The Crystal Structure and the Phase Transition of a Metastable Phase in the Au-37.8% Zn Alloy

The alloy of the composition of nearly Au:Zn=5:3 forms the β'-phase with the ordered β-brass type structure above 500°C. When this alloy is rapidly cooled from 600°C to room temperature, a metastable phase is formed, and if it is annealed at 200°C for more than an hour, it transforms into the stable phase, Au 5 Zn 3 . The crystal structure of the metastable phase has been determined by X-ray diffraction using single crystals. The unit cell is orthorhombic with the dimensions of a m =6.332, b m =8.953 and c m =4.477 kX, and a probable space group is Pmc 2 1 . The atomic arrangement is fundamentally of the body-centered cubic lattice type, but atoms are located in positions slightly shifted from the lattice points of the fundamental lattice. All of atoms have components of shifts along the c m -axis which are considered to be due to a relative gliding of every two (011) atomic planes of the original β'-phase lattice along the [01\bar1] direction. There are six equivalent orientation relationships between the unit cell of the metastable phase and that of the original β'-phase. They are expressed as follows: