Nov 9 2010
Makoto Nanko and Katsumi Uemura
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1833-122X) Volume 6 November 2010
ProcedureResults & DiscussionConclusionsReferencesContact
Ni - 5 mol%Al alloys with 0.5 mol%Cr addition were prepared by an arc-melting
process. Oxidation process was conducted at temperature ranging from 1000 to
1200°C with Co/CoO buffer to avoid oxidation of Ni. In oxidation at 1000 and
1100°C, Al2O3 rod-like precipitates were formed with small
amount of coarse Cr2O3 rod- like precipitates being
heterogeneously in the internal oxidation zone. Al2O3
rod-like precipitates seem to be covered by Cr2O3.
Oxidizing at 1200°C, (Al,Cr)2O3 rod-like precipitates were
formed homogeneously. Morphology of internal oxidation zone implies nuclear
formation of Cr2O3 in internal oxidation zone consisting
of Al2O3 precipitates does not occur homogeneously at
lower oxidation temperatures.
Internal oxidation, Ni(Al) solid solution, Cr, Al2O3,
Recently nano-rod array structures have received great attention for
high-tech applications such as electronics, optics and optoelectronics [1-6].
Their components and devices were produced by applying various semiconductor
production processes such as vapor phase processing [6-12] and etching processes
[13-16]. These production processes of nano-rod array structures require
expensive equipments such as vacuum chambers. Also, oxide ceramic materials for
nano-rod array structure are limited, such as ZnO, TiO2 and
SiO2. The template techniques, the direct nano-rod array fabrication
by selective deposition in a mold with the nano-pore array, have been also
applied to fabricate nano-rod array structures [1, 5, 17-19]. Bonding strength
to the substrate and mechanical strength of their ceramic rods produced by their
processes are not so high. The nano ceramic rods produced by their processes
were just loosely adhered to the substrate.
Internal oxidation is an oxidation mode where the oxide precipitates were
formed in the alloy inside. In high-temperature oxidation/corrosion resistance,
internal oxidation is very important because chemical composition of alloys may
degrade alloy performance. On the other hand, internal oxidation has been
applied to fabricate oxide-dispersion strengthen alloys . In cases of Ni(Al)
alloys, precipitates of internal oxidation have rod-like or needle-like shapes
perpendicular to the alloy surface [21-24]. The oxide morphology of the
internally oxidized zone (referred to as IOZ) of these alloys can be applied to
prepare nano-rod array after removing Ni in the IOZ. The author’s group has
proposed a unique production process for oxide nano-rod array structure using
internal oxidation of Ni(Al) alloys . Nano-rod array structures were
successfully fabricated by internal oxidation and electropolishing of dilute
Ni(Al) alloys. Because the nano-rods are embedded in the substrate, high holding
strength of these nano-rods are expected in comparison with the conventional
nano-rod array structures prepared by the vapor deposition or etching
When small amount of Cr is added into Ni(Al) solid solution, Cr would be
oxidized after formation of Al2O3 precipitates. Because
Cr2O3 has the same crystal structure with a-
Al2O3, Cr-doped Al2O3 precipitates
(i.e., ruby) may be obtained. In the present paper, internal oxidation behavior
of Ni(Al) solid solution with small amount of Cr was discussed in order to
fabricate Cr- doped Al2O3 nano-rod array structures.
Ni - 5 mol%Al alloys with 0.5 mol%Cr addition (5Al-0.5Cr) were prepared by an
arc-melting process. Ni – 5 mol%Al alloys (5Al) were also prepared for
comparison. The alloys were annealed at 1300°C for 12 h in vacuum. The alloys
were cut and polished in mirror finishing with diamond slurry. The samples were
ultrasonically washed in ethanol and dryed at room temperature in laboratory
air. Oxidation process was conducted at temperature ranging from 1000 to 1200°C
in vacuum with Co/CoO buffer to avoid oxidation of Ni. Electropolishing process
with sulphuric acid solution with constant 4 V was applied to the oxidized
samples for exposing oxide precipitates on the alloy surface. Microstructure of
oxidized samples was observed by scanning electron microscopy (SEM). Phase
identification of the oxide precipitates was conducted for electropolished
surface by using X-ray diffraction (XRD).
Results & Discussion
Figures 1 – 3 show cross-sectional views of 5Al and 5Al-0.5Cr oxidized at
1000, 1100 and 1200°C, respectively. Rod-like oxide precipitates were developed
in 5Al and 5Al-0.5Cr. However oxide rod orientation was sometimes at random, in
particular, the Cr added ones. Coarse precipitates were often observed in
5Al-0.5Cr oxidized at 1000 and 1100°C.
Figure 1. Cross-sectional views of samples oxidized at
Figure 2. Cross-sectional views of samples oxidized at
Figure 3. Cross-sectional views of samples oxidized at
Figure 4 shows XRD patterns of 5Al and 5Al-0.5Al oxidized at temperature
ranging from 1000 to 1200°C. Oxide products in 5Al consists of mainly
θ-Al2O3 at 1000°C. With incrasing oxidation temperature,
α- Al2O3 becomes predominat in 5Al. Oxide products in
5Al-0.5Al Cr consists of mainly α-Al2O3 at 1000°C. Cr
addition leads to faster θ/α transformation. Because Cr2O3
is the same crystal structure with α-Al2O3,
Cr2O3 would act as a catalyst for the θ/α-transformation.
Figure 4. XRD patterns of samples oxidized at various
temperatures. (a) 5Al and (b) 5Al-0.5Cr.
Figure 5 shows high magnification images of oxide precipitates oxidized at
1100°C (a) and 1200°C (b). Oxide Precipitates formed at 1100°C consist of dark
grains covered partially by a light gray one. Although
Al2O3 was detected in all of the oxidized samples by XRD,
the oxide precipitates consist of Al2O3, probably, covered
by Cr2O3 at low temperatures. On the other hand, oxide
precipitates in the sample oxidized at 1200°C have the uniform contrast as shown
in Fig. 5 (b). Cr2O3 would be dissolved into
Al2O3 grains immideatly.
Figure 5. High magnification images of cross section of
the 5Al-0.5Cr oxidized at 1100°C (a) and 1200°C (b).
In internal oxidation of 5Al-0.5Cr, Al is oxidized on the front of the
internal oxidation and then Cr is oxidized in internal oxidation zone
consisiting of Al2O3 precipetates in Ni(Cr) solid solution
materix, because affinity of Al to oxygen is higher than that of Cr. However
their oxygen partial pressures in the equilibrium of
Cr/Cr2O3 and Al/Al2O3 are much lower
than that of the oxidation atmosphere, that is, Co/CoO equilibrium. The area in
which only Al is oxidized internally is negligible. At high temperatures such as
1200° C, interdiffusion between Al2O3 and
Cr2O3 is so fast that their solid soution is formed
immidiately. Figure 6 shows a schematic drawing of the oxide precipitates in the
IOZ of 5Al-0.5Cr.
Figure 6. A schematic drawing of internal oxidation of
Ni(Al, Cr) solid solution.
Nano-rod array structure can be obtained by using internal oxidation of
Ni(Al, Cr) solid solution. Figure 7 shows SEM images of nano-rod structure on
Ni(Al, Cr) solid solution. IOZ prepared at 1000 and 1100°C sometime consists of
larger oxide precipitates. Nano-rod array structure fabricated at 1200°C was
more homogeneous than ones made at lower temperatures.
(Al,Cr)2O3 nano-rod array can be produced by using Ni(Al,
Cr) solid solution oxidized at high temperatures such as 1200°C.
Figure 7. SEM images of nano-rod array structure on
5Al-0.5Cr by internal oxidaiton and electropolishing.
Internal oxidation of Ni - 5 mol%Al alloys with 0.5 mol%Cr addition was
studied for preparing nano-rod array structure by using internal oxidation and
electropolishing. In oxidation at 1000 and 1100°C, Al2O3
rod-like precipitates were formed with small amount of coarse
Cr2O3 containing rod-like precipitates being
heterogeneously in the internal oxidation zone. Fine rod precipitates were most
likely comprised of Al2O3 grains covered by
Cr2O3. Oxidizing at 1200°C, (Al,Cr)
2O3 rod-like precipitates were formed homogeneously.
Morphology of internal oxidation zone implies nuclear formation of
Cr2O3 in internal oxidation zone with
Al2O3 precipitates does not occur homogeneously at lower
temperatures. Nano-rod array structure can be obtained successfully on Ni(Al,
Cr) solids solution by using internal oxidation and electropolishing.
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Makoto Nanko and Katsumi Uemura
Department of Mechanical
Engineering, Nagaoka University of Technology
Kamitomioka, Nagaoka, Niigata
E-mail : [email protected]
This paper was also published in print form in "Advances in
Technology of Materials and Materials Processing", 12 (2010)