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A P R I L

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RECENT ADVANCES IN EBSD

DEFORMATION ANALYSIS

High-speed, high-fidelity techniques used to analyze texture and dislocation

content enable greater understanding of the influence of texture on

deformation of titanium alloys than previously possible.

John Foltz, ATI Specialty Alloys and Components, Albany, Ore.

Rick Hugo, RC Imaging & Analysis, Portland, Ore.

itanium alloys are increasing-

ly used in many value-added

applications due to their low

density, high strength, and compati-

bility with various biological and com-

posite systems. Most titanium alloys

are composed of hexagonally close-

packed (hcp) alpha (

) phase and body-

centered cubic (bcc) beta (

) phase. The

length scale and morphology of these

phases, including their preferred orien-

tations in a product form (texture), in-

fluences quasistatic and dynamic mate-

rial properties. The importance of many

microstructural features is now recog-

nized and routinely quantified. Howev-

er, the presence and degree of texture

and grain-level deformation mecha-

nisms have proven to be amore difficult

challenge to analyze until recently.

Over the past two decades, com-

mercialization of electron backscat-

tered diffraction (EBSD) and electron

backscattered pattern (EBSP) anal-

ysis has improved in terms of speed

and ease of use. Scans with millions

of points can now be collected quickly

due to automated data reduction at the

EBSD detector, while still maintaining

adequate angular resolution (

≈

0.5°) for

generic textural studies.

In contrast, dislocation level defor-

mation studies have been limited pri-

marily to two families of techniques—

transmission electron microscopy (TEM),

and electron channeling contrast im-

aging (ECCI) in the scanning electron

microscope (SEM). TEM analysis is lim-

ited by sample preparation to small

volumes, as well as free-surface effects

from the thin foil. ECCI, in turn, is based

on theory similar to dark-field TEM, and

is conducted in specialized SEMs with

limited sample preparation

[1]

These two techniques can resolve

individual dislocations, but the great-

er pattern of dislocation generation or

motion is often worth studying. Three

bulk techniques used for this work

include conventional misorientation

maps generated from Hough-transform

based EBSD, graphical slip band analy-

sis combined with conventional EBSD,

and an emerging technique called high

resolution EBSD (HR-EBSD). The latter

two techniques are applied in this work

to elucidate the deformation mecha-

nisms in three titanium alloys in differ-

ent microstructural conditions.

In graphical slip band analysis, a

sample is polished for EBSD followed by

subsequent deformation to induce slip

band traces at the sample surface. Surface

deformation is visible in the SEM using

conventional secondary, backscattered,

and forward-scattering electron detec-

tors, and is also cross referenced with

EBSD data collected while imaging

[2]

. For

hcp and other low-symmetry deformable

materials, this can often identify the slip

plane upon which dislocations moved in

response to the applied deformation.

In high resolution EBSD (HR-EBSD)

analysis, EBSPs from a crystal are com-

pared via image cross correlation to a

reference pattern, yielding a misorien-

tation resolution of 0.006°,

≈

80× more

accurate than Hough transform EBSD

analysis. This allows the software to

accurately deconvolute the misorien-

tation into separate slip systems and

directly quantify the full deformation

state of the material

[3]

In this study, specimens were

examined using graphical slip band

analysis to understand the influence

of microstructure, alloy content, and

loading orientation on the surface

deformation response of titanium. In

addition, one sample was subsequent-

ly analyzed using HR-EBSD to com-

pare results with bulk deformation of

titanium.

SPECIMEN PREPARATION

AND DATA ACQUISITION

Wire and plate made of ATI 6-4,

ATI 3-2.5, and ATI 425 alloys were used

in the study. Two heat treated micro-

structures analyzed were

mill-

annealed structure and

solution

quenched and aged (Q + A) microstruc-

ture (see Table 1).

Heat treated specimens were me-

tallographically polished to create a

flat on the circumferential surface of

the material. Polishing titanium is typ-

ically performed using silicon-carbide

papers through 1200 fine grit and sub-

sequent polishing using colloidal silica.

After polishing, materials were strained