Elsevier

Corrosion Science

Volume 88, November 2014, Pages 209-214
Corrosion Science

Formation of amorphous oxide in Al82Ni13Zr5 and Al88Ni7Ca5 alloys

https://doi.org/10.1016/j.corsci.2014.07.035Get rights and content

Highlights

  • The stability of amorphous oxide is different depending on the alloy composition.

  • The stability of amorphous oxide in Al88Ni7Ca5 is higher than that in Al82Ni13Zr5.

  • The presence of calcium ion in oxide stabilizes amorphous structure of the oxide.

Abstract

Formation and thermal stability of amorphous oxide in Al82Ni13Zr5 and Al88Ni7Ca5 alloys during holding at 873 K have been investigated. The thickness of amorphous oxide layer upon heating up to 873 K is ∼8 nm in Al82Ni13Zr5, while ∼15 nm in Al88Ni7Ca5. After exposure for 15 h, the amorphous oxide still remains in Al88Ni7Ca5, indicating that the thermal stability of the amorphous oxide in Al88Ni7Ca5 is higher than that in Al82Ni13Zr5. Such a phenomenon is discussed from the points of view of structural stability and oxygen ion mobility in the crystalline and amorphous oxide phases.

Introduction

Recently, metallic glass receives a great attention as a potential candidate for functional applications due to their superior characteristics such as high thermoplastic forming ability and good oxidation, corrosion and electrochemical properties [1], [2], [3], [4], [5]. One of the outstanding merits of metallic glass is that it can flow with low viscosity in the supercooled liquid (SCL) region. Therefore, metallic glass can be utilized as thermoplastic or binding materials for various purposes [6], [7]. However, in such cases, one of the critical issues to be considered carefully is the oxidation resistance. Although it is well understood that formation of oxide layer affects various properties of metallic glasses such as wettability in the SCL state and electrical conductivity after crystallization, it is hard to avoid the oxidation during casting or binding process. Therefore, it is important to select proper alloy system and composition which not only enhances the glass forming ability but also minimizes the oxidation for metallic glass to be used as thermoplastic or binding materials.

In general, aluminum exhibits good oxidation resistance due to formation of passive Al2O3 layer. It is expected that Al-based metallic glass containing more than ∼80 at.% aluminum would also have higher oxidation resistance than other metallic glass systems. However, the oxidation behavior of Al-based metallic glass may be quite different from that of pure Al, since multicomponent system is required to assure enough glass forming ability. In the case of metallic glasses, it has been also shown that minor alloying element can affect the oxidation behavior, for example, minor alloying of Be in Cu–Zr–Al metallic glass can significantly improve the oxidation resistance in the SCL region [8].

One of the important points when considering the enhancement of oxidation resistance in metallic glass is the formation of the oxide layer with an amorphous structure at the early stage of the oxidation process [8], [9]. Since the atoms are more densely packed in the amorphous structure than in the counterpart crystalline structure, the transport of oxygen ion in the amorphous oxide layer is more difficult than in the counterpart crystalline oxide layer. Therefore, the thermal stability of the amorphous oxide has a crucial effect on the oxidation resistance of metallic glass at high temperature. Recently, it has been reported that ∼100 nm thick amorphous oxide layer forms during annealing Al87Ni3Y10 metallic glass at 873 K (far above the crystallization onset temperature (Tx), Tx: 542 K) for 15 h [10]. Such a large thickness of the amorphous oxide layer can be compared to ∼4 nm thickness of the amorphous oxide layer which forms on the surface of pure aluminum [11]. The presence of a small amount of yttrium in the amorphous oxide layer indicates that the diffusivity of oxygen ion is greatly suppressed by minor alloying effect. Therefore, minor alloying in the oxide layer can significantly affect the thermal stability of the amorphous oxide layer. From this point of view, it is strongly required that the effect of other minor alloying elements (X) in Al–Ni–X amorphous system on the oxidation behavior should be investigated. In the present study, we select calcium (Ca) and zirconium (Zr) as minor alloying elements which have been reported to influence the glass forming ability. Substitution of rare earth element with Ca has been reported to enhance the glass forming ability [12], while minor alloying of Zr has been reported to extend the SCL region up to ∼50 K in Al-based amorphous system [13]. Since the previous study on Al87Ni3Y10 metallic glass shows that the oxide layer retains the amorphous structure up to far above the crystallization onset temperature, the thermal stability of the amorphous oxide has been investigated after annealing at 873 K [10]. Following the previous result on Al87Ni3Y10 alloy, the thermal stability of the amorphous oxide in Al88Ni7Ca5 and Al82Ni13Zr5 alloys has been investigated after annealing at 873 K.

Here, it is to be noted that amorphous or crystalline nature of the matrix may have an effect on the oxidation behavior at high temperature. It has been reported that the amorphous matrix has a lower interfacial energy with the amorphous oxide due to bond flexibility, leading to easy formation of the amorphous oxide in Cu–Zr based metallic glass system [9]. Therefore, depending on the amorphous or crystalline nature of the matrix, the oxidation behavior may be different Al-based system. However, amorphous alloys are selected for the present investigation, since understanding of oxidation behavior of amorphous Al-based alloys is required, when they are used as precursor of crystallization product. For example, it was reported that Al-based amorphous alloys are promising candidates for highly conductive electrode formation in Si solar cells [6]. In such case, amorphous alloys are exposed to high temperature (far above the crystallization temperature) after thermoplastic forming in the SCL region.

Section snippets

Experimental procedure

Ingots with a nominal composition of Al88Ni7Ca5 and Al82Ni13Zr5 were prepared by arc-melting the mixture of high purity metals (>99.9%) in a water-cooled copper hearth in an argon atmosphere. The alloy ingots were re-melted by high-frequency induction and then rapidly solidified into a ribbon-type samples (thickness: ∼40 μm) by ejecting the liquid melt on the surface of a Cu wheel rotating with the speed of 40 m/s. The amorphous structure of the as-melt-spun samples was confirmed using X-ray

Result

Fig. 1 displays the cross-sectional images obtained from the melt-spun Al88Ni7Ca5 and Al82Ni13Zr5 samples heated up to 873 K. High-resolution electron microscope (HREM) images shown in Fig. 1(a) and (b) indicate that the oxide layer still retains the amorphous structure, as verified by Fourier transformed (FT) halo patterns inserted in Fig. 1(a) and (b), although crystallization of the amorphous matrix occurred in both samples. It can be noticed that the thickness of the amorphous oxide layer is

Discussion

The oxide layer on the surface of metals can have an amorphous structure when the surface energy decrease due to the amorphous structure overwhelms the corresponding bulk free energy increase [14]. Although the critical thickness of the amorphous oxide layer can be affected by temperature and crystallographic orientation of the substrate, it is generally very thin, for example, ∼4 nm on pure Al [11]. However, the present study indicates that the amorphous oxide in Al base alloy can have very

Conclusions

The oxidation behavior of Al82Ni13Zr5 and Al88Ni7Ca5 amorphous alloy during holding at 873 K has been investigated in the present study. The main conclusions are as follows:

  • (1)

    The amorphous oxide has very much different stability depending on the alloy chemistry. The amorphous oxide formed on Al82Ni13Zr5 melt-spun ribbon upon heating up to 873 K has ∼8 nm thickness, while that formed on Al88Ni7Ca5 has much larger thickness of ∼15 nm. The growth rate of the amorphous oxide layer in Al88Ni7Ca5 is higher

Acknowledgement

This work was supported by the Global Research Laboratory Program of the Korean Ministry of Education, Science and Technology.

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