Investigations on the directional solidification of the eutectic LiF–LiBaF3

Abstract

Abstract Important physico-chemical properties of the regular, lamellar eutectic LiF–LiBaF3 and its components have been investigated and the phase diagram LiF–BaF2 was redetermined. These investigations have been carried out partially to prepare and develop experimental concepts for the directional solidification of the eutectic LiF–LiBaF3 under microgravity conditions. Eutectic and off-eutectic LiF–LiBaF3, melts have been solidified under different growth conditions (growth velocity ν, temperature gradient G) and the correlation between the G/ν ratio and the eutectic structure was determined. The correlation between the average phase spacings λ and the growth rate ν (quantitative image analysis data) resulted in the calculated relation λ 2.08 ν = constant (regions of exact eutectic compositions) which is in good agreement with the Zener criterion λ 2 ν = constant. The high melt viscosity leads to low Rayleigh numbers, Ra, which is orders of magnitude lower than the critical Rayleigh number, Ra c , and therefore a strong reduction of convective influences during the eutectic growth is expected. It can be concluded that the growth from eutectic LiF–LiBaF3, melts is mainly governed by diffusion and this confirms the growth theories by Jackson and Hunt. During eutectic growth the G/ν ratio plays an important role in respect to the eutectic structure. Low growth rates and high temperature gradients lead locally to a transition of the eutectic structure from lamellae to fibers. We found empirically a correlation between fibers, lamellae and eutectic cells. But at present no quantitative theory of modification in the eutectic structure has been formulated yet that a given temperature gradient with a given growth rate will cause lamellae to fiber transition in a given normal eutectic. Furthermore the reason for the observed deviations of the volume fractions in the eutectic structure from exact eutectic melt compositions was studied. The enrichment of the LiBaF3, component at the growth interface of a seed with eutectic composition in dependence of the soaking time of the eutectic melt could be attributed to the Soret effect (thermodiffusion) and is not primary a consequence of sedimentation effects (influence of gravity). Our experiments under microgravity conditions are expected to give unequivocal informations concerning the influence of gravity and Soret effect on the directional solidification. The directional solidification of hypo- and hyper-eutectic melts indicate that the LiF (phase with higher undercooling) acts as nucleus for LiBaF3, but not vice versa (‘non-reciprocal nucleation’). Hyper-eutectic melts have no detectable negative influence on the quality of the eutectic structure whereas the crystallization of hypo-eutectic melts leads to disturbances of the regularity in the lamellar structure.