Introduction

Experimental studies of the Al-Pd-Co system comprising complex metallic alloys (CMA) were done by Yurechko et al.[1,2] and Cernickova et. al.[3] As a result, the authors proposed partial isothermal sections of the Al-Pd-Co phase diagram at 790, 940, 1000, 1050,[1,2] and 700 °C.[3] CMAs are mostly binary, ternary or quaternary systems containing mainly phases with giant unit cells and quasicrystals.[4-6] Many CMAs are not sufficiently known yet, inclusive of those related to the Al-Pd-Co system. Crystallographic parameters of binary and ternary phases reported for this system are summarized in Table 1.[1-3]

Table 1 Crystallographic data of phases in Al-Pd-Co system and corresponding binaries[1-3]
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In this work, several phases of the ε-family (denoted jointly as εn),[7-9] F, and U were studied in more detail. Phases of the ε-family are classified as orthorhombic approximants of the decagonal quasicrystal.[1,2] In the Al-Pd-Co system, ε6 and ε28 were classified as binary phases alloyed with the third element, and ε16, ε22 and ε34 as ternary phases.[7] Structures of all of them consist of two different clusters, i.e. PMI (pseudo-Mackay icosahedra) and/or LBPP (large bicapped pentagonal prism).[8] Central points of the PMI clusters correspond to vertices of phason tiles of three different shapes (hexagon, pentagon, and banana-shape nonagon) if projected into a plane oriented perpendicularly to the phason plane. Recently, three types of tiling were reported, consisting of only hexagons (ε6), pentagons and banana-shape nonagons (ε16), and hexagons, pentagons and banana-shape nonagons (ε22, ε28 and ε34).[7-9] Ternary monoclinic U-phase with lattice parameters a = 1.9024 nm, b = 2.9000 nm, c = 1.3140 nm, and β = 117.26° was identified by Yurechko et al.[10] The metal composition of this phase was reported between Al69.1Pd18.5Pd12.4 and Al70.2Pd11.4Co18.4. In alloys Al68Pd14.6Co17.4 and Al69.8Pd13.8Co16.4, several isostructural mutations of the U-phase differing from each other in metal compositions were observed, also after long-term isothermal annealing.[11] It was shown that each of the mutations originated from other parent phase. The structure of cubic F-phase with lattice parameter 2.4397 nm was derived by Sugiyama et al.[12] This phase was experimentally observed by Yurechko et al.[10,13] in the triangular composition range between Al71.2Pd11.6Co17.2, Al71.4Pd12.3Co16.3, and Al71.9Pd11.5Co16.6. In the long-term annealed Al72Pd9Co19 alloy, two isostructural mutations of the F-phase were identified with very similar metal compositions.[14]

According to the findings of Yurechko et al.,[1] phases εn, F, and U could form single-phase areas in the temperature range 940-790 °C in the Al-Pd-Co phase diagram. However, changes of these areas with temperature were not proved experimentally till now. In the present work, therefore, the alloys Al68Pd14.6Co17.4, Al70Pd25Co5, Al71.2Pd15Co13.8, Al71Pd9Co20, Al72.5Pd21Co6.5, Al72.8Pd15.6Co11.6, Al73.3Pd12.8Co13.9, and Al76Pd11Co13 were long-term annealed at 850 °C and subsequently characterized. The experiments were done with the intention to propose a partial isothermal section of the Al-Pd-Co phase diagram at 850 °C, still missing in the literature. The attention was focused on the area delimited by homogeneity ranges of phases εn, U, and F.

Experimental Procedures

The investigated alloys were prepared by arc melting of pure components (Al, Pd, and Co) under argon atmosphere. After casting, the samples were annealed at 850 °C for 500 h and rapidly cooled in water to preserve their high-temperature microstructures. In the investigation, the scanning electron microscopy (SEM) including energy dispersive x-ray spectroscopy (EDX), and the x-ray diffraction (XRD) were used.

Particular microstructure constituents were observed and their metal compositions were determined by a JEOL JSM-7600F scanning electron microscope operating at the acceleration voltage of 20 kV in regimes of secondary electrons (SEI) or back-scattered electrons (BEI). The microscope was equipped with an Oxford Instruments X-max50 spectrometer for EDX analysis including INCA software. At least 10 measurements per constituent were done to obtain mean values of metal compositions. Volume fractions of the constituents were determined by means of the ImageJ software.

An x-ray Panalytical Empyrean PIXCel 3D diffractometer with Bragg-Brentano geometry was used for the phase identification. The characteristic CoKα1,2 radiation was generated at 40 kV and 40 mA. Measurements were done in the angular range 10° to 140°, with the step size of 0.0131°, and the counting time 98 s/step.

Results

Microstructures of all the investigated alloys are documented in Fig. 1. They are formed with single-phase constituents only; to each constituent the respective phase identified by XRD is assigned. XRD patterns corresponding to particular alloys are shown in Fig. 2. Metal compositions and volume fractions of the microstructure constituents coupled with the identified phases are given in Table 2.

Fig. 1
figure 1

Microstructures of investigated alloys. To each microstructure constituent corresponding phase is assigned. Documented by BEI/SEM

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Fig. 2
figure 2figure 2

Powder x-ray diffraction patterns corresponding to investigated alloys

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Table 2 Metal compositions and volume fractions of observed microstructure constituents
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In the Al68Pd14.6Co17.4 alloy, three microstructure constituents were observed (Fig. 1a). Two of them were identified as phases U and β (Fig 2a), the third constituent showed metal composition close to binary Al5Co2 with the maximum solubility of Pd about 2.9 at.% (Table 2). In the Al70Pd25Co5 alloy, two microstructure constituents were observed (Fig. 1b) corresponding to phases δ and εn (εn is the mixture of ε6 and ε28), Fig. 2(b). The maximum solubility of Co in binary δ (Al3Pd2) was determined as about 0.9 at.% (Table 2). The microstructure of the Al71.2Pd15Co13.8 alloy consists of two constituents identified as U and εn (ε6 + ε28), Fig. 1(c) and 2(c). In the microstructure of the Al71Pd9Co20 alloy, two constituents were found, identified as F and Al5Co2 (Fig. 1d, 2d). The maximum solubility of Pd in binary Al5Co2 was determined to be 3.0 at.% (Table 2). Single-phase microstructures of alloys Al72.5Pd21Co6.5, Al72.8Pd15.6Co11.6, and Al73.3Pd12.8Co13.9 are formed with εn (Fig. 1e-g, 2e-g). The mixture of two binary phases ε6 + ε28 was identified in alloys Al73Pd20Co7 and Al73.5Pd15Co11.5 (Fig. 2e, f). The ternary ε16 phase was found in the Al73.8Pd12.8Co13.4 alloy (Fig. 2g). The double-phase microstructure of the Al76Pd11Co13 alloy contains ternary (ε16) and binary (Al9Co2) phases, Fig. 1(h) and 2(h). The maximum solubility of Pd in the latter phase was determined as about 1.8 at.% (Table 2).

Discussion

In the investigated alloys, various combinations of phases β, U, F, ε6, ε16, ε28, δ, Al5Co2, and Al9Co2 were identified after long-term annealing at 850 °C (Fig. 1, 2; Table 2). The phases classified under the ε-family (ε6, ε16, and ε28) were found in six of eight investigated alloys. For the alloys with a lower bulk Al content and the dominance of Pd over Co, the doublet ε6 + ε28 is characteristic. On the other hand, the single ε16 phase was found in alloys showing a higher Al content and the dominance of Co over Pd (Fig. 2; Table 2). The U-phase was present in alloys Al68Pd14.6Co17.4 and Al71.2Pd15Co13.8 with the bulk ratios Al:Pd:Co equal to about 70:15:15. The F-phase was identified in the Al71Pd9Co20 alloy with the highest bulk Co content, Table 2. Thus, cobalt was considered to stabilize this phase. Similarly, the presence of δ in the Al70Pd25Co5 alloy containing the highest amount of Pd (Table 2) shows δ has to be stabilised by Pd.

Experimentally determined partial isothermal section of the Al-Pd-Co phase diagram at 850 °C is illustrated in Fig. 3. In the isothermal section, homogeneity ranges of six phases (εn, U, F, β, δ, and Al5Co2) are sketched. According to positions of the homogeneity ranges, F, U, and εn can be characterized as ternary phases at 850 °C. As follows from the earlier results of Yurechko et al.,[1] F and U are ternary phases in a wider temperature range, at least between 790 and 940 °C. The expectation following from the comparison of isothermal sections at 790 and 940 °C about the shrinkage of F and U homogeneity ranges with decreasing temperature[1] was also confirmed in the present work; more evidently for the F-phase. On the other hand, εn classified as a binary family of phases at temperatures below 790 °C[1,15] should expand into triple-phase space at temperatures above 790 °C that was also confirmed in the present work for phases ε6, ε16, and ε28. Moreover, ε6 and ε28 containing up to 14.6 at.% Co (Table 2) considered earlier as binary Al-Pd phases (Table 1) were now identified at 850 °C in Al-Pd-Co alloys. Based on this finding, a strict distinguishing between binary (ε6 and ε28) and ternary (e.g. ε16) phases inside the ε-family seems to be questionable. As follows from the comparison of the isothermal section at 850 °C, Fig. 3, and isothermal sections at 790 and 940 °C,[1] the homogeneity range of εn shrinks with increasing temperature. The results obtained in this work confirmed that β, δ, and Al5Co2 are binary phases alloyed with the third element (compare Tables 1, 2).

Fig. 3
figure 3

Experimentally determined partial isothermal section of Al-Pd-Co phase diagram at 850 °C. Squares represent positions of investigated alloys, half-solid circles positions of phases in double-phase alloys, and third-solid circles positions of phases in triple-phase alloy. Bold solid lines characterize positions of phase homogeneity ranges. Fine solid and dashed lines are precise and estimative boundaries between multi-phase areas, respectively. Triple- and double-solid circles corresponding to the Al5Co2 area are overlapped, thus the former circle is not observable

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Conclusions

Eight alloys with metal compositions (at.%) ranging between (68-76)Al, (9-25)Pd and (5-20)Co were investigated after annealing at 850 °C for 500 h. The results obtained can be summarized as follows:

  1. (1)

    Various combinations of phases β, U, F, ε6, ε16, ε28, δ, Al5Co2, and Al9Co2 were identified in the investigated alloys.

  2. (2)

    Phases ε6 and ε28 containing up to 14.6 at.% Co were identified at 850 °C in alloys Al70Pd25Co5, Al71.2Pd15Co13.8, Al72.5Pd21Co6.5, and Al72.8Pd15.6Co11.6. Earlier they were considered as binary phases stable below 790 °C in the Al-Pd system.

  3. (3)

    The single ε16 phase was found in alloys Al73.3Pd12.8Co13.9 and Al76Pd11Co13, showing higher Al contents and the dominance of Co over Pd.

  4. (4)

    The U-phase was found in alloys Al68Pd14.6Co17.4 and Al71.2Pd15Co13.8 with the approximate bulk ratios Al:Pd:Co=70:15:15.

  5. (5)

    The F-phase was identified in the Al71Pd9Co20 alloy, having the highest bulk Co content of the investigated alloys.

  6. (6)

    Partial isothermal section at 850 °C of the Al-Pd-Co phase diagram was proposed, containing homogeneity ranges of six phases (εn, U, F, β, δ, and Al5Co2).