Microstructural Characterization of the Face-Centered Cubic-to-Hexagonal Close-Packed Transition in a Cobalt-Base Alloy and its Effect on Mechanical Strength

It is shown that the fcc-to-hcp transition in a Co-base alloy occurs by nucleation and growth during thermal aging as well as by stress-induced martensitic mechanism. The transition is promoted by relatively high concentrations of strong hcp-stabilizing elements particularly W and Cr, and relatively low concentration of fcc-stabilizing elements particularly Ni. Such a composition is shown to produce stacking fault energy of about 7 mJ/m² in the fcc phase which is roughly equivalent to the free energy difference between the two phases and considered quite low in comparison with other Co-base alloys. Both the thermally induced and stress-induced transitions are shown to produce significant strengthening with substantial loss of ductility; however, the effect is more pronounced in the case of the stress-induced transition due to the formation of deformation twins in addition to the hcp phase. The results of the study can be of particular importance to designing Co-base alloys more stable toward the fcc-to-hcp transition.