[en] The synthesis of materials and the studies of crystal structure and ⁵⁷Fe Moessbauer effect were performed for Tb_0.27Dy_0.73(Fe_1-xCo_x)₂ intermetallics. Terfenol-D (Tb_0.27Dy_0.73Fe₂) is the starting compound of this Fe/Co-substituted series. X-ray measurements showed evidence of a pure cubic Laves phase C15, MgCu₂-type, and unit cell parameters were determined across the series. A Co substitution introduced local area, at sub-nanoscale, with random Fe/Co neighbourhoods of the ⁵⁷Fe atoms. Moessbauer effect spectra for the Tb_0.27Dy_0.73(Fe_1-xCo_x)₂ series at room temperature are composed of a number of locally originated subspectra due to the random distribution of Fe and Co atoms in the transition metal sublattice, and due to (111) an easy axis of magnetization. Isomer shift, magnetic hyperfine field and quadrupole interaction parameter were obtained from the spectra, both for local area and for the bulk sample. As a result of Fe/Co substitution, a Slater-Pauling-type curve for the average magnetic hyperfine field vs. Co content was observed. It was found that the magnetic hyperfine field corresponding to the local area also create a dependence of the Slater-Pauling-type vs. Co contribution in the Fe/Co neighbourhoods. (author)