Microstructural Characterization of the Intermetallic Compound
Formed in Re-based Diffusion Barrier on Nb Substrate
Eni Sugiarti
Candidate for the Degree of Master of Engineering
Supervisor: Prof. Somei OHNUKI
Division of Materials Science and Engineering
Introduction
film on Nb substrate, followed by pack cementation with
Cr. Details of the formation process of the coating were
presented elsewhere.4) Both argon ions slicing (IS) and
focused ion beam (FIB) were used to obtain
cross-sectional TEM (XTEM) specimens.
SEM combined with EDS was employed to
investigate a coating layer structure and composition in
order of macro scale. Further, TEM examination
combined with EDS and SAED pattern was utilized to
examine a microstructure comprising of interface and
defect structures as well as to characterize the crystal
structure in the coating system. Additionaly, HVEM
observation is also essential to verify the realibity of the
phase identification which has been established by
electron diffraction analysis.
Niobium has attracted attention as the basis for
new materials for high temperature applications in place
of Ni-based superalloys. In order to increase the
properties of Nb or Nb-based alloys for applications that
requires the highest creep, Re as one of the refractory
metals has been selected for coating element.1,2) It is well
known that Re forms intermetallics compound phases by
alloying with Nb or Cr
Intermetallic compounds that are beneficially
strong at high temperature are sought for aerospace and
engine applications. The recent renewed interest in these
compounds is based on the prospect of low density with
elevated temperature mechanical properties.
In particular, Re-based diffusion barrier coating on
Nb-based alloys possesses a good oxidation resistance
until 1200OC.3) Hence, this potential has given us strong
expectation to withstand to long term exposure at
elevated temperature. However, data on the structure of
the Cr-Nb-Re diagram has not been available and the
understanding of this system is extremely important for
predicting the possibility of creation of the coating
system.
The objective of this study is to understand the
microstructure of the barrier layer and to identify the
crystal structure of the various phases in the Cr-Nb-Re
coating system. The relationships between the
composition and the phases as well as the microstructure
of the coating are summarized to make some suggestions
on improving the formation process of the diffusion
barrier on Nb or Nb-based alloys.
Results and Discussions
Macro-Micro Structure of Re-based Diffusion
Barrier Coating (DBC) on Nb Substrate
Composition
analysis
and
macrostructure
characteristic of the barrier layer were performed by
SEM.
Meanwhile,
microstructure
characteristic
presenting the details of interface and kinds of defect
structures were examined by TEM.
The cross-sectional structure of three types of the
coating specimens obtained by SEM is shown in Fig. 1.
It was recognized from a SEM image in Fig. 1 (top side)
and a EDS result in Fig. 1 (bottom side) that each
coating system comprises three-layers: the first layer of
Cr(Re), the second layer of Re-Cr-Nb, the third layer of
Nb(Re), and then there is the Nb substrate (four in the
figure).
Ni strike prior to the several micrometers thick Re
film was electroplated on Nb substrate as shown in Fig.
1(#1). The diffusion barrier layer of Re-based alloy with
~2 m in thickness composed of (60-48 at%)Cr, (35-20
at%)Re, (18-8 at%)Nb.
Methodology
Three types of the coating processes have been
challenging the formation of a Re-based diffusion barrier
on Nb substrate and presented in table 1. In general, a
coating was developed involving electroplating of Re
Table 1. Description of three types of the coating specimens.
Specimen
#1
#2
Procedure
 Nb
 Nb
 Ni Strike
 Re Plating
 Re-Ni Plating
 HT 1400OC 1h
O
 HT 1400 C 1h
 Re Plating
 Re-Ni Plating (2
(2 times)
times)
 Cr Pack 1100OC 1H (with
 Cr Pack 1400OC 1h
activator NH4Cl)
iv






#3
Nb
Re Plating
HT 1400OC 1h
Re Plating
(2 times)
HT 1485OC 3h
Cr Pack 1400OC 1h
Fig. 1(#3) shows that the Re-Cr-Nb layer with a
thickness of about 6 m contains (60-22 at%)Re, (45-24
at%)Cr and (18-12 at%) Nb. The surface of the outer
layer is irregular and not flat, while the intermediate and
inner layers are flatter.
XTEM analysis was also conducted in order to
examine detailed microstructure analysis of the
Re-based
diffusion
barrier
layer.
It
is
very important because studying the microstructure of a
material provides information linking its composition,
nature of the interface and defect structures to its
properties and performance. Fig. 2 shows the
microstructure of the interface between an outer Cr(Re)
layer, an intermediate Re-based diffusion barrier layer
and an inner Nb(Re) layer. The compositions of all
coated layers were determined by EDS analysis in TEM
and these results are correspond to the layers previously
shown by SEM results (in Fig 1).
Based on the micrograph, it is clarified that
Re-based diffusion barrier layer composed of
polycrystalline with fine grain size. Further, the Cr rich
layer in Fig. 2(#1) exhibits a lamellae structure with
grain size in order of nm. There are many defects,
cracks, dislocations and stacking faults in the interface
and some areas of each coated layer, influencing the
durability of the coating system.
Understanding the formed phases in the coating
system by the process is critical for the design of the
Re-based diffusion barrier layer on Nb Substrate as one
of ultra high temperature compound. Emphatically, the
phase of the Re-rich layer should really be considered
because this layer is expected to act as a diffusion
barrier between the alloy substrate and an outer
reservoir layer for both inward and outward diffusion,
its phase is directly related to stability of the coating
system.
Figure 1. Cross-sectional microstructures and
composition gradients for typical three layers after Re
and/or Ni plating on Nb substrate followed by Cr-pack
cementation (1) outer Cr(Re), (2) intermediate
Re-Cr-Nb, (3) inner Nb(Re), and (4) Nb substrate.
It was confirmed that the barrier layer formed
homogenously, concentrations varied sharply at the
interfaces between the outer layer, barrier layer, and
substrate. Many voids were observed in the outer
Cr(Re) and Nb substrate.
Fig. 1(#2) shows that the surface of the outer layer
is irregular and possess a thickness of about 1 m with
the composition nearly contains pure Cr and it did not
show in EDS profile. Consequently, the outer Cr(Re)
layer is slightly formed on the top of Re-riched layer.
Further, the Re-Cr-Nb layer thickness of about 6 m
was divided into segments by the cracks formed
probably during Re electroplating process. A typical
structure is observed due to it was prepared at a
relatively low temperature of 1100OC. In addition, many
voids were formed and accompanied by precipitate in
the Re-Cr-Nb layer.
Figure 2.Bright field XTEM images of three types of the coating specimens. (#1), (#2) and (#3) were obtained from
specimens in Fig. 1, respectively.
v
Complex Intermetallic Phase Constitution of the
DBC on Nb Substrate
Phase formation in all three types coating specimens
were examined by selected area electron diffraction
(SAED) pattern. SAED is a useful technique for the
characterization of crystalline materials and phase
identification in the case of limited experimental phase
equilibrium data. The binary phase diagram can be
utilized for determining the crystal structures of the outer
Cr(Re) and inner Nb(Re) layers, because the phases
almost comprised of two elements. According to these
phase diagrams, the phases were identified as solid
solutions with a body-center-cubic (BCC) structure.
The lattice constants determined from the patterns
are shown in table 2, and the results are in accord with
the lattice constants of Cr 70Re30 and Nb60Re40 phases of
bcc crystal structure in Powder X-ray database [5].
Because the atomic radius of Cr, Re and Nb in crystal
are different as RCr = 1.25 Å < RRe = 1.37 Å < RNb = 1.43
Å, the lattice constant of Cr(Re) phase is slightly larger
than that of pure Cr, on the contrary, the lattice constant
of Nb(Re) is slightly smaller than that of Nb.
For the phase of the Re-Cr-Nb layer, an intermetallic
compound phase was suggested to be formed on Nb
substrate in this study. According to Cr-Re, Cr-Nb and
Nb-Re binary diagrams [8], Cr-Re σ phase, Cr2Nb α
phase, Cr2Nb β phase, Nb-Re σ phase and Nb-Re χ phase
are possibly formed as the Re-Cr-Nb phase. Fig. 3 shows
a  phase for the indicated composition. It seems that the
lattice constant similar to cubic NbRe2 and Nb37Re63-
phases which have a lattice constant of a = 9.67 Å and a
= 9.76 Å, respectively [7].
Notified from the SAED patterns in Fig.4, the
Re-Cr-Nb diffusion barrier layer also consisted of a
hexagonal Laves C14 phase (specimen #1 and #3). The
phase can be determined by comparing the lattice
constants of the -Cr2Nb phase which has a = 4.98 Å and
c = 8.06 Å [5]. According to the SAED patterns, the
lattice constant calculations of the Re37Nb32Cr31 and
and Nb42Re33Cr25 phase are a = 5.12 Å and c = 8.23 Å,
which are somewhat larger than those in -Cr2Nb phases
[7].
Figure 3.SAED patterns showing a cubic  phase crystal
structure. (a) Re50Cr31Nb19, (b) Re73Nb17Cr10, (c)
Re68Nb27Cr5 and (d) Re63Cr20Nb17 in Fig.2.
Figure 4. SAED patterns showing a hexagonal Laves
C14 phase crystal structure. (a) Re37Nb32Cr31, (b)
Nb42Re33Cr25 in Fig.2.
Crystalline Feature of the Frank – Kasper
Phases and Nano – Scale Structure of the DBC
on Nb substrate
A stabilized  phase has been established in the
coating specimen at temperature of 1100OC. It is still
Table 2. Lattice constants determined from the coated layer.
Spec.
#1
#2
#3
Crystal
Structure
Phase
Cr66Re30
BCC
Re50Cr31Nb19
Re37Nb32Cr31
Layer
Lattice Constant (Å)
-Cr(Re)
a
2.97
c
-
Cubic
Hexagonal
 phase
Laves (C14)
9.53
5.12
8.23
Re73Nb17Cr10
Re68Nb27Cr5
Nb50Re49
Nb52Re44
Re63Cr20Nb17
Cubic
Cubic
BCC
BCC
Cubic
 phase
 phase
-Nb(Re)
-Nb(Re)
 phase
9.45
9.70
3.18
3.18
9.53
-
Nb42Re33Cr25
Nb70Re27
Hexagonal
BCC
Laves (C14)
-Nb(Re)
5.12
3.22
8.23
-
vi
References
Cr70 Re30
a
2.95
c
-
No.
[5]
Re63Nb37
-Cr2Nb
9.67
4.98
8.06
[6]
[7]
Re63Nb37
9.67
-
[6]
Nb50Re50
3.18
-
[5]
Re63Nb37
9.67
-
[6]
-Cr2Nb
Nb60Re40
4.98
3.23
8.06
-
[7]
[5]
worthy that the study of nano-structure can be essential
for understanding the crystal lattice image of a cubic 
phase formed in specimen #2. HRTEM image of the
selected area is illustrated in Fig.5. It is shown that the
dissimilar contrast of lattice fringe indicating an ordered
structure of cubic  phase.
HRTEM image of the selecte
Figure 7. Schematic illustration of cross-sectional
structure and established phases of Re-based diffusion
barrier layer on Nb substrate.
Figure 5. HRTEM image of cubic  phase formed in
Re60Nb34Cr6.
HRTEM image of the sele(2
A new ternary Nb(Cr,Re)2 laves phase (C14)
hexagonal structure (type MgZn 2) has clearly been
detected as shown in Fig.6. Distance between lattice
streaks in three different directions of the selected area
HRTEM image are all 0.440 nm corresponding to the
interplanar distances for [001] crystal direction of the
hexagonal structure.
Figure 8. Approximation of ternary diagram of Cr-Nb-Re
based on the established phases in the coated layer.
The established phases in this study were plotted in
a ternary Cr-Nb-Re phase diagram as illustrated in Fig. 8.
The crystal structure of the phases in the approximated
ternary system are listed in table 3.
Conclusions
The crystal structure data analysis is used to make
some recommendations. In the absence of experimental
data, the results here provide a guideline and will be
useful for improving the performance of a Re-based
diffusion barrier layers on Nb and Nb-based alloys,
Figure 6. HRTEM image of hexagonal C14 Laves phase
formed in Nb42Re33Cr25.
References
1)
Synthesis Discussion
2)
A structure modeling with phase characterization
was developed as illustrated in Fig. 7, which describes
any of Frank–Kasper structures formed in the
intermetallics compound of the diffusion barrier layer.
As a final point, the relationships between the
composition and the phases as well as the microstructure
of the coating layer are summarized.
3)
4)
5)
Table 3 Crystal structure of ternary Cr-Nb-Re system
Lattice constant (Å)
Pearson
Space
Phase
Symbol
Group
a
c
c/a
(Cr)
cI2
Im3m
2.97
(Nb)
cI2
Im3m
3.18 – 3.22
(Re)
hP4
P63/mmc
N/A

CI58
I43m
9.45 – 9.70
C14
hP12
P63/mmc
5.12
8.23 1.61
Laves
6)
7)
8)
vii
R. C. Reed: The Superalloy: Fundamentals and
Applications,(Cambridge University Press 2006).
T. Jin, W. Wang, X. Sun and Z. Hu: Mater. Sci.
Forum 638-642 (2010) 2257-2262.
Y. Matsumura, M. Fukumoto, S. Hayashi, A.
Kasama, I. Iwanaga, R. Tanaka, and T. Narita: Oxid.
Metals 61 (2004) 105-124.
K. Saito, S. Hayashi, T. Narita, I. Iwanaga and R.
Tanaka, Mater. Sci. Forum 522-523 (2006)
309-316.
J.M. Joubert: Progress in Mater. Sci. 53 (2008)
528-583.
J.M. Joubert and M. Phejar: Progress in Mater. Sci.
54 (2009) 945-980.
D. J. Thoma, J.H. Perepezko, D.H. Plantz and R.B.
Schwartz: Mat. Sci. Eng. A156 (1992) 97-108.
T.B. Massalski: Binary Alloys Phase Diagram, 2nd
ed. ASM International, 1990.