Enhancement of the icosahedral quasicrystalline phase forming ability with Be addition to an Al 62.5 Cu 25.5 Fe 12 alloy
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Enhancement of the icosahedral quasicrystalline phase forming ability with Be addition to an Al62.5Cu25.5Fe12 alloy S.M. Lee1, B.H.Kim2, W.T. Kim2, and D.H.Kim2 1 Korea Institute of Industrial Technology, Inchon, Korea 2 Yonsei University, Center for Noncrystalline Materials, Dept. of Metallurgical Engineering, Seoul, Korea
ABSTRACT The quasicrystalline phase forming ability, solidification structure, thermal stability, and phase relationships of a series of conventionally-cast Al62-xCu25.5Fe12.5Bex (x=0,1,3,5,7) alloys were investigated. Single icosahedral quasicrystalline phase was successfully obtained by partial replacement of Al with Be (7 at.%) in an Al62Cu25.5Fe12.5 alloy during crucible cooling. The Iphase in an Al55Cu25.5Fe12.5Be7 alloy has the same face-centered icosahedral structure as that in an Al62Cu25.5Fe12.5 alloy, with less phason strain. Hardness of the I-phase increases with x, i.e. from Hv 710 for x=1 to Hv 809 for x=7.
1. INTRODUCTION The discovery of stable quasicrystalline phases in Al-Cu-TM (TM=Fe, Ru, Os, Co or Ni), Al-Pd-TM (TM=Mn or Re) systems has provided an opportunity to make bulk quasicrystalline alloys, which encourages the studies of quasiperiodic structure and physical properties [1,2]. The icosahedral phase (I-phase) in the Al-Cu-Fe system is of particular interest because it has been shown to have a very perfect quasicrystalline structure [3]. Since most of the stable quasicrystals form through a diffusion-controlled peritectic reaction, there exists a kinetic obstacle for obtainment of an equilibrium quasicrystalline phase alloys by conventional casting. Therefore, it has been considered that heat treatment at relatively high temperature is required in order to complete the peritectic reaction in the solid state [4]. Addition of fourth alloying elements sometimes can be effective in forming single quasicrystalline phase alloys by modifying the solidification path [5]. In the present study, the effect of partial replacement of Al with Be (maximum of 7 at.%) in an Al62Cu25.5Fe12.5 alloy has been investigated by means of optical, scanning, and transmission electron microscopy (OM, SEM, and TEM), X-ray diffraction (XRD), and differential thermal analysis (DTA).
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2. EXPERIMENTAL PROCEDURES Alloys were prepared by high frequency induction melting appropriate amounts of high purity Al, Cu, Fe, and Cu-4wt.%Be master alloy under a dynamic Ar gas atmosphere. Commercially available Cu-4wt.%Be alloy was used to ensure the proper amount of Be in the designed alloy, since pure Be is not only dangerous for handling but also cannot be easily used for melting. Alloy composition was confirmed by inductively coupled plasma – emission spectroscopy (ICP-ES). Some of the alloy ingots were heat treated at 750 oC up to 1 hour in a vacuum-sealed quartz tube up to 10-2 torr. Some mechanical properties were also examined as a function of Be content.
3. RESULTS AND DISCUSSION The change of solidification path can be clearly explained by the observation of microstructures as shown in Fig. 1. The
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