Materials Transactions, Vol. 47, No. 8 (2006) pp. 2106 to 2108
#2006 The Japan Institute of Metals
RAPID PUBLICATION
Effect of Rhenium Addition on Tungsten Diffusivity in Iron-Chromium Alloys
Tomonori Kunieda1; * , Koji Yamashita1; *, Yoshinori Murata1 ,
Toshiyuki Koyama2 and Masahiko Morinaga1
1
Department of Materials, Physics, and Energy Engineering, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japan
2
National Institute for Materials Science, Tsukuba 305-0047, Japan
The Alloying effect of Re on the diffusivity of W in Fe-15 mol%Cr based alloys was investigated experimentally using the Fe-15Cr/Fe15Cr-5W and Fe-15Cr-1Re/Fe-15Cr-5W diffusion systems. In these systems, a single ferrite phase existed stably at 1473 K, and Fe and W atoms
interdiffused at 1473 K without any attendant changes in the Cr concentration of 15 mol% in them. The measured diffusion length of W atoms
was shorter in the Re-containing diffusion system than the Re-free one. Following the binary Boltzmann-Matano method, an apparent
interdiffusion coefficient at 1473 K was estimated to be 7:1 1015 m2 /s in the Re-free diffusion system and 1:5 1015 m2 /s in the Recontaining diffusion system. Thus, the presence of Re in the alloy worked to suppress the atomic diffusion of W in a ferrite phase.
[doi:10.2320/matertrans.47.2106]
(Received March 28, 2006; Accepted July 3, 2006; Published August 15, 2006)
Keywords: Diffusion, heat resistant steel, refractory element, rhenium, tungsten
1.
Introduction
The refractory elements such as Mo, W and Re play an
important role in strengthening high temperature creep of
high Cr heat resistant ferritic steels.1–3) In these steels, it is
known that the (Fe, Cr)2 (W, Mo) type Laves phase
precipitates together with M23 C6 carbide and the MX
carbonitride.4,5) Recently, it has been shown that the longterm creep strength at 923 K becomes much smaller than the
extrapolated strength from the short-term creep test.6) This is
related mainly to the microstructure changes in the steel
during creep. Recently, it has been reported that the addition
of a small amount of Re is effective in improving the longterm creep strength of the W-containing steels.7–9) This
implies that the presence of Re in the steel lowers the W
diffusivity in some ways, because the microstructure evolution during creep is subjected more or less to the coalescence
of the Laves phase, (Fe, Cr)2 (W, Mo), and M23 C6 carbide.
The diffusivity of refractory elements in heat resistant
steels is crucial for the basic understanding of the microstructural stability during creep.10) The purpose of this study
is to investigate the alloying effect of Re on the W diffusivity
in the ferritic steel. For this aim, Fe-15 mol%Cr was chosen
as a base alloy for the bcc matrix phase in high Cr ferritic
steels. This alloy is suitable for the present diffusion
experiment, as a bcc phase is stable up to the melting
temperature in it.
2.
were cut from the ingots. The homogenization and the grain
growth of each specimen were accomplished by 50% cold
rolling, followed by the annealing at 1473 K for 24 h. The
grain size of each specimen was estimated by an optical
microscope (OM) and a scanning electron microscope (SEM)
equipped with an instrument for taking an electron backscattered diffraction pattern (EBSD). The grain size of each
alloy was as large as about 1 mm after the annealing. The
diffusion temperature, 1473 K, employed in this study was
about 0.8Tm (Tm : melting temperatures of the experimental
alloys). In general, it is believed that when the temperature is
higher than 0.75Tm , there is little effect of interfacial
diffusion on lattice diffusion in the alloy.11)
A diffusion couple shown in Fig. 1 was prepared in the
following way. First each side of the plate specimen was
ground, and then one surface of each specimen was polished
mechanically with emery papers and 0.25 mm diamond
Fe-15Cr-5W
W
Alumina fiber
Experimental Procedures
Fe-15Cr
Fe-15Cr binary alloy, Fe-15Cr-1Re and Fe-15Cr-5W
ternary alloys were used in this study. Here, the alloy
compositions are given in mol% units. According to the FeCr and Fe-Cr-W phase diagrams, these three alloys should
consist of a bcc phase at 1473 K. The button ingots of these
alloys were prepared by arc melting in a high purity argon gas
atmosphere. Plate specimens with about 1 cm in thickness
*Graduate
Student, Nagoya University
Fe-15Cr-1Re
W
Fig. 1
Schematic illustration showing a diffusion couple.
Effect of Rhenium Addition on Tungsten Diffusivity in Iron-Chromium Alloys
2107
0.06
(a)
1.01
Mole fraction (W)
Alumina fiber
0.05
1.00
0.04
0.99
0.03
Fe+Cr
W
0.02
0.98
0.97
0.01
0.96
0.00
0.95
-0.01
-1500 -1000
(b)
-500
0
Distance, l /
500
1000
Mole fraction (Fe+Cr)
(a)
0.94
1500
m
1.01
0.06
20
m
Fig. 2 Typical SEM microstructures showing the initial interface (alumina
fiber) in (a) Fe-15Cr/Fe-15Cr-5W and (b) Fe-15Cr-Re/Fe-15Cr-5W
diffusion system, both annealed at 1473 K for 100 h.
Mole fraction (W)
Alumina fiber
0.05
1.00
0.04
0.99
0.03
3.
Results and Discussion
3.1 Concentration profiles and diffusion path
It was confirmed from the optical microscopic observation
that the as-annealed specimens contained only the ferrite
(bcc) single phase without any precipitates in them. Figure 2
shows typical SEM images taken from the diffusion couple
annealed at 1473 K for 100 h. In this figure, the alumina fiber
is seen as a dark circle image, and its position is on the initial
interface between the diffusion couple. The concentration
profiles of W and (Fe þ Cr) are shown in Fig. 3(a) for Fe15Cr/Fe-15Cr-5W and in Fig. 3(b) for Fe-15Cr-1Re/
W
0.02
0.98
0.97
0.01
0.96
0.00
0.95
-0.01
-1500
slurry. Subsequently, the diffusion couple of Fe-15Cr/Fe15Cr-5W and Fe-15Cr-1Re/Fe-15Cr-5W was assembled as
shown Fig. 1. There were the two diffusion systems in this
assembly. Alumina fibers were sandwiched between the plate
specimens as a marker of the initial interface.
This couple was held tightly with a molybdenum holder.
Then, it was encapsulated in a quartz tube with argon gas and
annealed at 1473 K for 100 h. After the annealing, the cross
section of the diffusion couple was examined by the scanning
electron microscopy (SEM) and the energy dispersive X-ray
spectroscopy (EDX) to measure concentration profiles across
the diffusion interface. The origin of the concentration profile
was set at the position of the alumina fiber.
Fe+Cr+Re
-1000
-500
0
500
Distance, l / m
1000
Mole fraction (Fe+Cr+Re)
(b)
0.94
1500
Fig. 3 Concentration profiles of W and (Fe þ Cr) obtained from (a) Fe15Cr/Fe-15Cr-5W diffusion system and (b) Fe-15Cr-1Re/Fe-15Cr-5W
diffusion system, both annealed at 1473 K for 100 h.
Fe-15Cr-5W. These concentration profiles exhibited the S
curves without any discontinuity, indicating that the diffusion
occurred in the single ferrite phase without precipitating any
phases in the alloy. From Fig. 3, it was found that W atoms
diffused into the region of about 800 mm apart from the
interface in the Re-containing system. Such a diffusion
region of W atoms was further extended over above 1000 mm
in the Re-free system. This result indicates that W atoms
diffuse more easily in the Re-free alloy than in the Recontaining alloy.
The diffusion path of each diffusion system was obtained
by plotting the data shown in Fig. 3 in the Gibbs triangle. The
result is shown in Fig. 4. The concentrations of Cr did not
change during annealing in both the diffusion systems. A
very small difference in the diffusion paths between the two
systems was attributable to the existence of 1 mol% Re in the
Fe-15Cr-1Re/Fe-15Cr-5W, but the absence in the Fe-15Cr/
Fe-15Cr-5W. This result implies that the presence of Cr
scarcely affects the W diffusion in the Fe-15Cr based
diffusion couples. In other words, W atoms diffuse in the
2108
T. Kunieda, K. Yamashita, Y. Murata, T. Koyama and M. Morinaga
Re-free
Re-containing
alloys. The reason why the presence of Re retards the W
diffusion in Fe-15Cr alloys is not clear at the moment, but
there are two possibilities. One is the existence of attractive
force between W and Re due to the trend of the formation of
an intermetallic compound between them. The other is the
vacancy trapping effect of Re in the alloy.
20
Cr
ol)
(m
4.
The alloying effect of Re on the W diffusivity was
examined experimentally using both the Fe-15Cr/Fe-15Cr5W and the Fe-15Cr-1Re/Fe-15Cr-5W diffusion systems.
The apparent interdiffusion coefficient for the Re-containing
alloy was found to be about one fifth of that for the Re-free
alloy. It was concluded that the existence of Re retarded
significantly the W diffusion in Fe-15 mol%Cr based alloy.
10
0
10
20
Acknowledgments
W (mol)
Fig. 4 Diffusion paths in Fe-15Cr/Fe-15Cr-5W and Fe-15Cr-1Re/Fe15Cr-5W diffusion systems.
Table 1 Apparent interdiffusion coefficients obtained from Fe-15Cr/Fe15Cr-5W and Fe-15Cr-1Re/Fe-15Cr-5W diffusion systems.
diffusion system
Fe-15Cr/Fe-15Cr-5W
Fe-15Cr-1Re/Fe-15Cr-5W
Conclusion
diffusion coefficient
(m2 s1 )
7:1 1015
15
1:5 10
Fe-Cr (-Re) alloy without distinguishing between Fe and Cr,
owing mainly to the equi-composition, 15 mol%, of Cr
between the two diffusion systems.
3.2 Interdiffusion coefficients
In order to compare the W diffusivity quantitatively in
each diffusion system, interdiffusion coefficients were estimated by assuming that the diffusion occurs in the (Fe, Cr)-W
pseudo-binary system. The apparent interdiffusion coefficient was calculated using the binary Boltzmann-Matano
method.12)
The calculated interdiffusion coefficients at 1473 K in the
two diffusion systems are shown in Table 1. The interdiffusion coefficient was about 5 times larger in the Re-free alloy
than in the Re-containing alloy. This result indicates that the
existence of Re suppresses the diffusion of W atoms in Fe-Cr
The authors would like to thank Associate Prof. Numakura
in Kyoto University for his valuable discussion. Also, this
work was supported in part by the Grant-in-Aid for Scientific
Research of Japan Society for Promotion of Science (JSPS),
Japan.
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