A model of the formation of the crystal structure of the Z phase in the CuAu alloy

A model of the crystal structure of the metastable Z phase that arises upon a low-temperature regime of ordering in the equiatomic CuAu alloy in the f orm of plates with a (110) habit is suggested. The shear mechanism of the f ormation of the Z phase is caused by the relaxation of strong internal stre sses that arise in a specific single-domain structure of the two-phase matr ix Al + L1(0) that is formed upon the nucleation of the tetragonal L1(0) ph ase at the initial stage of ordering. This mechanism, represented as a nonu niform shear on the twinning plane of the L1(0) phase, is effected via a re gular motion of dislocations with Burgers vector 1/2(($) over bar 01) on (1 01) planes and leads to the formation of regular twin defects of order that determine the structure of the Z phase. The reciprocal lattice and the spe ctrum of X-ray diffraction lines were calculated in the angular range 2 the ta = 15 degrees -85 degrees for a unit cell of the Z phase with a "superper iod" 4d(202) An experimental confirmation of the model was obtained upon th e analysis of the intensities of X-ray diffraction lines in the 220 doubler and upon the analysis of the position of superlattice reflections obtained from CuAu alloy samples quenched from the field of the uniform solid solut ion and annealed for a short time at a temperature of 260 degreesC.