Early-time particle dynamics and non-affine deformations during microstructure selectionin solids

Abstract

Solid–solid transitions are invariably associated with groups of particles whosedeformations cannot be expressed as an affine strain about a reference configuration. Thedynamics of these non-affine zones (NAZ) determine the subsequent microstructure,i.e. the mesoscale patterning resulting from the structural transition. Here, wefocus on early-time dynamics of individual particles within an NAZ associatedwith a nucleation event. We show that the early-time behavior of these particleshave distinctive characteristics depending on the transition temperature. Thedynamics is heterogeneous, consisting of a few active particles exhibiting complexintermittent jamming and flow in response to internal stresses generated during thetransformation. At low temperatures, the dynamics of these active particles is ballisticand the structural transformation proceeds via string-like correlated movementof active particles, along ridges in the potential energy topography set up byinactive particles. On increasing temperature, the dynamics of active particlesshow an abrupt transition from ballistic to diffusive behavior with a diffusioncoefficient which appears to be independent of temperature. This dynamical transitionin the nature of the trajectories of particles is coincident with a discontinuoustransition in the microstructure of the solid. Finally, we characterize this transition interms of a dynamical order parameter in the space of trajectories and discuss itsconnection with the glass transition and rheology of soft and granular matter.