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

Abstract

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