δ⁶⁰Ni₅₈ vs δ⁶⁴Ni₅₈ from MS SiC grain data (dark grey circles with 1σ error bars from Trappitsch et al. 2018) compared to the surface evolution of Set 0 and Set 2 stellar models of solar (top panel) and of double-solar metallicity (bottom panel) of different masses (see legend). Circles (Set 0) and triangles (Set 2) on the lines represent the composition after each TDU after the envelope becomes C-rich. The dashed black lines represent the solar composition (δ = 0 by definition).