Spherulitic growth and recrystallization in barium silicate glasses

Abstract

The growth and recrystallization of spherulites formed in barium disilicate glasses between 700 and 900° C has been studied by electron microscopy and electron diffraction. Spherulites formed at 700° C consist of fibrillar (∼ 100 Å diameter) monoclinic crystals in confocal arrangement with preferred crystallographic growth axes. High temperature (900° C) spherulites are composites of radially oriented plate-shaped orthorhombic crystals with lateral growth of epitaxially nucleated fibrillar monoclinic crystals. At intermediate temperatures “axialites”, consisting of a single orthorhombic “midrib” crystal with monoclinic fibrillar side-growths, grow in competition with the low temperature spherulite morphology. The monoclinic fibrillar phase is believed to be an intermediate metastable structure which is able to grow more rapidly than the orthorhombic phase via cellular transformation in the presence of impurities. Brief comparison is made between the observed morphologies and theories for interface instability and cellular crystallization. Recrystallization, induced mainly by the large interfacial area of spherulite fibrils, produces faulted and twinned monoclinic grains which transform slowly to the orthorhombic stable crystal phase. A glassy intercrystalline residue becomes more prominant with grain growth.