Quasicrystalline phase formation in Al 62 Cu 22.5 Fe 12.5 and Al 55 Cu 22.5 Fe 12.5 Be 7 alloys
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B.H. Kim, D.H. Kim, and W.T. Kim Center for Noncrystalline Materials, Department of Metallurgical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-ku, Seoul, 120-749, Korea (Received 7 April 2000; accepted 30 January 2001)
Formation of the icosahedral quasicrystalline phase in conventionally cast Al62Cu25.5Fe12.5 and Al55Cu25.5Fe12.5Be7 alloys were investigated. The icosahedral phase (I-phase) forming ability was greatly improved by partial replacement of Al by 7 at.% Be. The as-cast Al55Cu25.5Fe12.5Be7 alloy consisted of dendritic primary I-phase and interdendritic -phase, whereas that of an as-cast Al62Cu25.5Fe12.5 alloy consisted of various phases such as the -, I-, and -phases, together with a small amount of the - and -phases. The kinetic barrier for transformation into single I-phase by heat treatment was greatly reduced in an Al55Cu25.5Fe12.5Be7 alloy. The I-phase in an Al55Cu25.5Fe12.5Be7 alloy has the same face centered icosahedral structure as that in an Al62Cu25.5Fe12.5 alloy and is of high quality without phason strain. I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Thermodynamically stable icosahedral phase (I-phase) in the Al–Cu–Fe system is of interest because it has been shown to have a perfect quasicrystalline structure and is considered for practical application.1 However, since the Al–Cu–Fe I-phase forms through a peritectic reaction,2,3 there exists a kinetic obstacle to obtain a singlequasicrystalline-phase alloy by conventional solidification. To complete the peritectic reaction in solid state, heat treatment at high temperature is required. Addition of fourth alloying elements can be effective in forming single-quasicrystalline-phase alloys by modifying the solidification path.4 It is known that partial substitution of Fe by Cr or Co leads to the formation of the D-phase rather than the I-phase.5,6 Meanwhile, the I-phase forming ability was deteriorated by partial substitution of Al with B,7 and an 1/1 approximant phase replaces the Iphase by partial substitution of Al with Si.8 In this study, the solidification behavior and the effect of heat treatment in the I-phase-forming Al62Cu25.5Fe12.5 alloy are compared to those of an Al55Cu25.5Fe12.5Be7 alloy by means of optical and transmission electron microscopy (OM and TEM), x-ray diffraction (XRD), and differential thermal analysis (DTA). Through the combined analysis, the structure and thermal stability of the I-phase are discussed.
The Al62Cu25.5Fe12.5 and Al55Cu25.5Fe12.5Be7 alloys were prepared by high-frequency induction melting of high-purity Al, Cu, Fe, and Cu–4 wt% Be master alloy under a dynamic Ar gas atmosphere. Commercially available Cu–4 wt% Be alloy was used to ensure the proper amount of Be in the designated alloy, since pure Be is not only dangerous for handling but also is not recommended for direct addition to the melt. Approximately 30 g of the alloy melt was homogenized at around 1200 °C for 5 min before casting. Conventional casting was carried out with a Cu mold with a 12-mm-diameter cavity under a protecti
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