Dislocation based modelling of engineering alloys

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

The plastic deformation of metals and alloys is generally governed by the generation and motion of dislocations through the crystal lattice. Microstructural features such as grain boundaries, precipitates and inclusions impede the dislocation motion causing strengthening but often limiting ductility. Dislocations often accumulate in the vicinity of these microstructural features and their mutual interactions and reactions lead to further increased hardening and local hot spots in stress than can in turn lead to failure initiation. Understanding the behaviour of the dislocation ensemble is complex due to the many body interactions that take place. This project will continue development of discrete dislocation plasticity simulations with the particular aim of extending present capabilities from simple single phase, single crystal models to more complex geometries incorporating the grain boundaries and precipitates that are more representative of real engineering alloys.

The project will involve assisting in the development of a 3D discrete dislocation plasticity code. This will require learning Matlab, C and CUDA for acceleration of the computation using parallelisation on graphics processing units. The student will also learn the finite element method (FEM), discrete dislocation dynamics (DDD) and how to couple these together so that FEM captures external boundary conditions while DDD captures the shorter range dislocation-dislocation interactions responsible for the material hardening. Training will be provided to the student in these areas although self-study will also be required. The main tasks are as follows:

1) Accelerating the code through parallelisation on graphics processing units
2) Performing comparisons with available analytic results for simple well-defined sample and dislocation line geometries to check for errors
3) Debugging and development of the code to accurately simulate micromechanical testing
4) Simulating micro-cantilever single crystal bend tests and comparing the model prediction with measured load-displacement curves, the predicted elastic strain fields and lattice rotation fields with high resolution electron backscatter diffraction (HR-EBSD) measurements and the predicted dislocation structure with diffraction contrast observations in the transmission electron microscope (TEM), including 3D dislocation tomography results if available.
5) Incorporating complex microstructure such as precipitates or grain boundaries. We will begin with simple building block configurations of single planar impenetrable grain boundaries in bi-crystals, tri-crystal geometries to investigate triple junctions, and quadruple points at the junction of four grains before moving on to larger groups of grains. We will also consider isolated precipitates and groups of precipitates incorporating them in different ways: as impenetrable to dislocations, as elastically different but penetrable, and with an initial misfit strain to the matrix lattice. This will require an elegant approach to incorporate the geometry. Different possibilities will be investigated such as the level set method. This will also require dislocation remeshing algorithm will also need to be developed to account for dislocations interacting with the geometry.

This project falls into the Engineering theme

Publications

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Bromage B (2018) Calculating dislocation displacements on the surface of a volume in Modelling and Simulation in Materials Science and Engineering

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1802042 Studentship EP/N509711/1 01/10/2016 31/03/2020 Bruce Bromage
 
Description Development of an improved discrete dislocation dynamics based code that can consistently predict the plastic behaviour of single crystal materials. Results demonstrating the efficacy of the code still pending.
Exploitation Route The code has been designed so that it may be used as a foundation for creating simulations of more complicated materials. With further improvements, it should be possible to simulate multi-crystal microstructures, include precipitates and have a reasonably complex geometry of the test specimen being modelled. Further work could improve understanding of how dislocations behave and help design new materials and components for specific purposes.
Sectors Aerospace, Defence and Marine,Construction,Energy,Manufacturing, including Industrial Biotechology

 
Title Improved Dislocation Remeshing Algorithm 
Description Improves the remeshing of dislocation network structures for handling more complicated geometries 
Type Of Material Computer model/algorithm 
Year Produced 2019 
Provided To Others? No  
Impact Allows for the simulation of more complicated stress states in a model without creating a non-physical resultant dislocation structure 
 
Title Improved Mobility Law 
Description And improved code for the simulation of dislocation motion in body centred cubic materials 
Type Of Material Computer model/algorithm 
Year Produced 2020 
Provided To Others? No  
Impact Allows for more accurate prediction of dislocation behaviour while requiring less real time to simulate. 
 
Title Improved Time Integrator 
Description Improves the time step selection protocol used to allow for more efficient simulations 
Type Of Material Computer model/algorithm 
Year Produced 2019 
Provided To Others? No  
Impact Causes simulations to take less real time to run to completion 
 
Title Surface Displacement Boundary Condition Calculator 
Description Calculates the displacements on the surface of a finite volume due to the movement of dislocations within the volume for use in finite element boundary conditions 
Type Of Material Computer model/algorithm 
Year Produced 2018 
Provided To Others? No  
Impact Allows for more accurate modelling of plastic behaviour in single crystals using discrete dislocation dynamics