CHARACTERIZATION OF SEVERE MATRIX DISTORTIONS DURING PHASE-SEPARATIONFROM THE REDISTRIBUTION OF DIFFRACTED INTENSITIES

Severe matrix deformation has been examined at an early stage of age-h ardening in a polycrystalline Brush 25 alloy containing Cu-11.50 at % Be-0.23 at % Co aged at 200 degrees C. Simplified anisotropic elastic models for the atomic displacement field about coherent disc-shaped pr ecipitates provide quantitative estimates of the atomic displacement f ield in the surrounding matrix. This requires a separation of diffract ed intensity into Bragg peaks, static diffuse scattering, and quasilin es. The latter originates from the severely distorted zone about the p recipitates. Elastic models include single discs, and [101] stair-step pairs. Ageing at 200 degrees C introduces larger changes in the diffr action profiles than at 315 degrees C. This is observed mainly as larg e variations in quasiline intensities, as well as in their relative pe ak positions. Comparisons are made at about one-half the maximum hardn ess. These variations result from the response of the coherent anisotr opic copper matrix to large tetragonal Bain strains in disc-shaped pre cipitates, and a strong preference for the largest deformations to be perpendicular to the free surface. Quasiline shifts are used along wit h Vegard's Law to extend the metastable Guinier-Preston (GP)-zone boun dary to 200 degrees C. This boundary extension is smooth and continuou s with published data, and thereby relates the metastable GP boundary to a highly distorted matrix about disc-shaped precipitates. Disc diam eters range from 4.8-6.4 nm with a thickness of 0.29 nm after 16-64 h at 200 degrees C. The [101] stair-step pair model best fits the experi mental results.