Faceted etch pits formed on surfaces of an icosahedral Al₇₀Pd₂₀Mn₁₀ quasicrystal

An icosahedral Al₇₀Pd₂₀Mn₁₀ quasicrystal was etched under several etching conditions with solutions composed of HF and H₂O plus HCl and/or HNO₃. The formation of etch pits was examined closely by scanning electron microscopy and replica electron microscopy. When etched for 5 s at 293 K with the etchants of different compositions, the faceted pits were often observed except for a few exceptions. All those pits revealed the similar shape of pentagonal dodecahedron where the pentagonal faces were curved slightly near their corners and edges. By etching for 5 s with a solution of HF+HCl+H₂O (1:3:5 in volume ratio), the average size of the pits decreased from 4.5 to 2.5 μm with an increase of etching temperature from 273 to 333 K, and the pentagonal faces tended to be flattened above 313 K. The pentagonal faces near the corners and edges of the pits became more curved and the average size remained unchanged (∼4.5 μm) with increasing etching time in the range of 1 and 20 s at a temperature of 293 K. By comparing the surfaces before and after etching at the same areas, it was concluded that the faceted pits originated from irregular small holes which preexisted on the polished surface.