LONG-RANGE ORDERED STRUCTURE IN BETA-PHASE OF AG-ZN-AL ALLOY

The ordered structure and its change by heat-treatments in the beta ph ase (bcc structure) of an Ag-22.3 at%Zn-8.9 at%Al alloy were investiga ted by means of electron microscopy and electrical resistivity measure ments. This alloy can be brought to the disordered state by quenching after heating to a little above the order-disorder transition temperat ure (about 340 K). The ordering in disordered specimens by aging at a temperature near room temperature proceeds in two stages. The first st age is the transition to C11(b) structure, whose unit cell is thrice a s large as the unit cell of the bcc structure. This structure correspo nds to a binary alloy of type A(2)B. The atomic composition of the pre sent alloy suggests that Ag atoms should be arranged at A atomic sites and Zn or Al atoms at B sites. By prolonged aging, ordering moves to the second stage. The ordered structure in this stage, which we call C 11(b) structure, has the unit cell 3 x 3 times as large as the bce un it cell, and it is composed of three kinds of atomic sites ii, B, and C in the ratio of 6 : 2 : 1. It may be reasonable to assign Ag, Zn and Al atoms to A, B and C sites, respectively. Measurements of lattice p arameters have shown that the transition to the C11(b) structure acco mpanies a transformation strain. Regularly arranged twin plates, which have been observed in the C11(b) phase, seem to accommodate the tran sformation strain. Disordered specimens transform martensitically by l owering the temperature to the vicinity of the liquid nitrogen tempera ture, but ordered ones do not transform.