Reading EBSD Data

To read experimental electron backscatter diffraction (EBSD) data three components are needed

  • EBSDMesh Mesh object

  • EBSDReader UserObjcet

  • Initial conditions (such as the ReconVarIC action provides)

Reconstructed microstructure with stress, created using the combined module example file EBSD_reconstruction_grain_growth_mech.i

Reconstructed microstructure using EBSDReader, created using the phase_field module example file IN100-111grn.i

Mesh

The mesh is generated from the EBSD information in the specified EBSD data file to get an optimal reconstruction of the data. This is accomplished in the mesh block using the EBSDMesh type. The mesh is created with one node per data point in the EBSD data file. If you wish to use mesh adaptivity and allow the mesh to get coarser during the simulation, the uniform_refine parameter is used to set how many times the mesh can be coarsened. The block takes the form:

[Mesh]
  type = EBSDMesh
  filename = IN100_120x120.txt
  uniform_refine = 2
[]
(modules/phase_field/examples/ebsd_reconstruction/IN100-111grn.i)

EBSD Reader UserObject

The UserObject reads in the data file, using the name supplied in the mesh block, and stores a data object with the local data at each material point as well as the average data about each grain. The block syntax is very simple:

[UserObjects]
  [./ebsd_reader]
    type = EBSDReader
  [../]
  [./ebsd]
    type = PolycrystalEBSD
    coloring_algorithm = bt
    ebsd_reader = ebsd_reader
    enable_var_coloring = true
  [../]
  [./grain_tracker]
    type = GrainTracker
    flood_entity_type = ELEMENTAL
    compute_halo_maps = true # For displaying HALO fields
    polycrystal_ic_uo = ebsd
  [../]
[]

[ICs]
  [./PolycrystalICs]
    [./PolycrystalColoringIC]
      polycrystal_ic_uo = ebsd
    [../]
  [../]
[]
(modules/phase_field/examples/ebsd_reconstruction/IN100-111grn.i)

Applying Initial Conditions

The initial condition for the variables is set from the EBSD data. There are three possible use cases summarized below.

Case 1: Create grain structure from the grain numbers in the EBSD data, ignoring the phase number

A grain structure is created from the EBSD data by assigning initial condition values for order parameters. Many more grains can be represented than the number of order parameters. The required blocks are

[Mesh]
  # Create a mesh representing the EBSD data
  type = EBSDMesh
  filename = IN100_001_28x28_Marmot.txt
[]

[GlobalParams]
  # Define the number and names of the order parameters used to represent the grains
  op_num = 4
  var_name_base = gr
[]

[UserObjects]
  [./ebsd_reader]
    # Read in the EBSD data. Uses the filename given in the mesh block.
    type = EBSDReader
  [../]
  [./ebsd]
    type = PolycrystalEBSD
    coloring_algorithm = bt
    ebsd_reader = ebsd_reader
    output_adjacency_matrix = true
  [../]
  [./grain_tracker]
    type = GrainTracker
    # For displaying HALO fields
    compute_halo_maps = true
    # Link in the ebsd userobject here so that grain tracker can extract info from it
    polycrystal_ic_uo = ebsd
  [../]
[]

[Variables]
  [./PolycrystalVariables]
    # Create all the order parameters
    order = FIRST
    family = LAGRANGE
  [../]
[]

[ICs]
  [./PolycrystalICs]
    [./PolycrystalColoringIC]
      # Uses the data from the user object 'ebsd' to initialize the variables for all the order parameters.
      polycrystal_ic_uo = ebsd
    [../]
  [../]
[]
(modules/phase_field/test/tests/reconstruction/1phase_reconstruction.i)

Case 2: Initialize a variable from a specific phase number in the EBSD data, ignoring the grain numbers

Here, the value for a single variable is initialized from the EBSD data corresponding to a single phase number. The required blocks are

[Mesh]
  # Create a mesh representing the EBSD data
  type = EBSDMesh
  filename = 'Ti_2Phase_28x28_ebsd.txt'
[]

[UserObjects]
  [./ebsd]
    # Read in the EBSD data. Uses the filename given in the mesh block.
    type = EBSDReader
  [../]
[]

[Variables]
  # Creates the two variables being initialized
  [./c1]
  [../]
  [./c2]
  [../]
[]

[ICs]
  [./phase1_recon]
    # Initializes the variable info from the ebsd data
    type = ReconPhaseVarIC
    ebsd_reader = ebsd
    phase = 1
    variable = c1
  [../]
  [./phase2_recon]
    type = ReconPhaseVarIC
    ebsd_reader = ebsd
    phase = 2
    variable = c2
  [../]
[]
(modules/phase_field/test/tests/reconstruction/2phase_reconstruction.i)

Case 3: Create an initial grain structure from the EBSD data only corresponding to one phase number Here, the grain and phase numbers are used. The order parameters are initialized from the EBSD data, but only using those grains with a given phase number.

[Mesh]
  type = EBSDMesh
  filename = Ti_2Phase_28x28_ebsd.txt
[]

[GlobalParams]
  op_num = 2
  var_name_base = gr
[]

[UserObjects]
  [./ebsd_reader]
    type = EBSDReader
  [../]
  [./ebsd]
    type = PolycrystalEBSD
    coloring_algorithm = bt
    ebsd_reader = ebsd_reader
    phase = 1
    output_adjacency_matrix = true
  [../]
[]

[Variables]
  [./PolycrystalVariables]
  [../]
[]

[ICs]
  [./PolycrystalICs]
    [./PolycrystalColoringIC]
      # select only data for phase 1 from the EBSD file
      polycrystal_ic_uo = ebsd
    [../]
  [../]
[]
(modules/phase_field/test/tests/reconstruction/2phase_reconstruction2.i)

Using EBSD Crystal Info

The EBSDReader local grid data is extracted using the getData(Point) function call, where you pass in location of the point where you want the data. The available data that can be extracted for a given point is

  • phi1 - The first Euler angle

  • phi - The second Euler angle

  • phi2 - The third Euler angle

  • grain - The index of the grain

  • phase - The index of the phase

  • symmetry - The symmetry class (from TSL)

An example of using this function is shown here


const EBSDReader::EBSDData & d = _ebsd_reader.getData(p);
_euler_angles(0) = d.phi1;
_euler_angles(1) = d.phi;
_euler_angles(2) = d.phi2;

The EBSDReader average grain data is extracted using the getAvgData(unsigned int) function call, where you pass in the grain number for which you want the data. The available data that can be extracted

  • phi1 - The average first Euler angle

  • phi - The average second Euler angle

  • phi2 - The average third Euler angle

  • phase - The index of the phase of the grain

  • symmetry - The symmetry class (from TSL)

  • p - Point with centroid location

An example of using this function is show here, taken from ReconVarIC


const EBSDReader::EBSDAvgData & d = _ebsd_reader.getAvgData(grn_index);
_centerpoints[gr] = d.p;

Plotting Color Representation of Crystal Orientations

Reconstructed microstructure with the color representation of the inverse polefigure description of the crystyal orientations. Image created using the phase_field module example file IN100-111grn.i.

It is common to use an inverse pole figure representation of the crystal orientations to color the grains to represent EBSD data. To simplify the comparison with experiments, MOOSE has a tool for outputting color values for the inverse pole figure representation that can then be visualized using Paraview. The spatially varying red, green, and blue (RGB) values are outputted as auxvariables that are automatically read by Paraview as a vector.

Two Auxkernels can be used to output the RGB values. The first, EulerAngleProvider2RGBAux is the simplest but requires the entire domain to have the same crystal structure. The second, EulerAngleVariables2RGBAux requires various other auxvariables that contain the Euler angles, the crystal structure, and the phase number.

The easiest way of outputting the values is to use a custom action block in the input file that is available. The syntax is


[Modules]
  [./PhaseField]
    [./EulerAngles2RGB]
      crystal_structure = cubic
      euler_angle_provider = ebsd
      grain_tracker = grain_tracker
    [../]
  [../]
[]

We recommend you plot the colors using Paraview. The EulerAngle2RGB action will create three auxvariables with default names RGB_x, RGB_y, and RGB_z. Paraview will automatically create a vector variable of name RGB_. To correctly represent the colors,

  1. Select RGB_ as the visualization variable.

  2. In the properties section with the advanced properties toggled on, uncheck Map Scalars under Scalar Coloring.