Icosahedral Quasicrystal Layer Observed on λ Phase in Al-Cu-Fe Alloy

Icosahedral quasicrystals display irregular shape if it is embedded in bulk material. If it has free surface, it has well-defined facets, reflecting its unique 5-, 3-, and 2-fold rotational symmetries. In this study, an Al-Cu-Fe alloy with nominal composition of Al65Cu20Fe15 was prepared by arc melting and the microstructure was studied by using Scanning Electron Microscope, Energy Dispersive X-ray Spectroscopy, and Electron Back Scattering Diffraction (EBSD). On the surface of λ crystalline phase, an extra layer is found. EBSD from this layer revealed 5-, 3-, and 2-fold rotational symmetries, demonstrating the icosahedral quasicrystalline structure. Further, it has been found that the icosahedral quasicrystalline extra layer and the λ substrate have orientation relationship revealed by the coincidences of Kikuchi bands and poles on the EBSD patterns. This report is important to future studies regarding the formation of icosahedral quasicrystalline phase and thin film preparation related to icosahedral quasicrystalline phase.


Introduction
Stable icosahedral quasicrystals (IQC) were discovered in an Al-Cu-Fe alloy by Tsai et al. in 1987 [1]. This particular Al-Cu-Fe alloy has several advantages such as low cost for mass production and it can be prepared relatively easily by arc melting. In 1993, Balzuweit et al. conducted a study investigating various phases of Al-Cu-Fe alloy prepared by arc-melting [2]. They reported a λ 2 phase with column-shaped morphology. This phase is called λ phase in later reports and the present paper. It has monoclinic crystal structure [2] [3] with unit cell dimen-sions of a = 1.558 nm, b = 0.796 nm, c = 1.251 nm, and β = 108.14˚. This phase is believed to play an important role in the IQC phase formation [4] [5]. It is worth to mention that Balzuweit et al. further reported that the λ phase does not have a smooth surface. In some regions, extra layers were observed. No detailed discussion about the crystal structure of the extra layer and its relationship to the IQC phase formation was given.
IQC phase has been reported to display irregular bulky shape [6] [7], typically for the case when the grains are embedded in an alloy matrix. If the IQC grains have free surface, decahedral pentagonal shape [8] [9], and rhombic triacontahedron [10] [11] shapes are observed. These morphologies reflect the 5-, 3-, and 2-fold rotational symmetries. There had been efforts to prepare IQC thin films on some substrata with limited success [12] [13] [14] [15] [16]. been shown clearly that this extra layer has IQC structure, which is different from the λ phase of the substrate. Further, it has been revealed that the two phases have orientation relationship.

Materials and Methods
The alloy was prepared by arc melting pure Aluminum (99.99%), Copper (99.9%) and Iron (99.9%) from Good fellow with nominal composition of Al 65 Cu 20 Fe 15 . The vacuum chamber was evacuated to a vacuum of 10 −5 torr, purged with pure Ar gas, and finally refilled with Ar gas. The alloy was cast into a water-cooled copper crucible. Therefore, the cooling rate at the bottom of the ingot is higher than on the free surface. The hemisphere shaped alloy ingot has a diameter of two centimeter and height of one centimeter. The as-prepared alloy was then fractured and the naturally exposed surfaces are examined by using a TESCAN Vega-3 XMU Scanning Electron Microscope (SEM) with secondary electron detector. The electron beam is accelerated with a voltage of 30 kV. Elemental analysis was carried out using Oxford Aztec Energy Dispersive X-ray Spectrometer (EDS). Electron Back Scattering Diffraction (EBSD) was done using an OxfordNordlysMax3 EBSD detector. X-ray diffraction (XRD) was performed on a Bruker-NoniusD8 Advance Powder Diffractometer with copper k α line.   correspond to these three phases are identified, confirming the above phase identification. As shown in Figure 3, a detailed SEM observation of the columnlike structure revealed that the surfaces are not smooth. There are places where the substrate is exposed while in others the substrate is covered with extra layer. EBSD patterns taken from the substrates of adjacent column-like structures are the same and a typical one is shown in Figure 4(a). This implies that these adjacent column-like structures are parts of a single crystal. Two types of EBSD patterns are observed from the extra layer and are shown in Figure 4(b) and Figure   4(c), respectively. They are different from that in Figure 4   It is concluded that the extra layer has an IQC structure based on the following analysis. Figure 6 shows the two types of EBSD patterns from the extra layer again, where Figure 6  These results and analysis indicate that the substrate λ phase and the extra layer has an orientation relationship defined by the coincidence of Kikuchi bands I-1 and I-3 and pole I on EBSD patterns. It is worthy to point out that, for both types, the shared pole corresponds to the 2-fold rotational axis.

Results and Discussion
Two types of orientation relationship between the λ substrate and the IQC extra layer are found. As mentioned previously, adjacent column-like structures are parts of a single crystal of λ phase. As a result, EBSD patterns from different area of substrate in Figure 3 are the same. In contrast, two types of EBSD patterns from the extra layer are observed, although both of them correspond to IQC phase. The first type is represented by the EBSD pattern shown in Figure   4  However, it is reasonable to consider that certain orientation relationship allows better atomic site matching between substrate and IQC extra layer, and hence reduced interface energy. It should be pointed out that on the interface of crystal to crystal, usually there is only one type of orientation relationship that corresponds to the lowest interface energy or best atomic site matching. However, in the present crystal to IQC interface, there are, at least, two orientation relationship as discussed in the previous paragraphs. This is probably because the interface is the matching between crystal with translational symmetry and IQC structure without translational symmetry. Therefore, more than one orientation relationships are comparable in term of lowered interface energy. It is worth to mention that, in both cases, the shared pole is the 2-fold rotation axis.
This orientation relationship explains several experimental observations and is valuable for future studies. First, it explains that the extra layer tends to spread relatively large areas. For λ phase, it is reasonable to consider that, in atomic level, the arrangement of atoms repeats in a distance comparable to its lattice constant, which is in the order of one nanometer. For IQC phase, we use the size of icosahedron to estimate the "repeating" distance [14]. The size of an icosahedron is less than 1 nm. As shown in Figure 3 and Figure 7, the extralayer spreads over several ten micrometers, well beyond the repeating distance for both phases. It is known that when a thin film is formed on a substrate, its morphology depends on the interface energy. If the interface energy is low, the thin film tends to adhere well on the substrate and spread over large area. If the interface energy is high, the thin film tends to form an island and grow in thickness direction [21]. The fact that the present IQC extra layer spreads over relatively large area is considered as the evidence of lowered interface energy. Second, it helps the studies of preparing IQC related thin film. Icosahedral quasicrystalline alloys are reported to be hard and brittle. The brittleness makes it unsuitable for structural purpose. However, the high hardness makes it an appealing material for coating purpose. There have been some studies to create a quasicrystalline coating [12] [13] [14] [15] [16]. There have also been attempt to grow crystalline thin film on top of IQC surface [21] [22]. In an attempt to grow Aluminum thin film on IQC surface, Cai et al. [22] reported that although Aluminum atoms can find favorable atomic sites at low coverage, lateral growth stops and island forms as coverage increases, probably due to the relatively high interface energy or the lack of long-range atomic site matching. Present report indicates that semi-epitaxial growth of IQC thin film on selected crystalline surface is possible, which is a strategy that can be adopted in future studies. Second, it is observed very often that IQC grains with distinct pentagonal facets line-up with each other and the facets from different grains have coordinated orientation, as shown in Figure 9. These are explained as the result that the growth of IQC phase uses the column-shaped λ phase as template and the IQC phase has orientation relationship with the substrate.
There are several questions remain unanswered regarding the formation of the IQC extra layer. First, it is not clear about the thickness of the interface layer. Is there a transitional layer spreading several atomic layers? Or, the transition is an abrupt switch from one phase to another in atomic level? Second, the orientation relationship between the λ substrate and the IQC extra layer is not determined quantitatively in the present work, due to the limited EBSD instrument time. A study of the interface by Transmission Electron Microscopy (TEM) will be helpful to answer some of these questions. Diffraction analysis by TEM can also determine quantitatively the orientation relationship between the substrate and IQC extra layer, the result of which can be used to compare with previous theoretical predictions [23] [24].

Conclusion
The microstructure of Al 65 Cu 20 Fe 15 alloy prepared by arc melting was studied.
Column-shaped λ phase, sphere-shaped β phase, and IQC particles with well-defined pentagonal facets were observed. The surface of column-shaped λ phase was found to be covered with IQC extra layer. EBSD studies revealed that the λ substrate and the IQC extra layer have orientation relationship. Different from the epitaxial growth of crystalline thin film on crystalline substrate, there are at least two types of orientation relationships, which are considered to be unique for the interface between crystal and IQC. Lowered interface energy is considered to be the reason for the orientation relationship. A study of the interface by TEM will provide data to have better insight.