INHIBITED COARSENING OF SOLID-LIQUID MICROSTRUCTURES IN SPRAY CASTINGAT HIGH-VOLUME FRACTIONS OF SOLID

Experimental studies on coarsening in fine grained solid-liquid micros tructures at high volume fractions of solid (f(s)) have been carried o ut to determine if inhibited coarsening under these circumstances coul d account for the anomalous fine cell sizes observed in spray castings . The materials investigated included a chill-cast dendritic binary al loy of Al-Cu, two spray cast alloys-AA2014 and Cu-Ti, whose grain size was the segregate spacing, and a d.c.-cast alloy Al-4.5 wt% Cu-1.5 wt % Mg in both coarse-grain and grain-refined conditions. The observed s egregate spacings after coarsening were smaller than that predicted by empirical correlations of dendrite arm spacing and freezing time. In all cases, the coarsening was found to become slower as the temperatur e was reduced and f(s) increased. Conventional coarsening theories and experiments predict the opposite, i.e. faster coarsening at higher vo lume fractions of solid. Two additional coarsening models were develop ed for the grain growth at high volume fractions of solid by processes whose rates are limited by migration of liquid at grain boundaries as liquid films on 2-grain surfaces or liquid rods on 3-grain triple poi nts. In both models, the conventional diffusion-limited t(1/3) coarsen ing law was reproduced, but the rate constant K contained the term 1/1 - f(s) and so also predicted accelerated coarsening as f(s) --> 1. Th ree possible explanations for the observed lower K values at increasin g f(s) are proposed. The first is the effect of the increasing differe nce between the solute contents of solid and liquid as the temperature is reduced. This produces a 1/X(1) dependence of the coarsening rate constant K. The second inhibiting effect, specific to dendritic struct ures, is in-grain coalescence of dendrite arms at high f(s) which prod uces isolated liquid particles within the grains. The final possibilit y is particle-inhibition of grain boundary migration by minority (impu rity) particles at the grain boundaries. Such particles were seen, how ever, for only two of the alloys, viz. the grain defined d.c. cast Al- 4.5 wt% Cu-1.5 wt% Mg and the spray case AA2014, but they or gas-fille d pores are proposed as strong possibilities to account for the fine g rain sizes observed in all spray cast microstructures.