MODEL OF BANDING IN DIFFUSIVE AND CONVECTIVE REGIMES DURING DIRECTIONAL SOLIDIFICATION OF PERITECTIC SYSTEMS

The formation of banded microstructure in peritectic systems is examin ed theoretically in both diffusive and convective regimes. A rigorous model is developed in the diffusive regime that describes the non-stea dy-state growth of alternate solid alpha and beta phase bands with a p lanar solid-liquid interface. The model is extended to incorporate the effect of convection by assuming that solute diffusion takes place wi thin a boundary layer of constant thickness, with a uniform compositio n in the mixed liquid zone outside this layer. The model predicts that convection effects in a semi-infinite sample narrow the composition r ange over which extended banding can occur, and the spacing of bands i s reduced compared to the diffusive growth model. In a finite length s ample, convection is shown to lead only to the transient formation of bands. In this transient banding regime, only a few bands with a Varia ble width are formed, and this transient banding process can occur ove r a wide range of compositions inside the two-phase peritectic region. Directional solidification studies in the Pb-Bi system show transient bands and agree qualitatively with these predictions. However, the ba sic mechanism of band formation observed in this system is found to be significantly different from the one assumed in the model. A new mech anism of banding is proposed in which continuous growth of both phases is present instead of nucleation at the boundary of the pre-existing phase. This mechanism yields an oscillatory structure with a shorter s patial periodicity than the band spacing predicted by the purely diffu sive or boundary layer convective models.