Pseudo-Mackay clusters in the Al-Pd-Co System

A great number of complicated intermetallic compounds were reported in binary and ternary alloy systems of Al with transition metals. Al–Co–Pd is one of the most interesting alloys, since a variety of crystalline phases associated with quasicrystals has been reported [1]. Among these crystalline phases, the structures of ε-phases are closely associated with Al3Pd, which is an important crystalline approximant for the decagonal phase with a period of 1.6 nm. W-phase is another approximant for the decagonal phase and its structural information is useful for understanding the columnar unit in the decagonal phase with a periodicity of 0.8 nm [2]. On the other hand, some crystalline phases associated with the icosahedral phase were also found in this Al-Pd-Co system. C2–phase, R-phase (R¯3: a = 2.91 nm, c = 1.32 nm) and F-phase (Pa3: a = 2.44 nm) are classified into this category. In particular, the structures of R-phase and F-phase consist of a variety of pseudo-Mackay clusters similar to those found in 1/1-AlCuRu and the trigonal χ-AlPdRe. As an example, the structure of R-phase shows two types of pMCs. These pMCs can be ranked by their atomic arrangements of the first shells, nevertheless every outer shell is a harmony of an Al-icosidodecahedron and a Co/Pd-icosahedron. These pMCs interpenetrate each other by sharing edges of Co/Pd-icosahedra and the interstitial space is subsequently filled by the smaller Al-icosahedra around Pd/Al sites [3]. The characteristic structural motifs for R-phase and F-phase readily suggest the importance of pMC as a fundamental structural unit for icosahedral quasicrystals.

Coarsening Process of Decomposed Phases in Cu-Ni-Cr Alloys

A study of the coarsening process of the decomposed phases was carried out in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys using transmission electron microscopy. As aging progressed, the morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100> Cu-rich matrix directions. Prolonged aging caused the loss of coherency between the decomposed phases and the morphology of the Ni-rich phase changed to ellipsoidal. The variation of mean radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The coarsening rate was faster in the symmetrical Cu-45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates. The activation energy for thermally activated growth was determined to be about 182 and 102 kJ mol-1 in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys, respectively. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the modified LSW theory for ternary alloys.

Preparation of the Al-Cu-Co and Al-Cu-Ni Ternary Intermetallic Powders via a Solid-Liquid Reaction Ball Milling Technique

Al-Cu-Co and Al-Cu-Ni ternary alloy nanopowders were prepared via a self-developed solid-liquid reaction milling technique. The milling balls were made of the metals with highest melting point for each alloy systems. And the binary alloy melts of other two elements were used as raw materials. Formation regularities of Phases in the milling process were investigated and solid-liquid reaction mechanism of ternary alloy systems was discussed.