or as a microdomain structure, depending on the detailed arrangement of the defects. High-resolution electron microscopy is shown to be a powerful technique in the direct interpretation of both periodic and non-periodic defect structures.