THE PHASE-TRANSITION IN PALLADIUM MANGANESE ALLOYS WITH UP TO 33.3 AT-PERCENT MN

Phase transitions in Pd-Mn alloys with 10.0-33.3 at.% Mn were studied by metallographic observations, X-ray and electron diffraction, and electrical resistance measurements. The annealed alloys with 12.5-21.5 at.% Mn obtained by slow cooling from 1153 K were found to exhibit a diffuse scattering in their electron diffraction patterns due to short-range order of the alpha-L1(2-s) (Ag3Mg type) superstructure with a domain size M = 2, and the annealed alloys with 23.0-31.0 at.% Mn had a relatively high degree of alpha-L1(2-s)(M = 2) order. Furthermore, for the 29.0-31.0 at.% Mn alloys, in addition to the alpha-L1(2-s) structure, another two structures were observed: one a beta-1-L1(2-s) superstructure, which also has a one-dimensional long-period antiphase domain with M = 2 derived from CuAu(I) (L1(0) type), i.e. a beta-1 phase with an f.c.t. structure and a c/a ratio of less than unity, and the other an L1(2)-type structure of the beta-2 phase, which is also an f.c.t. structure but has a c/a ratio greater than unity. The beta-1-L1(2-s)(M = 2) and beta-2 phases were also observed for an annealed 33.3 at.% Mn alloy. The phase transitions are discussed in the light of the previously reported phase diagram.