Synthesis and Investigation of Quaternary Quasi-Crystalline Phase in Al – Cu – Fe – Cr Alloys

Formation of a quasi-crystalline phase under rapid solidification and heat treatment of alloys of the Al – Cu – Fe – Cr system is studied. The study is performed by x-ray diffractometry, optical, scanning and transmission electron microscopy and differential scanning calorimetry. It is shown that the quasi-crystalline Al₆₅Cu₂₀Fe₁₀Cr₅ phase is a mixture of icosahedral and decagonal phases. The substitution of iron with chromium destabilizes the icosahedral I-phase and promotes formation of a decagonal d-phase. After quenching from 880°C, the Al₆₄Cu₂₄Fe₁₀Cr₂ alloy acquires a pure I-phase, and the Al₆₄Cu₂₄Fe₈Cr₄ alloy acquires a d-phase.

Introduction

After the discovery of icosahedral quasicrystal formed under rapid solidification of melts of the Al – Mn system [1], quasicrystals have been discovered in alloys of many other systems [2]. In addition to two-component quasicrystals near the Al₆Mn, Al₅Pd and Al₄Ni compositions, quasicrystals have been reported to occur in alloys of the Al – C – Fe, Mg – Zn – Y, and Al – Ni – Si ternary systems [3]. Many aluminum alloys with quasicrystalline phases have found practical application [4]. An icosahedral phase in alloys of the Al – Cu – Fe system with stable and perfect quasicrystalline structure attracts special interest [5].

Several publications have been devoted to the effect of alloying on the stability of ternary icosahedral I-phase Al₆Cu₂Fe. However, works on the phase equilibria in quaternary systems with quasicrystalline phases are quite scarce. Some authors assume that quaternary quasicrystals do not exist, and three components are enough for formation of a quasicrystalline phase [6]. According to the data of [7], alloys of the Al – Cu – Fe system acquire ternary compounds and quasicrystalline phases at high cooling rates. However, thin regions of a wedge ingot of the Al – Cu – Fe – Cr quaternary system exhibit a tendency to amorphization [7]. In some cases, substitution of iron with cobalt results in formation of a decagonal phase [8]. Addition of a specific content of a fourth element into the Al – Cu – Fe alloys yields a mixture of icosahedral and decagonal phases [9].

The aim of the present work was to continue the investigation of quasicrystalline phases in alloys of the Al – Cu – Fe and Al – Cu – Fe – Cr systems.

Methods of Study

We obtained quasicrystalline phases by partial substitution of iron with chromium in alloys subjected to rapid solidification and subsequent heat treatment. The alloys studied had nominal compositions of Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ prepared from elements with purity 99.99% in an induction furnace in argon atmosphere. The ready melt was poured into a metallic mold.

The microstructure of the alloys was studied with the help of a ZEISS optical microscope, a SU-1500 scanning electron microscope (SEM) equipped with an energy dispersive attachment for chemical analysis, and a JEM2100 transmission electron microscope at an accelerating voltage of 200 kV. The Vickers hardness was measured using a HVS-1000 digital microhardness meter. The x-ray diffraction analysis was performed using a Rigaku D/max-rB diffractometer in copper radiation. The differential scanning calorimetry (DSC) was conducted using a NETZSCH STA 449C device; the powder specimens were heated at a rate of 20 K/min.

Results and Discussion

By the data of the scanning electron microscopy (Fig. 1), both cast alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ contained four phases, i.e., an (Al-Cu)₁₃Fe₄ λ-phase (colored black), an Al₆₅Cu₂₀Fe₁₀Cr₅ phase (dark-gray), an Al₆₄Cu₂₄Fe₁₂I-phase (light gray), and an AlCu(Fe) η-phase (white). The compositions of the alloys determined by the energy dispersive analysis are presented in Table 1. The x-ray diffraction patterns in Fig. 2 also show the presence of some Al₂Cu. The light-gray regions correspond to an Al₆₄Cu₂₄Fe₁₂ icosahedral quasicrystalline I-phase. A similar phase has been described in [10,11,12,13]. Data on the structure of phase Al₆₅Cu₂₀Fe₁₀Cr₅ are quite scarce. We studied it by the method of TEM.

Fig. 1.

Microstructure of alloys Al₆₄Cu₂₄Fe₁₀Cr₂ (a) and Al₆₄Fe₈Cr₄ (b ) in cast condition (SEM): 1 ) λ-phase (Al-Cu) ₁₃Fe₄; 2 ) phase Al₆₅Cu₂₀Fe₁₀Cr₅; 3 ) I-phase Al₆₄Cu₂₄Fe₁₂; 4 ) η-phase AlCu(Fe).

Table 1. Chemical Composition of Phases in Alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ (Fig. 1)

Fig. 2.

X-ray diffraction patterns of cast alloys Al₆₄Cu₂₄Fe₁₀Cr₂ (a) and Al₆₄Cu₂₄Fe₈Cr₄ (b ) (n is the number of pulses).

Figure 3a presents a gray banded region of alloy Al₆₄Cu₂₄Fe₁₀Cr₄, which is represented by phase Al₆₅Cu₂₀Fe₁₀Cr₅ with composition Al_67.14Cu_18.13Fe_11.2Cr_3.53.

Fig. 3.

Microstructure with diffraction pattern (a) and selective electron diffraction patterns (b – f ) from the region of Al_67.14Cu_18.13Fe_11.2Cr_3.53 in alloy Al₆₄Cu₂₄Fe₁₀Cr₄ showing coexistence of icosahedral and decagonal phases: a) light-background image (TEM); b, c, d ) 5th-, 3rd-, and 2nd-order axes of the icosahedral quasicrystalline phase, respectively; e, f ) 10thand 2nd-order axes of the decagonal quasicrystalline phase, respectively.

It can be seen from Fig. 3b – f that the Al₆₅Cu₂₀Fe₁₀Cr₅ phase contains both quasicrystalline phases, i.e., an icosahedral one and a decagonal one. The electron diffraction patterns in Fig. 3b and d present the diffraction patterns with quintuple and double symmetry, respectively.

The diffraction pattern in Fig. 3c seems to correspond to a third-order symmetry axis of an icosahedral phase. However, we have detected numerous reflections from a rhombic approximant with a great period, which means that the third-order symmetry may be only an approximation. In addition, this region also bears a decagonal quasicrystalline phase, which follows from Fig. 3e and f. The diffraction pattern in Fig. 3f has been obtained upon a turn, which confirms two-dimensional structure of the decagonal quasicrystal. Therefore, as compared to the Al₆₄Cu₂₄Fe₁₂ alloy, which consists of only a stable icosahedral I-phase, the quasicrystalline phase in the Al – Cu – Fe – Cr alloy should be a mixture of icosahedral and decagonal phases. Exact compositions of these phases are hard to determine.

Figure 4 presents the results of the electron microscopic analysis of the light-gray region 3 (Fig. 1). This region is represented by an I-phase. The icosahedral symmetry of the I-phase is confirmed by the electron diffraction pattern in Fig. 3b with symmetry of the second order. Comparative analysis shows that the homogeneous light-gray region in Fig. 4a is an I-phase, and the dark-gray phase uniformly alternating with the I-phase (Fig. 3a ) should be a decagonal d-phase. This becomes more obvious from Fig. 4c, where the arrows point at the d-phase, the I-phase, and their mixture. In many cases, the d-phase and the I-phase obey an orientation relation determined in [14]. Thus, the d-phase may form from the I-phase destabilized by the introduction of chromium, which agrees with the data of [15].

Fig. 4.

Structure of I-phase (a), electron diffraction pattern of the icosahedral quasicrystalline phase showing the 2nd order axis (b ), and structure of the conjugation (c) of the dark-gray (d ) and light-gray (I ) phases and their mixture (d + I ) in cast alloy Al₆₄Cu₂₄Fe₁₀Cr₄.

Figure 5 presents the structures of alloys Al₆₄Cu₂₄Fe₁₂, Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ with the indents used to determine the average values of their microhardness, i.e., 790, 692 and 781 HV, respectively. It can be seen that the Al₆₅Cu₂₀Fe₁₀Cr₅ phase has a microhardness close to that of the icosahedral phase in alloy Al₆₄Cu₂₄Fe₁₂, i.e., the replacement of a part of iron with chromium affects little the mechanical properties of the quasicrystalline phase.

Fig. 5.

Structure with indents (at a load of 200 g) obtained due to measuring the microhardness of alloys Al₆₄Cu₂₄Fe₁₂ (a), Al₆₄Cu₂₄Fe₁₀Cr₂ (b ) and Al₆₄Cu₂₄Fe₈Cr₄ (c) (optical microscopy).

Figure 6 presents the DSC curves for alloys Al₆₄Cu₂₄Fe₁₂, Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ in heating and cooling at a constant rate of 20 K/min. The peak temperatures are indicated only on the heating curves. The transformations due to heating of alloy Al₆₄Cu₂₄Fe₁₂ have been studied in [16,17,18]. This alloy exhibits two successive endothermic effects corresponding to melting of the copper-rich low-melting phase η-AlCu(Fe) (694.85°C) and of the I-phase (887.81°C) [17]. The DSC curves of alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ also exhibit several endothermic peaks and one exothermic peak. Since the quenching from 800°C (6 h) should yield a pure quasicrystalline phase, we studied the effect of heat treatment on the Al – Cu – Fe – Cr alloys. In accordance with the DSC curves, these alloys have a wide endothermic peak at 827°C, which corresponds to melting. The cooling curves exhibit an exothermic peak at 800°C (in the dashed ovals) which should be connected with formation of a phase melting at 827°C. This phenomenon resembles that occurring in the Al₆₄Cu₂₄Fe₁₂ alloy. In this connection, we studied the phase transformations in the alloys by the method of x-ray diffractometry. Specimens of cast alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ were subjected to a hold at 750 and 880°C and then cooled in water and in air.

Fig. 6.

DSC curves of heating and cooling (indicated by the arrows) at a rate of 20 K/min for alloys Al₆₄Cu₂₄Fe₁₂, Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄.

Figure 7 presents the x-ray diffraction patterns of alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ after different treatments. The most considerable changes in these curves are disappearance of phase Al₇Cu₂Fe and growth in the content of the quasicrystalline I-phase. Therefore, the wide endothermic peak at 827°C may be connected with melting of phase ω-Al₇Cu₂Fe. In addition, in accordance with Fig. 7a and c, after holding at 880°C and air cooling, Al₇Cu₂Fe becomes one of the main phases, which differs radically from the condition after the hold at 880°C and water quenching. This may be explained by repeated precipitation of the molten Al₇Cu₂Fe phase under slow cooling in air. This agrees with the exothermic peaks near 800°C on the cooling curves of alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ (Fig. 6). On the whole, quenching from 880°C (30 min) in water provides formation of only quasicrystalline phases (icosahedral and decagonal ones). It should be noted that the intensity of the peaks from phase Al₆₅Cu₂Fe₁₀Cr₅ remains high, which indicates its high thermal stability. In addition, the x-ray diffraction patterns of the two alloys reflect the following little difference: phase ω-Al₇Cu₂Fe forms more easily in alloy Al₆₄Cu₂₄Fe₁₀Cr₂ at 750°C (Fig. 7).

Fig. 7.

X-ray diffraction patterns (n is the number of pulses) of alloys Al₆₄Cu₂₄Fe₁₀Cr₂ (a, b ) and Al₆₄Cu₂₄Fe₈Cr₄ (c, d ) after holding at 750 and 880°C and cooling in air (a, c) and in water (b, d ): ♦) peaks from Al₆₅Cu₂₀Fe₁₀Cr₅; ○) ω-Al₇Cu₂Fe; ■) Al₂Cu; ●) AlCu; ∆) Al₂Cu₃; ▲) Al₈₆Cr₄; ★) FeCr; ☆) AlCu₃.

Figure 8 presents the structure of alloy Al₆₄Cu₂₄Fe₁₀Cr₂ after holding at different temperature and cooling in air. It can be seen that phase λ -(Al-Cu)₁₃Fe₄ disappears with growth of the temperature giving place to a gray quasicrystalline phase or an ω -Al₇Cu₂Fe phase. The microstructure of the two alloys after water quenching looks similarly and is not presented in the paper. The composition of the phases (Table 2) in the regions shown in Fig. 9 for alloy Al₆₄Cu₂₄Fe₁₀Cr₂ was determined with the help of an energy dispersive attachment to the scanning electron microscope. It turned out (Table 2) that after the heating to 900°C and air cooling phase ω-Al₇Cu₂Fe in region 3 has composition Al_68.01Cu_21.96Fe_10.03. However, for the light-gray region 2 the composition is Al_64.09Cu_26.73Fe_9.18, which means that the I-phase and the ω-Al₇Cu₂Fe phase in this test look approximately similarly. Figure 9b presents the Al₆₅Cu₂₀Fe₁₀Cr₅ phase and the I-phase obtained after water cooling. According to the data of Table 2, the dark-gray region corresponds to phase Al₆₅Cu₂₀Fe₁₀Cr₅, and the light-gray region corresponds to the I-phase. The uneven region marked with the arrow in Fig. 9b may be explained by melting of phase λ-(Al-Cu)₁₃Fe₄.

Fig. 8.

Structure of alloy Al₆₄Cu₂₄Fe₁₀Cr₂ cooled in air after heating to different temperatures (optical microscopy): a) 570°C; b) 610°C; c) 675°C; d ) 710°C; e) 750°C; f ) 880°C; g ) 900°C.

Table 2. Chemical Compositions of Phases Entering Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄

Fig. 9.

Microstructure of alloy Al₆₄Cu₂₄Fe₁₀Cr₂ after heating to 880°C and cooling in air (a) and in water (b ) (SEM). The description of the phases at points 1 – 3 is given in Table 2.

Conclusions

1. The structure of alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄ in cast conditions contains a quaternary quasicrystalline phase Al₆₅Cu₂₀Fe₁₀Cr₅ with rectangular morphology. This phase is a mixture of icosahedral and decagonal quasicrystalline phases. Substitution of iron with chromium destabilizes the icosahedral I-phase and stabilizes the decagonal d-phase.

2. The studies by the methods of DSC and x-ray diffractometry have shown that heat treatment may produce pure quasicrystalline phases (icosahedral and decagonal ones). When the temperature of the exposure is increased to 880°C, the λ-(Al-Cu)₁₃Fe₄ phase disappears progressively yielding a quasicrystalline I-phase and a ω-Al₇Cu₂Fe phase. After the slow air cooling the molten ω -Al₇Cu₂Fe phase precipitates again, while the water quenching yields an I-phase.

3. We have detected some differences in the properties and transformations in alloys Al₆₄Cu₂₄Fe₁₀Cr₂ and Al₆₄Cu₂₄Fe₈Cr₄, i.e., the microhardness of phase Al₆₅Cu₂₀Fe₁₀Cr₅ in the Al₆₄Cu₂₄Fe₈Cr₄ alloy is higher than in the Al₆₄Cu₂₄Fe₁₀Cr₂ alloy; phase ω-Al₇Cu₂Fe forms more easily under heating of the Al₆₄Cu₂₄Fe₁₀Cr₂ alloy.