The Behavior of Grain Boundaries During Recrystallization of Dilute Aluminum-Gold Alloys

The early stage of isothermal recrystallization of five polycrystalline alloys of zone-refined aluminum containing different amounts of gold and deformed 40% by rolling at 0°C was studied by quantitative metallography. All results could be interpreted in terms of a site-saturated, matrix grain edge nucleation at zero time with subsequent growth controlling the recrystallization kinetics. The length of the matrix edges producing recrystallized colonies depended on the penultimate grain size and was longer the larger that grain size. A modification of the original phenomeno-logical model was proposed which accounted for the observed discrepancies between the measured recrystallization parameters and the ones calculated from the earlier model. The presence of AuAl₂ precipitates had no apparent effect on the nucleation mode. However, the apportionment of gold into a precipitated state caused an enormous (104) increase in boundary migration rate (recrystallization rate). With the exception of an alloy containing 3.4 ppm gold, the activation energy for recrystallization was 25,700 cal/ mole, did not vary with the extent of recrystallization, and was in approximate agreement with the solute drag theory of boundary migration. The migration behavior of the 3.4 ppm gold alloy suggested the possibility of a grain boundary structure transformation. A theory of vacancy-enhanced grain boundary migration during recrystallization of dilute alloys was proposed to account for the observations.