Quantitative Prediction and Observation of Grain Boundary Dynamics
Abstract
Most natural and technological important crystalline materials are polycrystalline (aggregates of single-crystal grains of distinct crystallographic orientations). Grain boundaries (GBs) are interfaces between pairs of contiguous grains. GB properties play an important role in determining many physical and mechanical properties of polycrystals including mechanical strength, ductility, creep rate, fatigue strength, radiation damage resistance, diffusivity, susceptibility to corrosion, thermal and electrical resistivity, magnetic hysteresis loop shape, magnetoresistance, superconducting critical current density, ... Therefore, one effective approach for tailoring material properties is through exploitation of GB structure-property-processing relationships to control the spatial, chemical and crystallographic distribution of GBs. We have existing models for many of these GB properties (typically one for each) and such models are usually not tied to the underlying GB structure. This proposal addresses the development and experimental verification of unifying descriptions of GB dynamical phenomena based upon a GB geometric and temperature-sensitive, mechanistic model. Our approach is built around the concept of line defects that lie in the GB, i.e., disconnections, that have both dislocation and step characteristics. The propagation of steps along a GB produces GB migration. The propagation of a dislocation leads to sliding of one grain with respect to the other. Since disconnections are generally associated with BOTH a finite step height and Burgers vector, their motion results in both GB migration and GB sliding (they are coupled). GB dynamics may be influenced by a wide variety of driving forces (including mechanical stresses/strains, jumps in the chemical potential across a GB, e.g., associated with GB curvature, lattice defects within the grains and anisotropic elastic, magnetic, surface,..properties). GBs play an important role in plastic deformation of polycrystalline materials; they block, absorb, transmit, and/or emit dislocations. The rates of these processes are ultimately determined by the dynamics of disconnections within GBs. Our project develops on a disconnection description of GB dynamics including GB migration, GB sliding, and the reaction of lattice dislocations at GBs. Our research combines analytical theory (based on bicrystallography, dislocation theory, elasticity, and statistical mechanics), multi-scale simulations (molecular dynamics, nudged elastic band, kinetic Monte Carlo) and state-of-the-art electron microscopy. It addresses 5 main questions: A. Is the motion of disconnections the mechanism that controls GB migration and GB sliding? B. Which disconnections are active in GB migration/sliding in specific GBs in specific materials? C. How can we predict the GB mobility and GB sliding coefficient for any specific GBs? D. How do lattice dislocations interact with a GB? E. What are the conditions and mechanisms by which GBs emit lattice dislocations into the grains?Answering these questions involve a tightly coupled integration of theory, simulation and experiment. This is largely made possible by our recent developments in the kinetic theory of disconnections and the application of recent developments in electron microscopy. Our experimental methods include: static atomic resolution imaging of GBs and determination of disconnection structure via time-resolved high-resolution transmission electron microscopy, atomic resolution scanning transmission electron microscopy and diffraction); 3D structure characterization of GBs and disconnections using STEM depth-sectioning; strain and composition distribution via fast, high-resolution spectroscopy and in-situ characterization of defect motion in the TEM.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 22, 2019
- Source ID
- W911NF1910263
Entities
People
- David J Srolovitz
Organizations
- Army Contracting Command
- United States Army
- University of Pennsylvania