Stress induced pore formation and phase selection in a crystallizing stretched glass

In this article we experimentally investigate and theoretically describe phase selection and the nucleation of pores in small samples of undercooled diopside liquid when it is enclosed by a hard crystalline surface layer. The formation of the surface crystalline layer begins with nucleation and growth of highly dense diopside crystals. At the moment of impingement of these crystals on the sample surface, the crystallization pathway switches from diopside to a wollastonite-like (WL) phase. The WL crystal produces less elastic stress energy than diopside due to its lower density, which is closer to the liquid density. The relative content of the two crystalline phases can be changed by varying the sample size. Due to the density misfit, the growth of the WL crystalline layer leads to uniform stretching of the encapsulated liquid and finally to formation of one pore, which rapidly grows up to a size that almost eliminates the elastic stress and, therefore, dramatically reduces the driving force for pore nucleation. This nucleation process occurs in a very narrow range of negative pressures indicating that it proceeds via homogeneous nucleation. This result is corroborated by theoretical calculations of elastic stress fields and their effect on nucleation. Good qualitative and partial quantitative agreement between experiment and theory is found. The findings of this research are quite general because the densities of most glasses significantly differ from those of their isochemical crystals, and are thus of technological significance for glass–ceramic development and sinter-crystallization processes.