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

Abstract

The crystalline phase coexisting with the icosahedral quasicrystal (point group m3\ifmmode\bar\else\textasciimacron\fi{} 5\ifmmode\bar\else\textasciimacron\fi{}) 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}$${\mathrm{Cr}}_{7}$ are parallel to the 5\ifmmode\bar\else\textasciimacron\fi{} and 3\ifmmode\bar\else\textasciimacron\fi{} axes, respectively, of the icosahedral phase. Fivefold and threefold rotational twins of ${\mathrm{Al}}_{45}$${\mathrm{Cr}}_{7}$ have been found around these axes; this is discussed from the group-subgroup point of view.