Materials Sciences and Applications, 2011, 2, 355-363
doi:10.4236/msa.2011.25046 Published Online May 2011 (
Copyright © 2011 SciRes. MSA
A TEM Study of Cobalt-Base Alloy Prototypes
Fabricated by EBM
Sara M. Gaytan, Larry E. Murr, Diana A. Ramirez, Brenda I. Machado, Edwin Martinez,
Daniel H. Hernandez, Jose L. Martinez, Francisco Medina, Ryan B. Wicker
University of Texas at El Paso, El Paso, USA.
Received February 1st, 2011; revised March 15th, 2011; accepted April 1st, 2011.
A novel microstructural architecture consisting of Cr23C6 carbide spatial columns was created in solid components of
Co-26Cr-6Mo-0.2C fabricated from powder b y additive manufacturing using electron beam melting. These columns of
carbides extended in the build direction and are formed by the x-y rastering of the electron beam to pre-heat and melt
powder layers using CAD models. Th ese microstructural architectures are similar to rapidly so lidified/unidirectionally
solidified structures created by heat extraction in the direction perpendicular to the build plane. These columnar car-
bide architectures were observed by optical metallography and transmission electron microscopy (TEM) and compared
with intrinsic stacking fau lt microstruc tures observed in annealed components. The TEM analysis allowed the details of
the carbide crystal structure and corresponding cubic morphology to be observed.
Keywords: Electron Beam Melting, Stacking Faults, Precipitates
1. Introduction
There have been significant developments in the creation
and applications of metals and alloys over the past dec-
ades (since about 1960) when the discipline of materials
science and engineering emerged, embodying the struc-
ture-properties-processing-performance paradigm [1,2].
Despite the discoveries and associated improvements in
the properties and performance of metals and alloys,
there remain limitations in structural/(microstructural)
control inherent in conventional (bulk) processing and
processing routes. These are consistent with the corres-
ponding materials science and engineering paradigm
limitations in the range of achievable properties and per-
formance. While it is common to modify material micro-
structures to achieve predictable property variations such
as hardness, strength (and elastic modulus), toughness,
thermal and electrical conductivity and the like, the con-
trolled development of spatial arrangements of micro-
structural features (or architectures) such as directional
solidification has produced columnar arrays of fibers,
especially eutectic fibers, which provide directional
properties [3-5]. Similar processing routes have recently
been demonstrated by additive manufacturing (AM) us-
ing electron beam melting (EBM) for the fabrication of
Co-base alloy (Co-26Cr-6Mo-0.2C) components [6], and
a Ni-base superalloy where columnar Ni3Nb precipitates
were formed [7]. In this prior Co-base alloy study [6]
Cr23C6 precipitates were observed to form spatial arrays
of carbide columns generally in the EBM build direction
as a consequence of unique thermodynamic zone arrays
characteristic of the electron beam preheat and melt-raster
geometry which was conducive to carbide precipitate
formation. It has been stated by Stoloff and others that
cobalt-base alloys are hardened by precipitation of either
carbides or intermetallic compounds [8].
This paper examines the microstructural architecture
characterized mainly by spatial columns of Cr23C6 car-
bides and dislocations which occur in solid Co-base alloy
prototypes fabricated by EBM. CoCrMo has a high cor-
rosion resistance due to the high chromium content that
forms a thin passive oxide layer, which makes it an ideal
candidate for metal-on-metal bearing surfaces in ortho-
paedic implants. This alloy also provides a high wear
resistance depending on the size, shape and distribution
of carbide precipitates [9]. A study conducted by Cawley
et al., basically consisted in comparing the microstruc-
tures of the same Co-Cr-Mo alloy as cast, wrought and
with different heat treatments, finding that the carbides
obtained from all the thermally processed samples were
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
richer in chromium than molybdenum and no difference
in hardness was obtained from the as-cast and the ther-
mally treated materials [10]. We demonstrate these ar-
chitectures by constructing and comparing 3D image
composites from optical microscopy and transmission
electron microscopy (TEM). Intrinsic stacking faults
characteristic of the Co-rich fcc, low stacking fault ener-
gy crystal structure are also observed in the background
of these microstructures, and become the dominant mi-
crostructure upon appropriate annealing to cause carbide
dissolution and dislocation annihilation.
2. Experimental Methods and Procedures
Although we have described the EBM-AM process in
detail elsewhere [6,11], it will provide a necessary con-
text if we outline the process briefly herein. We begin by
providing an overview of the Arcam A2-EBM system as
illustrated in Figure 1. The major components illustrated
in Figure 1 are the electron-optical column (1) which
focuses and scans the electron beam (in vacuum) over the
building component (3) as powder is supplied by gravity
feed from the cassettes shown at (2), and raked into a
layer roughly 100 µm thick. Figure 2(a) illustrates the
Co-26Cr-6Mo-0.2C powder (with nominal size of 40 µm)
used in this study while Figure 2(b) shows some simple
component geometries fabricated.
Figure 2(a) shows Co-26Cr-6Mo-0.2C powder obser-
ved in the SEM. As implicit in Figure 1, the electron
beam is rastered orthogonally (x-y) in multiple passes to
preheat each powder layer to roughly 830˚C. This is fol-
lowed by a melt scan (x,y) at reduced beam current and
scan rate which builds component geometries layer-by-
Figure 1. Arcam A2 EBM system.
Figure 2. (a) Precursor powder; (b) Components fabricated
by EBM.
layer in a direction shown by the arrow in Figure 2(b).
Horizontal and vertical planes (perpendicular to and pa-
rallel to the build direction) are noted by H and V, res-
pecatively in Figure 2(b). The rectangular block and
cylindrical components fabricated as illustrated in Figure
2(b) were observed to be essentially fully dense, with a
density of 8.4 g/cm3.
Optical metallography (OM) on extracted sections
from rectangular block and cylindrical builds illustrated
in Figure 2(b) was a precursor characterization prior to
developing TEM thin foils.
We employed a Reichert MEF4 A/M metallographic
system for extracted specimens which were ground, po-
lished, and etched with a 6:1 HCl: H2O2 (3%) etchant for
16 h at room temperature (~22˚C). Annealed samples
were etched in much less time (~2 min.). The annealing
conditions involved HIP at ~1200˚C for 4 h in Ar fol-
lowed by a quench from a homogenizing treatment at
1220˚C for 4 h in Ar at a quench rate of ~7.5 ˚C/min. As
illustrated in Figures 3 (a) and (b), OM was conducted
for both the horizontal (H) and vertical (V) planes.
As illustrated in Figure 3(c), specimens extracted from
the EBM-fabricated components as in Figure 3(a) and 3(b)
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
Figure 3. TEM samples obtained from block component in
vertical and horizontal plane.
were also sliced using a diamond saw along the H and V
planes and smaller coupons cut from these slices, ground
to ~0.2 mm thickness, and 3 mm discs punched from
these ground coupons. These discs were mechanically
dimpled and electropolished in a Tenupol-5 dual jet unit
at temperatures ranging from 30˚C - 40˚C using an elec-
tropolishing solution consisting of 15% perchloric acid
and 85% acetic acid, at 20 V and 3 - 30 mA. Punched
and electropolished 3 mm discs are shown in the inserts
in Figure 3(c).
3. Results and Discussion
3.1. Block Component Fabricated by Electron
Beam Melting
Figure 4 shows an OM 3D composite for an extracted,
polished, and etched specimen as illustrated in Figures
3(a) and (b) forming Cr23C6 carbide columns with spac-
ings determined largely by the electron beam focus and
scan geometry, including the spatial dimensions of the
X-Y scans. Figure 5 illustrates this processing feature in
the EBM. The columns are generally parallel to the build
direction marked by the arrows in Figures 4 and 5.
The microstructure (microstructural architecture or
carbide columns in Figure 4 is similar to structures de-
veloped by rapid solidification or directional solidifica-
tion [3-5], by heat extraction in the build direction. As
the electron beam scans and melts the powder the spatial,
Figure 4. OM 3D composite for a block component.
Figure 5. Carbide precipitate architecture as fabricated by
small liquid volume rapidly cools, solidifying with a
plane front or cellular liquid/solid interface (Figure 5).
The carbon within these zone segregates to the bounda-
ries of the cellular interface, resulting in the columnar of
aligned Cr23C6 precipitates. These features can be varied
somewhat as described for precipitate columns formed
by EBM in a Ni-base (Inconel 718)) alloy [7], and in the
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
production of -phase columnar grains in the selective
laser melting (SLM) of Ti-6Al-4V [9]. In the SLM study
[12], energy density, scanning velocity, and hatch spac-
ing (shown schematically in Figure 5) were referred to
as a scanning strategy which can be used to manipulate
associated microstructures and microstructural architec-
Figures 6 and 7 show 3D section views observed in
the TEM at low and high magnification, respectively.
The TEM views show the carbide columns in Figure 4 to
be spaced Cr23C6, incoherent carbide precipitates. Figure
7 in particular shows clearly the incoherent cubic (fcc: a
= 1.07 nm) Cr23C6 particles spaced roughly 100 to 200
nm in the EBM fabricated columns which are correspon-
dingly spaced ~2 - 3 µm as illustrated in both Figures 4
and 6. Contrast fringes indicting linear stacking- fault
features are apparent in both the horizontal (H) and ver-
tical (V) plane views in Figure 7 along with dense dis-
location arrangements intermingled with the carbide pre-
cipitates. These features are also observed at higher
magnifications as shown in Figure 8 which illustrates
individual precipitate cubes and related crystal geome-
tries measuring ~100 to 200 nm on a side.
Figures 9 to 11 show several examples of the dense
intrinsic stacking-fault arrangements intercalated with
dense dislocation structures. The selected-area electron
diffraction (SAED) pattern insert in Figure 9 shows mi-
cro-twin and double-diffraction spots indicative of re-
gions of overlapping intrinsic stacking faults on every
{111} plane in the Co-rich fcc matrix (a = 0.355 nm).
The magnified views for dense stacking-fault arrays for
stacking-faults on
111 and
111 planes intersecting
at 90˚ in the (100) surface orientation are illustrated by the
SAED pattern insert in Figure 10. The magnified view of
these intersecting stacking faults is shown in Figure 11
where dense stacking faults intersecting at 90˚ for faults
on {111} planes inclined ~55˚ to the (100) surface plane,
and in the <022> trace directions are shown. Similar but
far less dense intrinsic stacking-fault arrays were demon-
strated in the prior work of Atamert and Bhadeshia [13]
for nominal compositions of Co-28Cr-4.8W-1.3C (Stellite
6). In addition, the observations in Figures 8 to 11 illu-
strate stacking faults on all {111} variant planes in con-
trast to observations of the favoring of a particular variant
by Atamert and Bhadeshia [13]. The corresponding
Rockwell C-scale hardness (HRC) values for the hori-
zontal/vertical planes as in Figure 4 were observed to be
3.2. Cylindrical Component Fabricated by
Electron Beam Melting
Figure 12 shows similar Cr23C6 carbide column archi-
tectures in a representative cylindrical specimen as arrays
Figure 6. 3D TEM representation of carbide columns.
Figure 7. 3D TEM representation at higher magnification.
observed in both the horizontal and vertical planes are
essentially the same as shown for the fabricated block in
Figure 4. The associated HRC values are correspondingly
47/48. In Figure 13, the horizontal (OM) carbide array in
Figure 13(a) is compared with corresponding TEM im-
ages for the same horizontal orientation. The SAED pat-
tern insert in Figure 13 (c) shows a (100) plane orientation
with superimposed Cr23C6 carbide reflections, and these
features are shown for aggregated carbide precipitates in
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
(a) (b)
(c) (d)
Figure 8. Cr23C6 precipitates.
Figure 9. Stacking faults and double-diffraction spots.
Figure 14 which range in particle size from ~50 to 500 nm,
with their (100)-fcc diffraction pattern superimposed on
the Co-rich matrix SAED pattern insert in Figure 14.
Figure 10. Intersecting intrinsic staking faults.
Figure 11. Magnified image of stacking faults.
Figure 14 also shows a high density of dislocations es-
pecially associated with the carbide precipitates, the
SAED pattern insert shows [100] Cr23C6 zone axis supe-
rimposed on [112] fcc Co-rich matrix zone axis. Figure
15 shows dense stacking-fault arrays similar to those
shown in Figures 9-11 for a block specimen (Figure 3).
Here again, the intersections of faults on {111} planes
which intersect at 90˚ produce diffraction contrast features
for these intersecting {111} planes at 45˚. The SAED pat-
tern insert in Figure 15 also shows the same (100) orien-
tation as observed in Figures 9-11, along with some extra
reflections characteristic of twin faults created by the
systematic, overlapping intrinsic stacking faults.
As shown previously by Gaytan, et al. [6], the fracture
surface for failed cylindrical samples illustrated a spatial
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
Figure 12. Columnar architecture in cylindrical component.
Figure 13. OM and TEM images of carbides.
Figure 14. Cr23C6 particles interacting with or generating
Figure 15. Dense, intersecting stacking faults.
array of ductile dimples matching the cubic-shape carbide
precipitate array illustrated typically in Figures 12 and 13
in the horizontal plane, coincident with the fracture sur-
face; this feature is illustrated in Figure 16. Correspond-
ing UTS and elongation were 1.45 GPa and 3.6%, re-
spectively, as previously determined by Gaytan, et al. [6].
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
Figure 16. Fractured surface from a cylindrical component
fabricated by EBM, (a) Low magnification and (b) High
magnification image.
Grain boundaries and regular grain structures were
difficult to differentiate in the carbide arrays but more
regular Cr23C6 precipitation and corresponding etching
made them somewhat apparent as noted in the etching
patterns of Figure 4. Here the average grain sizes are
observed to be about 20 µm, or an order of magnitude
larger than the carbide column arrays. There seems to be a
preference for [100] orientations parallel to the build
direction, which is also characteristic of orientation pre-
ference in commercial processed copper billets and plate.
3.3. Components Fabricated by Electron Beam
Melting after Annealing
In the annealed condition, the carbide column architectures
disappear, leaving an, fcc grain structure containing a pro-
pensity of annealing twin boundaries. While Cr23C6 car-
bides generally dissolve in the grain matrix, there are resi-
dual carbides in the grain boundaries, and these promote
exaggerated etching, especially at high angle or high energy
grain boundaries. In this regard, note that there is no car-
bide-associated etching along the straight coherent twin
boundaries where the energy is roughly the same as the
stacking faults (~15 mJ/m2), and about 0.03 times the grain
boundary free energy [14]. These features are illustrated
typically in Figure 17 where the average grain size, in-
cluding twinned grains (light grains) was ~35 µm. This is
roughly a 75% increase over the as-fabricated Co-26Cr-
6Mo-0.2C components (Figure 2(b)). Correspondingly,
the residual hardness (HRC) was 40 in contrast to ~47 for
the as-fabricated components; a decrease of ~15%. The
high density of stacking faults and twin-faults declined
measurably as illustrated in comparative views of stacking
faults shown in Figures 18-20, where Figure 18 represents
a surface orientation of (112) with 110
direction shown
by the arrow. Figure 19 is a magnified view of stacking
faults in the (100) surface orientation with their traces in the
<022> directions. Figure 20, like Figures 18 and 19, illu-
strates intrinsic stacking faults on essentially all {111}
plane variants in contrast to the prior Co-base alloy TEM
observations by Atamert and Bhadeshia [13].
It is of interest to compare, retrospectively, the annealed,
equiaxed grain structure (Figure 17) and the associated
stacking fault microstructure (Figures 18-20) with the
more dense stacking fault microstructure and unique car-
bide microstructural architectures created by EBM fabri-
cation as illustrated in Figures 9-11 and 15, and Figures 4,
6-8, and 12-14. Figure 18 shows a TEM bright-field im-
age of intrinsic stacking faults in the (112) orientation for
the annealed Co-base alloy in Figure 17. The arrow in-
dicates the trace of the 110
direction, while Figure
19 is a TEM image of stacking faults intersecting at ~90˚
in a (100) grain orientation and Figure 20 shows multi-
variant {111} stacking faults in the annealed Co-base
alloy. These comparisons provide a compelling example
of a controlled microstructural architecture in contrast to
the conventional (carbide) microstructural reime. Alth-
ough the Cr23C6 column architectures are not continuous
or even proximate carbide particles, the concept as it can
be created through systematic, additive manufacturing
using EBM, suggests a revolutionary extension of the
conventional microstructure-property-processing-perfor-
mance materials science an engineering paradigm. Indeed,
in the same way compositions (in this case especially
carbon content) or chemistries of alloy systems can in-
fluence the microstructure-property relationships in par-
ticular, the prospects for microstructural architecture ma-
nipulation and the attendant property-processing-perfor-
mance variations, attest to the novelty and potential fea-
tures of the concept. Increasing the carbon content in the
Co-26Cr-6Mo-C alloy, especially to more contemporary
levels for aerospace superalloy applications (> 0.5C)
should have a controlling effect on the carbide particle
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
Figure 17. EBM fabricated components after annealing
Figure 18. Intrinsic stacking faults in the (112) orientation.
densities (or spacings) in the EBM-produced columns
(Figures 4, 6 and 7). This in turn should influence simple
mechanical properties such as residual hardness (even
directionally), strength, elongation, and even the elastic
Figure 19. Stacking faults intersecting at ~90˚ in a (100)
grain orientation.
Figure 20. Multivariant {111} stacking faults in the an-
nealed Co-base alloy.
(Young’s) modulus. The latter would be especially nota-
ble if the carbide particle columns began to act somewhat
like fiber-arrays in a matrix where the volumetric addition
A TEM Study of Cobalt-Base Alloy Prototypes Fabricated by EBM
Copyright © 2011 SciRes. MSA
of such reinforcing features alter the elastic modulus in
relation to their spatial relationship as it influences the
so-called rule of mixtures for the ideal isostrain condition
where the stress is applied in the direction of the fibers. Of
course these features are only speculative at this point.
4. Conclusions
Using TEM, we have illustrated the details of Cr23C6
carbide columns created by the systematic, spatial scans
of an electron beam in the additive manufacturing of
Co-26Cr-6Mo-0.2C prototypes using EBM. These col-
umns represent an example of a controlled microstructural
architecture. In this study, we have in principle compared
these uniquely fabricated architectures with more con-
ventional, stacking fault microstructures by annealing the
carbides and their attendant architectures. These archi-
tectures represent length scales ranging from microns-to-
nanometers which may be extended systematically to the
macroscale represented by millimeters-to-meters. The
TEM analyses have demonstrated Cr23C6 carbides to be
exclusive in the formation of column architectures with
spacings of 1 - 3 µm for carbide particles ranging nomi-
nally from <100 nm to >200 nm.
5. Acknowledgements
This work was supported in part by Mr. & Mrs. MacIn-
tosh Murchison endowments at The University of Texas
at El Paso.
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