Quasicrystals, crystalline phases, and multiple twins in rapidly solidified Al-Cr alloys

The crystalline phase coexisting with the icosahedral quasicrystal (point group m3¯ 5¯) in a rapidly solidified ${\mathrm{Al}}_7$Cr alloy was found to be the monoclinic ${\mathrm{Al}}_{45}$ ${\mathrm{Cr}}_7$ (2/m). A definite orientation relationship exists between these two phases which can well be accounted for by the orientation of some of the pseudoicosahedra in ${\mathrm{Al}}_{45}$ ${\mathrm{Cr}}_7$. In addition to the parallel relation of their twofold axes, the [101] and [145] axes of ${\mathrm{Al}}_{45}$ ${\mathrm{Cr}}_7$ are parallel to the 5¯ and 3¯ axes, respectively, of the icosahedral phase. Fivefold and threefold rotational twins of ${\mathrm{Al}}_{45}$ ${\mathrm{Cr}}_7$ have been found around these axes; this is discussed from the group-subgroup point of view.