Abstract
The grain boundary reaction phenomena were studied microscopically on Al-Zn alloys containing 10, 25% Zn respectively, which were solution heat-treated and aged at 70°, 85°, 95°, 120° and 160°. The rate of the growth of nodules which are characteristic of the grain boundary reaction was obtained from the estimation of the measured area of the total nodules. The micro-hardness of the interior of grains and nodules were measured, and the characteristics of the reaction were investigated. The results obtained were as follows: (1) The grain boundary reaction is observed in all alloys we prepared. (2) All the matrices of the alloys are not completely substituted by nodules in the reaction. (3) The total area changed by the nodules which is represented by the fraction of the reacted area to the initial one, changes with the temperature of aging, and it rises to the maximum value at a certain intermediate temperature. That is, when the temperature of aging is too high, the reaction does not occur. (4) Cold working accerelates the reaction. (5) The nodules which contain precipitates perpendicularly oriented to the boundary grow anomalously large. (6) On aging, the micro-hardness of the interior of the grains decreases, while that of the nodules does not change initially but decreases later from the coalescence of precipitates. (7) The grain boundary reaction is also observed at the boundaries of specimens which were furnace-cooled from the solid-solution temperature.
