ON THE CRYSTAL-STRUCTURE AND SOLUBILITY RANGE OF THE TERNARY PHI-PHASE IN THE MG-AL-ZN SYSTEM

The existence of the ternary intermetallic phi phase in the Mg-AI-Zn s ystem was first established by Clark [1]. However its crystal structur e remained undetermined until now. In this paper, a method using trans mission electron microscopy, initially developed by Steeds and Vincent [9], has been applied to determine the unit cell, possible space grou ps and lattice parameters. The method is based on the comparison betwe en the Laue zones of zero order and first order.The unit cell of the p hi phase is orthorhombic, space group Pbc2(1) (N degrees 29) or Pbcm ( N degrees 57) with large lattice parameters (a = 897.9, b = 1698.8, c = 1934 pm). Using this information, it was possible to find a way for indexing of the X-ray diffraction pattern. The solubility range of the phi phase is narrow along a composition line close to 53 to 55 at.% M g. Al and Zn contents vary approximately from 18 to 29 and from 28 to 17 at.%, respectively.