Kinetics of interfaces

A fundamental understanding of material response involves studying these systems at different length and time scales. Typical examples involve dislocations and deformation twins in metals, cracks in glasses and ceramics and ferroelectric domain walls. As these materials are pushed to their extremes (e.g. high strain rates, pressures, electric fields) their macroscopic response become non-intuitive i.e. existing material models begin to fail. A critical reason for this is our lack of understading of how these microscopic mechanisms evolve especially under these extreme conditions.
For the theoretical mechanician, an interesting way of formulating the problem is as concentrated defects inside a continuum (in our case the material of interest). Forces on these defects may be defined from (now) classical thermodynamics-based formulations. The nature of these forces deviate from our conventional intuitions of force at the macroscale, and are used to describe the irreversible motion of these concentrated defects. We call the relation between the forces and motion of these defects, "kinetics". A very interesting aspect of studying the kinetics of these mechanisms lies in understanding and predicting its effects on the macroscopic dissipation and hence the response of the material system under interest. This is out multi-scale problem with objectives towards robust physics-based descriptions of dynamic material response.
Studying kinetics at the small length and time scales is a a fundamental and nascent problem that by definition requires a synergy of experimental, theoretical and computational tools. Here is a brief account of my past, current and future plans for research in this area.

Kinetics of deformation twinning in magnesium

The strength of magnesium and its alloys during plastic deformation is significantly influenced by the activity of specific types of volume defects called deformation twins in the material. Some of our prior measurements on magnesium alloys have linked a strain-rate dependent increase in the strength of magnesium to the activity of deformation twins (see section). However, direct in-situ measurements of the kinetics of deformation twins have eluded us for a long time primarily due to complexities in combining high spatial and temporal resolutions in experiments.


For the first time, we were able to capture the dynamic motion of the interfaces between a deformation twin and its parent crystal (called a "twin boundary") "in-situ" during dynamic impact experiments on single crystal samples of magnesium. With spatial resolutions of 5 micrometer per pixel and temporal resolutions of 200 nanoseconds, we observed the growth of twin tips at velocities up to 1 km/s. Our measurements pointed to a transition in kinetics at high rates; while twin boundaries have been observed to grow slowly at velocities of the order of nanometers/second during a slow tension experiment, our measurements at high loading rates indicated orders of magnitude faster growth rates and faster nucleation of new twin boudnaries. This offers a mechanistic understanding of why material strength changes as we change loading rates in magnesium and similar hexagonal close packed crystals.

Deformation twin evolution during high strain rate compression of Mg single crystals (Duration: 36 microseconds; FOV: 3 x 5mm)
In-situ high speed microscopy (Time resolution: 200 ns; Spatial resolution: 5 micrometers/pixel)

Collaborators

Prof. K. T. Ramesh , Department of Mechanical Engineering and Hopkins Extreme Materials Institute, Johns Hopkins University

Relevant Publication

Kinetics of domain walls in ferroelectric materials

Some of our current research involves studying the kinetics of interfaces in electro-mechanical materials, specifically ferroelectrics. The polarization switching response, and associated mechanical actuation under large electric fields is due to the nucleation and growth of microscale defects called ferroelectric domains. The interface between the parent crystal and a domain is called a "domain wall" and is driven by both electrical and mechanical forces. We study the effects of these kinetics on the macroscopic switching response of ferroelectric ceramics.
Conversely, the question of how we can use the electrical loading history to control the spatio-temporal evolution of domain walls in the material is one of fundamental and technological interest. This understanding has potential applications in active damping control of ferroelectric ceramics. At much smaller length and time scales (in thin films), this problem is studied widely in the solid state physics community in the context of memory storage.

Collaborators

Prof. D. M. Kochmann , Dr. L. Guin , R. Indergand , Department of Mechanical and Process Engineering, ETH Zürich
Dr. M. Trassin , Department of Materials, ETH Zürich

Relevant Publications