Influences of Cr Addition on Internal Oxidation of Ni(Al) Solid Solution

Makoto Nanko and Katsumi Uemura

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AZojomo (ISSN 1833-122X) Volume 6 November 2010

Topics Covered

Abstract

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.

Keywords

Internal oxidation, Ni(Al) solid solution, Cr, Al₂O₃, Nano-rod array

Introduction

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 [20]. 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 [25]. 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 processes.

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.

Experimental Procedure

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 1000°C.

Figure 2. Cross-sectional views of samples oxidized at 1100°C.

Figure 3. Cross-sectional views of samples oxidized at 1200°C.

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.

Conclusions

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.

References

Contact Details

Makoto Nanko and Katsumi Uemura
Department of Mechanical Engineering, Nagaoka University of Technology
Kamitomioka, Nagaoka, Niigata 940-2188, JAPAN

E-mail : [email protected]

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 12[1] (2010) 13-18.