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 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 2nm 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. Al70Ni20Rh10 produces diffraction spots with the longest lattice spacing of 4 2nm among the quasicrystals examined by us, which indicates very goodlong-range order in the alloy. The present quasicrystals donot 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.