Discontinuous precipitation in cobalt-tungsten alloys

Abstract

Discontinuous precipitation in a Co32 wt%W alloy aged in the temperature range from 875 K to 1025 K has been investigated. Philips EM 430 STEM has been used to characterise the microstructure and to measure the composition profiles across individual lamellae of ϵ Co and Co 3W phases in partially transformed specimens. Two kinds of cellular precipitates have been found in the alloy. The initial transformation product, identified as “primary” lamellae with spacing of a few nanometres is replaced during prolonged ageing by “secondary” lamellae with a much larger interlamellar spacing, typically a few tens of nm. Line scans across cell boundaries of the “primary” lamellae revealed that, just behind the advancing cell boundary, the solute content is far from the equilibrium state. This solute excess within the cells is quickly removed at the ageing temperature. Calculations show that the diffusion process was too rapid to be identified as ordinary volume diffusion. Investigation of the kinetics showed that discontinuous precipitation is controlled by diffusion processes at the advancing cell boundary. This proposal has been confirmed by STEM analysis of tungsten profiles in the depleted ϵ Co lamellae.