GUINIER-PRESTON ZONES IN AU-FE ALLOYS

Transmission electron microscopy was employed to study aging effects in the AuFe system. Specimens containing 10.7 18, 25 and 35 at.% Fe in both as-quenched and aged conditions (from room temperature up to 500 °C for as long as 2 months) were investigated. Rod-shaped and/or lobe-like strain contrast images whose visibility was sensitive to specimen preparation and diffraction conditions were seen after aged specimens were further cleaned using an ion miller, whereas a dynamical flickering "streaming effect" was observed in as-electropolished specimens. Trace analysis as well as other evidence indicates that the images correspond to small precipitates rather than defects generated by ion milling. Dark-field and stereomicroscopy analyses show that the nature of the strain field is inwards. A model of Guinier-Preston (G-P) zone precipitation is proposed to interpret the various types of diffuse streaks present in the diffraction patterns. Theoretical calculations indicate that the habit of the iron-rich G-P zones might lie on planes with normal n0 = <102〉 or thereabouts. The present results indicate that a G-P zone model appears to be plausible, but are also consistent with the static concentration wave concept. This will be discussed in a subsequent paper. © 1990.