Short range order in Ni-Mo, Au-Cr, Au-V and Au-Mn alloys

Abstract

The nature of short range order (sro) has been investigated in the series of alloys Ni 4Mo, Ni 3Mo, Au 3Cr, Au 4Cr, Au 3Mn and Au 4V, which exhibit different long range ordered (lro) structures, by transmission electron microscopy and diffraction. The examination of the sro state, at temperatures above the critical temperatures ( T c ) for the order-disorder transformations and also in the samples that were quenched from above T c , showed diffuse scattering peaks centered near {1 1 2 0} positions in the f.c.c. reciprocal lattice, whose shape changed from one system to another. Evidence for diffuse scattering near superlattice positions corresponding to the D1 a structure was obtained in the sro state of Au-Cr, Au-V and Ni-Mo alloys. Additional diffuse scattering was observed near Ni 2Mo positons in the Ni-Mo alloys. The diffraction patterns of Au 3Mn above T c showed evidence for diffuse scattering near the superlattice positions corresponding to the Watanabe type two-dimensional long period superstructure. Inclusion of pairwise interaction parameters up to third nearest neighbors in the high temperature statistical mechanical model of Clapp and Moss partially accounts for the shapes of {1 1 2 0} sro spots found in Ni-Mo and Au-Mn alloys, but not in Au-Cr and Au-V alloys. The shapes of the diffuse sro scattering in the latter alloys, however, can be interpreted in terms of a distribution of various types of imperfectly ordered microdomains existing above the critical temperature. For a particular stoichiometry, the most frequently occurring microdomain may or may not correspond to the equilibrium long range ordered (Iro) structure of the alloy.