Structural relationship between the R and Z phases in the Al5CuLi3 alloy

The extensive research effort on Al-Cu-Li alloys was originally initiated to find materials with high mechanical properties/density ratio for aerospace use. Recently the system has been reexamined since it produces the icosahedral quasicrystals. The transformation behavior from quasicrystal to stable crystalline phases has drawn much attention due to the fact that the structures of the corresponding crystalline phases may provide some useful information for the understanding of the quasicrystalline structures. In the Al-Cu-Li system, several phases have been identified which are related to quasicrystals, such as R, C and r phase, among which the R phase has received the most attention and has served as starting point for solving other structures. Loiseau and Lapasset studied the C phase together with the quasicrystal and recognized that the C phase is a quasicrystal approximant. Audier et. al. described the R phase structure as consisting of 2 double triacontahedra (DTs) within a BCC unit cell. They have also carried out detailed studies on the structures of the Z, C and r phases and modeled these phases based on the structure unit of the R phase. In a recent study, the observation was made that during isothermal heat treatment the quasicrystalline phase in an Al5CuLi3 melt spun ribbon transforms mainly into the R phase. Within the R phase, several types of shear structures are present. The quasilattice vectors introduced for the study of quasicrystals are useful in interpreting these observed shear structures. The present paper focuses on the structural link between the hexagonal Z phase and the R phase. The two phases will be shown to have a well defined orientation relationship and habit plane, and that successive faults in one phase will produce the other phase if the faulted area is extended