journal menu
research papers
A three-dimensional X-ray diffuse scattering investigation of the short-range order in a-phase Cu-Al alloys, and its dependence on alloy composition, quenching temperature and isothermal annealing at 250°C, has been carried out. The Cowley-Warren order coefficients were determined after separation of the size effects using a procedure based on the Boric-Sparks quadratic approximation of atomic displacements. These short-range-order coefficients were analyzed in terms of the complete spectrum of nearest-neighbor atomic configurations, without the necessity of invoking a specific model for the characteristic feature(s) of the local-order structure. Although in the composition range investigated the equilibrium condition is a short-range-ordered structure, the nearest-neighbor configurations bear a marked resemblance to the long-period anti-phase-shift structures which have been found in alloys with more Al. Because essentially all of the most highly ordered configurations were found to exist isolated from each other in a 9.13 at.% Al alloy, it is concluded that they represent inherently stable, spatial arrangements of the atoms. In alloys containing 13.56 and 14.76 at.% Al, connected ordered configurations, or small ordered domains, were detected, and this effect increases markedly with increasing Al content. Isothermal annealing at 250°C of a Cu-14.76 at.% Al alloy quenched from 650°C results in a net disordering process; however, superimposed on this, in the early stages of annealing, is an enhancement of the population density of an atomic configuration confined to planes of the { 111 } type. This is interpreted as evidence for occurrence of the Suzuki mechanism. It is argued that the body of published experimental observations on this alloy system is better understood if one recognizes the existence of stacking faults, and their interaction with the matrix, in addition to that of short-range order.
