STRENGTHENING OF COBALT TUNGSTEN ALLOYS UPON DISCONTINUOUS PRECIPITATION

Precipitation and hardening were investigated in cobalt-tungsten alloys containing 5-13 at.% W. A significant hardening up to 860 HV in alloys aged at 600°C is caused by discontinuous precipitation. Transmission electron microscopy shows that as-quenched alloys consist of f.c.c. α-Co grains with frequent twins and martensitic plates. During ageing L12-type order develops within α-Co and simultaneously discontinuous precipitation starts at a grain boundaries. The microstructure consists of hexagonal ε{lunate}-Co and Co3W lamellae as found during in situ high temperature studies in the electron microscope. High resolution electron microscopy reveals coherency strains within a few atomic layers at the ε{lunate}-Co/Co3W interface and nonhomogeneous D19 ordering within the Co3W lamellae, resembling in some places nonstoichiometric B19 ordering. © 1990.