Influence of freezing rate oscillations and convection on eutectic microstructure

As discussed in our review paper (Wilcox, W. R. and Regel, L. L., Micrograv ity Quarterly, 1994, 4, 147-156), the influence of microgravity on eutectic microstructure has been rather erratic and largely unexplained. Directiona l solidification in microgravity sometimes coarsened the structure, sometim es made it finer, and sometimes, even on the same system, had no measurable effect. Theoretical models predicted no influence of the weak buoyancy-dri ven convection that occurs in the Vertical Bridgman technique on earth. Thu s, we hypothesized that freezing rate fluctuations due to irregular convect ion might be responsible. For example, with a fibrous microstructure an inc rease in freezing rate must cause new fibers to form, either by branching o r by nucleation. A decrease in freezing rate would cause fibers to terminat e by overgrowth of the matrix phase. If the kinetics of fiber formation dif fers from that for fiber termination, an oscillatory freezing rate would ca use the average fiber spacing to deviate from that at a steady freezing rat e. We have been investigating this hypothesis both experimentally and theor etically. Vertical Bridgman experiments were performed on the MnBi-Bi eutec tic with freezing rate oscillations caused by periodic electric current pul ses passed through the material. With increased current amplitude, more and more grains exhibited irregular microstructures. Of the grains with contin ued quasi-regular rod structure, the microstructure became finer. This resu lt was contrary to that expected from our hypothesis for this system. Numer ical modeling also predicted that an oscillatory freezing rate should yield a finer microstructure. It was also predicted that freezing interface osci llations should cause the average melt composition at the freezing rate to deviate from the eutectic. This results in the formation of a composition b oundary layer of sufficient thickness that it would become sensitive to con vection. Hence we have arrived at a revised hypothesis. On earth, irregular convection causes freezing rate fluctuations that change the interfacial m elt composition, leading to a thick composition boundary layer. Convection interacts with this boundary layer to change the interfacial melt compositi on, thereby altering the response of the system to freezing rate fluctuatio ns. (C) 2001 Published by Elsevier Science Ltd.