Crystalline defects in M7C3carbides

Abstract

Abstract Abstract M7C2, carbides prepared in various conditions have been observed by electron microscopy and diffraction. Examinations of the first-order Laue zone with crystals having the c axis parallel to the electron beam demonstrate that the unit cell describing the structure of these carbides is the orthorhombic cell proposed by Fruchart and Rouault. This cell can be considered as an ordered cell of an average hexagonal cell and can have three orientations at 120° from each other. It contains metal atom tetrahedra whose local environment in prismatic interstices containing carbon atoms is typical and is never destroyed, whatever the conditions of preparation may be. Thus, in primary carbides present in cast irons and steels, the state of crystallization is such that a long-range order of the environment is established : ortho-rhombic domains are separated by {1I00} twins and antiphase boundaries. In Fe7C3 carbides crystallized from amorphous iron-carbon alloys, the local environments are only short-range ordered, and domains and {1120} defects have the same size. Intermediate cases, as seen in ion-plating carbides, are also described.