CRITICAL-TEMPERATURE FOR ORDERING AND B2 DOMAIN-STRUCTURE IN AG-ZN BETA-PHASE ALLOYS

The critical temperature for the order-disorder transition, T-c, and t he ordered domain structure of the B2 phase have been investigated by electrical resistivity measurements and electron microscopic observati ons in Ag-Zn alloys containing 38, 40 and 42 at%Zn. Specimens quenched from the disordered beta phase were all found to be ordered in the B2 structure. Resistivity change measured during aging of the B2 phase s hows a plateau at the initial stage of aging. This indicates the degre e of order reaches the equilibrium value immediately after the start o f aging. The B2 domain size as measured after aging and quenching incr eases as the aging temperature increases. However, it takes a smaller value when the aging temperature is beyond T-c. Values of T-c were det ermined by monitoring the domain size as 441, 405 and 367 K for the 42 , 40 and 38Zn alloys, respectively. The ragging or fluctuating of anti -phase domain boundaries was recognized in the specimens aged at a tem perature just below T-c. The activation energy for the domain growth w as obtained by measuring growth rates as 49.3 kJ/mol, which was judged to correspond to the migration energy of excess vacancies.