On the growth and form of spherulites

TL;DR

This paper presents a unified field theoretic phase field model explaining the growth and diverse forms of spherulites in crystalline materials, incorporating anisotropies, heterogeneities, and grain boundary effects.

Contribution

It introduces the first phase field model that includes key ingredients like anisotropic surface energies and interface mobilities for spherulite growth.

Findings

Diversity of spherulite forms results from competition between crystallographic symmetry and randomization.

Large-scale isotropy is mainly due to growth front nucleation.

Model explains needle-like growth and trapping of local order.

Abstract

Many structural materials (metal alloys, polymers, minerals, etc.) are formed by quenching liquids into crystalline solids. This highly non-equilibrium process often leads to polycrystalline growth patterns that are broadly termed "spherulites" because of their large-scale average spherical shape. Despite the prevalence and practical importance of spherulite formation, only rather qualitative concepts of this phenomenon exist. The present work explains the growth and form of these fundamental condensed matter structures on the basis of a unified field theoretic approach. Our phase field model is the first to incorporate the essential ingredients for this type crystal growth: anisotropies in both the surface energy and interface mobilities that are responsible for needle-like growth, trapping of local orientational order due to either static heterogeneities (impurities) or dynamic heterogeneities in highly supercooled liquids, and a preferred relative grain orientation induced by a misorientation-dependent grain boundary energy. Our calculations indicate that the diversity of spherulite growth forms arises from a competition between the ordering effect of discrete local crystallographic symmetries and the randomization of the local crystallographic orientation that accompanies crystal grain nucleation at the growth front (growth front nucleation or GFN). The large-scale isotropy of spherulitic growth arises from the predominance of GFN.