TEM analysis of the deformation microstructure of polycrystalline cobalt plastically strained in tension

The microstructure evolution of hexagonal close-packed cobalt at different stages of work hardening has been studied. The deformation microstructure of polycrystalline samples upon a tension test was investigated by Transmission Electron Microscopy (TEM). Corresponding straining mechanisms were also identified by acoustic emission measurements. It was shown that dislocation glide is the predominant deformation mechanism during the first stage of plastic deformation, while twinning becomes the prominent strain accommodation mechanism in the later stage. A detailed TEM study using electron diffraction allowed identifying different types of twins activated during tensile testing. Four single twinning modes were identified, from which two types correspond to tensile strain and two types to compressive strain (with respect to the c direction), double twins were also observed. A relative importance of various systems is estimated and it is shown that the vast majority of twins correspond to the {10 (1) over bar2} twinning system.