DIFFUSION INDUCED GRAIN-BOUNDARY MIGRATION (DIGM) AND LIQUID-FILM MIGRATION (LFM) IN AN AL-2.07WT-PERCENT CU ALLOY

Diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM) in an Al-2.07 wt% Cu alloy during isothermal annealing after down and up quenching from an equilibrated state at 620-degrees-C in the alpha + liquid phase field has been studied by light and scanning electron microscopy. Down quenching to 520, 540 and 560-degrees-C resulted in grain boundaries migrating by DIGM against their curvature from one grain into another leaving behind alloyed zones. Down quenching to 590 and 605-degrees-C resulted in liquid films migrating against their curvature from one grain into another leaving behind alloyed zones. Boundaries were also observed to migrate at these temperatures. Up quenching to 630 and 640-degrees-C resulted in liquid films migrating from one grain into another leaving behind dealloyed zones. The migration distance, s, for both boundaries and liquid films was observed to decrease monotonically with annealing time (t) and to obey a power law relationship, s = Kt(n), with annealing time. The exponent n falls between 0.20 and 0.25. For a given annealing time the migration rates of liquid films by LFM were about twice as fast as those of grain boundaries by DIGM, With respect to driving force for boundary and liquid film migration, the coherency strain energy developed in the grain being consumed does not seem to be great enough to drive the reactions against the curvature observed.