FORMATION OF BANDS OF ULTRAFINE BERYLLIUM PARTICLES DURING RAPID SOLIDIFICATION OF AL-BE ALLOYS - MODELING AND DIRECT OBSERVATIONS

Rapid solidification of dilute hyper-eutectic and monotectic alloys so metimes produces a dispersion of ultrafine randomly-oriented particles that lie in arrays parallel to the advancing solidification front. We characterize this effect in Al-Be where Be-rich particles with diamet ers on the order of 10 nm form in arrays spaced approximately 25 nm ap art, and we present a model of macroscopically steady state but micros copically oscillatory motion of the solidification front to explain th is unusual microstructure. The proposed mechanism involves; (i) the bu ild-up of rejected solute in a diffusional boundary layer which slows down the growing crystal matrix, (ii) the boundary layer composition e ntering a metastable liquid miscibility gap, (iii) homogeneous nucleat ion of solute rich liquid droplets in the boundary layer, and crystall ization of these droplets, and (iv) growth of the matrix past the drop lets and its reformation into a planar interface. The size of the Be-r ich particles is limited by the beryllium supersaturation in the diffu sional boundary layer. A numerical model was developed to investigate this solidification mechanism, and the results of the model are in goo d agreement with experimental observations of rapidly solidified Al-5 at.% Be.