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

Decagonal quasicrystals of Al70Ni20Rh10 and Al70NixIr30-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 0.4 nm along the decagonal axis and belong to the centrosymmetric space group of P10(5)/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 el al. and Tsuda er al. Al70Ni20Rh20 produces diffraction spots with the longest lattice spacing of 4.2 nm 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.