1. ECCI Analysis Dislocation/Grain Boundary Interactions And Plasticity In Commercially Pure Ti
Songyang Han, Carl Boehlert, Thomas Bieler, Martin Crimp
Michigan State University, Department of Chemical Engineering and Material Science
Introduction
To better understand dislocation and grain boundary interactions,
nanoindentation is being used to activate dislocations near grain
boundaries in commercially pure Ti. The near surface dislocation
distributions associated with the indentations are being characterized
using electron channeling contrast imaging (ECCI), allowing the
dislocations to be characterized in terms of line directions and Burgers
vectors. The nature of the dislocations associated with grain boundary
interactions is compared with the distributions associated with
nominally single crystal indentations collected at large distances from
grain boundaries. The extent of slip transfer has been characterized at a
number of different grain boundaries and assessed in terms of a
number of criteria that describe the crystallographic compatibility
across the boundaries.
Electron backscatter diffraction (EBSD) mapping of the polycrystalline
material was carried out prior to nanoindentation.
Nanoindentations were placed in a selected area (Figure 1&2), EBSD
orientation mapping (Figure 3) of the selected area was performed.
Nominally single crystal (with grains) and bicrystal (near boundary)
indents were studied.
Experiment Set-up
Results
The Identification of dislocation Burgers vectors were carried out by
applying g∙b=0 invisibility contrast analysis, where g indicates the lattice
planes in which the electrons are channeling. The g is established by
tilting to the dark/bright edge of the particular channeling pattern.
Dislocations with g·b=0 will show reduced or zero contrast allowing the
dislocations to be identified. By applying invisible criteria, dislocations
with different Burgers vectors will then disappear under differnet
channeling conditions.
Dislocation Analysis of Near Boundary Indent
The indent in the blue circle (Figure 3 left) is a bi-crystal indentation. In addition to
dislocations being imaged near the pile-up, dislocations that have either slipped
across the boundary or that have been generated at the boundary or in the
neighboring grain have been imaged. Characterization of these dislocations is
ongoing.
Summary
Conclusions:
Dislocations involved in nanoindentation and grain boundary slip
transfer can be imaged in the near surface region of bulk crystals. The
Burgers vectors of these dislocations can be determined by channeling
condition contrast analysis.
Future Work:
• Characterized how dislocations generated by nanoindentation
interact with different grain boundaries, i.e. low angle boundary
and high angle affect shear transfer.
• Correlated grain boundary deformation characteristics with
geometric slip compatibility factors such as m’= cosψ cosK [2].
Dislocation Analysis of Nominally Single Crystal Indents
Figure 3. (left) orientation map of the area studied. (middle) secondary electron (SE) image of the
indented area showing grain boundaries. (right) backscattered electrons image showing the same area.
HV: 30kV, Beam Intensity: 18, Spot size: 20 nm, WD&Z: 9.97mm
Figure 1. Geometry of
the conical indenter [1].
References
[1] S. Zaefferer, N. Elhami, Acta Materialia, 2014, 20–50
[2] J. Lust, et al, Matall Mater. Trans. A, 1995, 26A, 1745-1756
For More Information,contact hansong7@msu.edu crimp@egr.msu.edu
Figure 2. ECCI image of the geometry of one of a nanoindents.
Dislocations can be visualized within 1μm from the margin of the
brightest field. HV: 30kV, Beam Intensity: 18, Spot size: 20nm,
WD&Z: 9.98mm
Figure 8. Geometric compatibilityfactor m’ ĸ
ψ
The indent in the red circle (Figure 3 left) is a nominally single crystal indent in a
large grain, as the boundary is well removed from the boundary. Such indentation
can be used to show the special distributions of the dislocations around an indent.
More detailed analysis allows the dislocation types to be satisfied.
Figure 4. (left) orientation map of the nominal single crystal
indent. Noise in the map suggests large lattice strain near
the indent. (right) Secondary electron (SE) image of the
indent shows large topography change associated with the
indentation pile-up.
Figure 5. ECC images of the
area below the large pile-up
in figure 4. These images
show visible dislocations
under different channeling
conditions, indicated by the
selected area channeling
pattern in the upper left
corners.
Figure 7. ECC image of the
indent area, area in red square
is used to observe dislocations.
Table 1. g∙b=0 invisible criteriafor Burgers vector analysis
Figure 5. (left) orientation map of the near boundary
indent. (right) Secondary electron (SE) image of the
indent