Investigation of relative interfacial free energies in 304 stainless steel by electron transmission and diffraction microscopy

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Coherent twin-grain boundary intersections in thin films of annealed 304 stainless steel were examined by transmission and diffraction electron microscopy. Standard electron diffraction and Kikuchi line techniques were combined with electron transmission image data in a computer program to obtain twin—grain boundary energy ratios. It is shown that a three-dimensional analysis of twin-grain boundary intersections yields a greatly reduced spread of individual energy ratio values when compared with 2-dimensional or optical data. It is further shown that negative values of the twin—grain boundary energy ratio obtained by optical measurement of boundary surface traces are invalid, and simply reflect the errors inherent in a two-dimensional approximation of the intersection of boundary planes in threedimensions. The mean twin-grain boundary energy ratio computed for stainless steel for a rigorous Kikuchi electron diffraction analysis was 0.034. An attempt was also made to further minimize measurement error by insuring that the specimen surface plane was normal to the electron beam, and therefore coincident with the crystallographic surface plane determined by selected area electron diffraction. The technique applied here consisted of a check of specimen surface orientations by the analysis of angles between the surface traces of coherent {111} twin boundary planes and {111} dislocation slip planes. Twin—grain intersections admitted as acceptable examples for detailed three-dimensional analysis were then determined on the basis of angular deviations between surface traces not exceeding three degrees from the value expected when the specimen surface plane is normal to the electron beam. Of the 87 twin-grain intersections of this type analyzed in stainless steel, only 35 fulfilled the above criterion. Computer analysis of these 35 electron transmission images gave twin-grain boundary energy ratios ranging from 0.002 to 0.07 with a mean energy ratio of 0.022 compared to the earlier result of 0.034 obtained for identical stainless steel samples where errors of specimen surface orientation were neglected.