DISCONTINUOUS PRECIPITATION IN COBALT-TUNGSTEN ALLOYS

Discontinuous precipitation in a Co32 wt%W alloy aged in the temperatu re range from 875 K to 1025 K has been investigated. Philips EM 430 ST EM has been used to characterise the microstructure and to measure the composition profiles across individual lamellae of epsilon(Co) and Co 3W phases in partially transformed specimens. Two kinds of cellular pr ecipitates have been found in the alloy. The initial transformation pr oduct, identified as ''primary'' lamellae with spacing of a few nanome tres is replaced during prolonged ageing by ''secondary'' lamellae wit h a much larger interlamellar spacing, typically a Few tens of nm. Lin e scans across cell boundaries of the ''primary'' lamellae revealed th at, just behind the advancing cell boundary, the solute content is far from the equilibrium state. This solute excess within the cells is qu ickly removed at the ageing temperature. Calculations show that the di ffusion process was too rapid to be identified as ordinary volume diff usion. Investigation of the kinetics showed that discontinuous precipi tation is controlled by diffusion processes at the advancing cell boun dary. This proposal has been confirmed by STEM analysis of tungsten pr ofiles in the depleted epsilon(Co) lamellae. (C) 1997 Acta Metallurgic a Inc.