Energetics of Grain-Boundary Triple Junctions and Corner-Twinned Junctions: Transmission Electron Microscope Studies

Abstract

Direct observations of grain‐boundary triple junctions and triple junctions containing a grain‐corner twin have been made in 304 stainless steel foils by transmission and diffraction electron microscopy. The distributions of true dihedral angles at pure triple junctions and the true dihedral angles in twinned grain corners have been computed by creating crystallographically identifiable slip or stacking‐fault traces in grains composing the junction through high‐intensity electron‐beam bombardment of the junction or areas in the immediate vicinity, or through the measurement of the image projection of coherent twin boundaries with the subsequent determination of the foil thickness. The mean dihedral angle for pure triple junctions was observed to be 120°, while that for the corner‐twinned grains was observed to be 105°. The characterization of pure triple junctions and corner‐twinned junctions on the basis of a cyclic crystallographic convention (hkl) for grain‐surface orientations yielded a mean dihedral angle of 123.3° for the pure junctions and 113.7° for the mean dihedral angle in the twinned grain. These results confirm the validity of the Fullman‐Fisher theory for corner‐twin formation in fcc metals. It is also demonstrated that the mean angle of inclination of grain boundaries with respect to the thin foil surfaces is 61.6°, compared with the probable mean of 63.2° for a random slice through solid polyhedra, a property suggesting the bulk representation of electrothinned films.