On the kinking nonlinear elastic deformation of cobalt

Recently cobalt was classified as a kinking nonlinear elastic, KNE, solid. Fully reversible incipient kink bands, IKBs, were invoked to explain both its microyielding and hysteretic stress-strain curves. Herein we present further evidence and insights in the KNE nature of cobalt by measuring its mechanical hysteresis as a function of grain size, pre-strain and testing temperature. Unlike previous work, in coarse-grained cobalt, something other than grain boundaries determine the domain size. The hysteresis loops were only obtained at temperatures where cobalt was hexagonal-close packed. In situ neutron diffraction strains could only account for ≈1/3 of the total strain measured and ruled out dislocation pileups as the source of the remaining strain suggesting that it is due to IKBs. The totality of our results can be successfully explained and quantified by our microscale IKB-based model, based on which we estimate the critical resolved shear stress of basal plane dislocations to be 13 ± 3 MPa and the reversible dislocation density to be 1.5-6 × 1013 m−2 in the ≈200-400 MPa stress range.