Directionally solidified Bi2O3-6Bi2O3 � SiO2 eutectic

Abstract

The phase diagrams of the systems Bi2O3-6Bi2O3 · SiO2 and (Bi2O3)0.85 (Nb2O5)0.15-6Bi2O3 · SiO2 were determined by a quenching method. Melts of the eutectic and neighbouring compositions in these systems were unidirectionally solidified at various rates of 0.5 to 10 mm h−1 under a temperature gradient of 100° C cm−1. At any solidification rate examined, the ingot was of irregular structure in the former system and was of well-aligned regular structure in the latter system at the eutectic composition and those deviated from that by 10 mol%. In the latter ingots, fibrousδ-(Bi2O3)0.85 (Nb2O5)0.15 crystals were embedded in parallel and with a uniform spacing in a matrix ofγ-6Bi2O3 · SiO2 crystal. The diameter,d, of the fibrous crystals and the interfibre spacingλ decreased from 3.8 to 0.7 μm and 10 to 1.9 μm, respectively, with increasing solidification rate,R, in the range 0.5 to 10 mm h−1, following the relationsd= 2.5R−1/2 andλ=6.9R−1/2. The volume fraction of the fibrous crystals increased from 0.14 to 0.23 with an increase in the (Bi2O3)0.85 (Nb2O5)0.15 content of the melt from 46 to 66 mol%. The elongation direction 〈110〉 of the fibrous crystal was parallel to the 〈111〉 direction of the matrix crystal. The difference in the microstructure of the ingots between the systems is discussed in terms of the solidification process.