Impact of uniformly mixing the composition within a cutoff velocity v_mix = 3750, 5000, 7500, and 15 000 km s⁻¹ on the Ni abundance profile (top) and Ni II/Ni III population ratio (bottom), illustrated using the M_Ch delayed-detonation model DDC15 of Blondin et al. (2015) at 190 d past explosion. The stable Ni mass for this model is ∼0.03 M_⊙ (see Table A.1). We show the angle-averaged Ni abundance profile of the 3D delayed-detonation model N100 of Seitenzahl et al. (2013) for comparison (grey dashed line, top panel). The inset in the lower panel shows the ⁵⁶Co abundance profiles, whose decay heating by positrons and γ-rays largely determines the ionization state at this time.