Recent developments on the understanding of plane-fault structures in b. c. c. metals are reviewed.
Computer simulation studies have shown that there are two possible structures (symmetrical type and displaced type) of a (112) twin boundary, each of which corresponds to a local minimum on an energy surface represented as a function of guide parameters. They have also shown that one type (I₂ type) of a stacking fault is stable. The stability of the other type (I₁ type) is, however, still equivocal because it depends upon the choice of the interatomic potential.
An electron microscopic study has evidenced that twin boundaries in a b. c. c. alloy (Fe-3% Si) are of the symmetrical type. It is pointed out that experimental studies on stacking faults performed so far are not satisfactory because they have not clarified the type of faults observed.

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The diffusion of Laue spots in high energy electoron diffraction with increasing the crystal thickness is summerized theoretically. We take into account multiple inelastic scat-terings with diffraction by coupled diffusion equations with asymmetrical diffusion coef-ficients. Numerical calculations are performed for Si single crystal and compared with the experiments.

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