Crystallographic Orientation of Nodules formed by the Grain Boundary Reaction of Aluminium–Zinc Alloys

Abstract

ALTHOUGH the grain boundary reaction phenomenon (discontinuous precipitation or cellular precipitation) of aluminium–zinc alloys was reported previously^1,2, the crystallographic orientations of nodules have not yet been determined. As the spacing between lamellar precipitates in nodules of aluminium–zinc alloys is very small, the precipitates cannot be observed by the ordinary optical microscope, but only by the electron microscope, as shown in Fig. 1. If the precipitates in the nodules are oriented in a definite direction, the relationship of the crystallographic orientations between nodules and matrix grains could be determined³. Or, if the specimens can be treated by an adequate etching technique to be sensitive to polarized light, the relationship of the orientations can be observed under the polarized illumination⁴; but this was also impossible for the existence of the small precipitates.