Convergent-beam electron diffraction and electron microscopy study of decagonal quasicrystals of Al-Ni-Rh and Al-Ni-Ir

Decagonal quasicrystals of Al₇₀Ni₂₀Rh₁₀and Al₇₀Ni_xIr₃₀-x (x= 15, 17 and 20) alloys prepared by the melt-quenching method have been examined by convergent-beam electron diffraction (CBED) and high-resolution electron microscopy (HREM). All these alloys were found to have a regular periodicity of about 0.4nm along the decagonal axis and belong to the centrosymmetric space group of P10₅/mmc, which is the same as the stable decagonal quasicrystals of Al-Ni-Co and Al-Cu-Co alloys. However, the HREM study has revealed that the present alloys have specific decagonal atom-cluster columns of about 2 nm diameter, while the stable Al± Ni± Co and Al± Cu± Co alloys have pentagonal atom-cluster columns according to the current work of Saitoh et al. and Tsuda et al. Al₇₀Ni₂₀Rh₁₀produces diffraction spots with the longest lattice spacing of 4.2nm among the quasicrystals examined by us, which indicates very good long-range order in the alloy. The present quasicrystals do not show diffuse streaks perpendicular to the decagonal axis in selected-area diffraction patterns, while the stable Al-Ni-Co and Al-Cu-Co alloys do show them.