The influence of buoyancy-driven convection on the lamellar spacing of an Al-Zn cutectic was studied under conditions of oriented crystallization. The convection due to huoyancy was affected by the mold position respective to thc direction of action of the force of gravity. It has been found that the buoyancy-driven convection does not oxcrt, any direct effect on the lamellar spacing. Rather, it only affects the rate of advance of tho crystallization front. The lamellar spacing 1 of the Al-Zn eutectic can be described as function of this rate of advance V by the relation 2 = I.'-0.46. 9.0

A study was made of the buoyancy convection of melt provoked by temperature gradient of an orientation identical to that of the gravity vector. The convection was assessed on the basis of temperature fluctuations in the melt. The critical Rayleigh numbers for periodic and turbulent flow were determined. Both periodic and turbulent flow gives rise to transverse "bands" in the structure of the eutection obtained by directional solidification. The intensity of unstable flow affects the number of eutectic crystallites in the sample. B pa6OMe HCCJIeAOBaHHa rpaBHTaUHOHHaR IEOHBeKUUR BbI3aHa rPPAHeHTOM TeMIlepa-TPbI OpHeHTHpOBaHHbIM COrJIaCHO C QaBPiTaUHea. ~OHBeKUHR 6b1na 0UeHHBaHa C no-'IHHLI qHCJIa Rayleigh AnFI B03HHkHOBeHHR IIepHOHHqeCKOrO I4 Typ6yJleHTHOrO ABHMfEeIlHFI paCIlJlaBa. H e c~a 6~n b~o e ABHmeHHe B PaCnJlaBe IlpH HaIIPaBJleHHOg KpHCTaJlJlH3aUHCi CIIOCO6CTByeT B03HHHHOBeHHlU nOIll2pC?sHbIX i'lIOJIOCbI'' €3 CTpykiType 3BTeHTHfFM. r/lHTeHCHB-HOCTb HeCTa6HJlbHOl' O HBHWeHHH PaCIIJlaBa BJIHHeT Ha qHCJI0 3BTeKTH9eCKHX HpHCTaJlJlOB Mouwo @JrloKTyaL@i TeMnepaTypbI B pacnaaee. BLIJIH ycTaHonneHLi KpHTnqecme nem-B o6pa3ue.