Cu3Pd Observed by High-Voltage Electron Microscopy

Cu-Pd samples of compositions varying from 16 to 26 at.% Pd were irradiated in situ in a 1.5 MeV electron microscope at various temperatures. Low temperature (90 K) irradiation produced completely disordered solid solutions. Irradiation at room temperature up to as high as about 500 K produced steady state short range order (SRO) which, for specimens of 18% or more Pd, is characterized by diffuse intensity at [1,±q,0] and equivalent positions in reciprocal space (modulated SRO). In general, q is a function of composition, temperature and irradiation dost. High temperature irradiation tended to produce the expected equilibrium long range order — either Ll2 or a long period superstructure depending on composition and temperature. The 18 and 20% samples irradiated at room temperature exhibited steady state modulated SRO even though the expected equilibrium structure is one of unmodulated order (Ll2). It is suggested that spinodal ordering is responsible for this latter effect. An f.c.c.-based Cu-Pd phase diagram is proposed incorporating ordering stability loci and a metastable Lifshitz point.