G. S. Song

Icosahedral quasicrystalline and hexagonal approximant phases in the Al±Mn±Be alloy system

Abstract We report the formation of a primitive icosahedral quasicrystal with increased stability in Al-Mn-Be alloys close to the compound Al15Mn3Be2 by melt spinning and injection casting. The crystal structure of this compound was unknown. We show that in as-cast as well as heat treated condition the intermetallic phase HI has a hexagonal structure with lattice parameters a = 1.2295 nm and c = 2.4634nm. The space group is P63/mmc. In the injection-cast samples, the quasicrystal coexists with another closely related hexagonal phase H2 with a = 1.2295nm and c = 1.2317nm with a possible space group of P63/mmc. This phase exhibits specific orientation relationships with the icosahedral quasicrystal given by [0001]hex//2f QC and [0110]hex//5f QC, where 2f QC and 5f QC represent twofold and fivefold axes respectively. Electron diffraction patterns from both phases exhibit a close resemblance to the quasicrystalline phase. It is shown that the H1 phase is closely related to μ-Al4Mn with the same c parameter while the a parameter is reduced by τ. Following Kreiner and Franzen, it is postulated that both structures (H1 and H2) can be understood by a simple hexagonal packing of 113 clusters.

Formation and stability of quasicrystalline and hexagonal approximant phases in an Al-Mn-Be alloy

Phase formation and thermal stability for an Al-Mn-Be alloy have been investigated by melt-spinning and conventional casting. Significant differences in the phase formation and the thermal stability of the microstructure were found as a result of the different cooling rates. In the melt-spun ribbons, a large volume fraction of a metastable icosahedral phase was found to coexist with an Al solid solution. In the bulk cast ingots, the primary phase formed in the two-phase microstructure was a hexagonal approximant phase of quasicrystals. This phase that solidified in the form of faceted particles embedded in the Al solid matrix proved to be thermodynamically stable during annealing at 540 °C for 100 h. The effect of Be addition on the formation of the stable approximant phase is discussed in terms of the Hume-Rothery mechanism.

Enhancement of the quasicrystal-forming ability in Al-based alloys by Be-addition

Abstract The influence of beryllium (Be) addition on the quasicrystal-forming ability (QFA) in Al–Cu–Fe and Al–Mn systems has been investigated using conventional solidification technique. For a series of as-cast (Al 62− x Be x )Cu 25.5 Fe 12.5 ( x =0,1,3,5,7) alloys, the Be addition modified the icosahedral phase formation mechanism from peritectic reaction to primary solidification and resulted in the increase in the volume fraction of the i-phase from 40% for x =0 to 90% for x =7. Such an enhancement of the QFA with the increase of Be content can be represented by using the reduced quasicrystal transition temperature proposed in the present paper. In Al–Mn alloy system, microstructural changes due to the addition of Be were examined in detail by comparing as-cast Al 90 Mn 2.5 Be 7.5 alloy with Al 97.5 Mn 2.5 bulk ingot. For this alloy system, the substitution of Al by Be was found to reduce substantially the critical Mn-content and cooling rate necessary for the formation of the i-phase.

Quasicrystal-forming ability of the icosahedral phase in Al–Cu–Fe–Be alloys

Abstract The effect of Be (beryllium) addition and the influence of cooling rate on the solidification path of the icosahedral (i) phase in conventional casting Al 62− x Be x Cu 25.5 Fe 12.5 ( x =0,1,3,5,7 at.%) alloys were investigated by means of X-ray diffraction, differential thermal analysis, SEM and EDX analyses. The addition of Be has been found to modify the i-phase formation mechanism from peritectic reaction to primary solidification. The volume fraction of the i-phase has been observed to increase from 45% for x =0 to 90% for x =7. These results suggested that the substitution of Al by Be is favorable for the increase of the quasicrystal-forming ability (QFA) of the i-phase. Two parameters have been proposed for defining and assessing the QFA for i-phase formed by peritectic reaction. These parameters can be expressed by the reduced quasicrystal transition temperature T rq = T p / T l and the reduced undercooling Δ T r =Δ T lp / T l , respectively, ( T l : liquidus temperature, T p : peritectic peak temperature, and Δ T lp = T l − T p : freezing range of primary phase).

On the role of carbon in the formation and properties of hot pressed icosahedral Al-Cu-Fe phase

Al-Cu-Fe icosahedral (i) quasicrystal phase exists only within a restricted temperature range between 750 ◦C and 850 ◦C or at a very sharply defined composition around Al62Cu25.5Fe12.5 [1–3]. As a stable i-phase, interests in this system have arised mainly to study its quasiperiodic structure, its bulk and surface properties, and the possibility of growing large single quasicrystals [4–7]. Recently, starting from the Al-Cu-Fe system, work has been performed to study the effects of nonmetallic elemental additions such as oxygen and boron on the structure and stability of the icosahedral phase [8–10]. To the authors’ knowledge, however, no information exists on the effects of metalloid carbon addition on the alloys. In this paper, we present in situ carburization of the i-AlCuFe quasicrystalline phase prepared by the hot-pressing of gas atomized powders at a temperature for which it is thermodynamically stable. The influence of carbon on the formation and mechanical properties of the i-AlCuFe quasicrystalline phase is also studied to contribute to the understanding of the carburization behavior of bulk icosahedral Al-Cu-Fe phase. The starting materials were powders of the composition Al62Cu26Fe12 prepared by gas atomization of the liquid alloy. The majority of the sieved powder particles were less than 38 microns in diameter. The corresponding as-atomized structure consists of primary icosahedral (i) dendrites with small amount of τ phase in the interdendritic region [11]. The bulk quasicrystalline sample, a three-dimensional body 40 × 40 × 8 mm3, was fabricated by hot-pressing the powder particles. The hot-pressing process was performed in two steps. The former was a degassing process at a low temperature of 600 ◦C for 30 min under high-vacuum condition (5 × 10−5 Torr); the latter was a sintering and densification process of the icosahedral phase formed at a high temperature of 800 ◦C for 120 min under a pressure of 35 MPa. In the whole hot-pressing process, a graphite paper of approximately 0.5 mm was used as a separator to protect both graphite mold and graphite punch. In this case, the graphite paper is expected to behave as a carburizer and consequently to induce the contamination of the quasicrystalline forming alloys during hot pressing. The characterization of the hot pressed samples was performed as shown in Fig. 1 on the sample surface and its cross-section, respectively. First, the carbon distribution and microstructure were detected perpendicular to the cross section of sample. Second, a mechanical polishing method was used to remove the surface of the carburized sample layer by layer so as to obtain the depth profiles of the phase constituents and mechanical properties. It should be noted that the measurement of the initial surface started from the freshly abraded surface following the removal of the carbon film. The phase constituents and microstructure were assessed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The chemical analysis was carried out by X-ray energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES) for qualitative purposes, respectively. The Vickers microhardness Hv was determined from at least 15 different indentations, and the fracture toughness KIC was evaluated from the crack length initiated at the corners of the Vickers microindentation by using an empirical equation proposed by Niihara et al. [12]. The SEM image and the elemental EDS X-ray scans of the cross-section of the carburized sample are presented in Fig. 2. A continuous carbon film developed during hot-pressing can be seen on the surface of sample in the upper left-hand side image. The film thickness is about 35 μm. In addition, a spare dispersion of Al element in the carbon film was detected, as evidenced by the X-ray mapping of Al (middle left-hand side). This illustrates that the formation of this film results in the selective consumption of aluminum. Beneath the film a multiphase microstructure in the carburized sample can be observed; the multiphase assemblages were later identified further using XRD technique. Owing

Solidification paths for an icosahedral quasicrystalline phase in cast Al62Cu25.5Fe12.5 and Al55Cu25.5Fe12.5Be7 alloys

Abstract The solidification behavior of the icosahedral (i) phase in (Al 62− x Be x )Cu 25.5 Fe 12.5 ( x =0,7) alloys has been investigated using a conventional casting technique. It was observed that the addition of beryllium (Be) modified the i-phase formation mechanism from peritectic reaction to primary solidification compared to as-cast Al 62 Cu 25.5 Fe 12.5 alloy. The establishment of the new solidification path is attributed to the narrowing of the freezing range resulting from the beryllium addition. However, the resolidification at a slow cooling rate could result in the formation of the i-phase by peritectic reaction. The conversion of the initial solidification path to the peritectic solidification was attributed to a cooling rate effect resulting from the elaboration process. A new decomposition process of the bulk single i-phase in the Be-containing alloy is also proposed in the present paper.

Synthesis of Bulk Quasicrystals by Spark Plasma Sintering

Spark plasma sintering method was applied to Al-Cu-Fe and Al-Si-Cu-Fe gas-atomized powders to prepare almost pore-free cylindrical specimens with icosahedral and 1/1 cubic approximant phases, respectively. This investigation has revealed that a high density could be obtained despite the short period and low temperature imposed during spark plasma sintering. In comparison to hot press technique, these conditions are favorable since they limit the formation of secondary phases and avoid exaggerated grain growth. The Vickers microhardness and fracture toughness of these two alloy systems were found to be larger than those obtained from cast and hot pressed samples, which could be attributed to a strong bonding between powder particles and the small-grained microstructure of the bulk SPS quasicrystalline specimens.