  At present, there are strong indications that white dwarf (WD) stars with
masses well below the Chandrasekhar limit (MCh ~ 1.4 Msun) contribute a
significant fraction of SN Ia progenitors. The relative fraction of stable
iron-group elements synthesized in the explosion has been suggested as a
possible discriminant between MCh and sub-MCh events. In particular, it is
thought that the higher-density ejecta of MCh WDs, which favours the synthesis
of stable isotopes of nickel, results in prominent [Ni II] lines in late-time
spectra. We study the explosive nucleosynthesis of stable nickel in SNe Ia
resulting from MCh and sub-MCh progenitors. We explore the potential for lines
of [Ni II] at 7378 \AA\ and 1.94 microns in late-time spectra to serve as a
diagnostic of the exploding WD mass, using nonlocal thermodynamic equilibrium
radiative-transfer simulations with the CMFGEN code. We find that the radiative
proton-capture reaction 57Co(p,gamma)58Ni is the dominant production mode for
58Ni in both MCh and sub-MCh models, while the alpha-capture reaction on 54Fe
has a negligible impact on the final 58Ni yield. More importantly, we
demonstrate that the lack of [Ni II] lines in late-time spectra of sub-MCh
events is not always due to an under-abundance of stable Ni; rather, it results
from the higher ionization of Ni in the inner ejecta. Conversely, the strong
[Ni II] lines predicted in our 1D MCh models are completely suppressed when
56Ni is sufficiently mixed with the innermost layers, which are rich in stable
iron-group elements. [Ni II] lines in late-time SN Ia spectra have a complex
dependency on the abundance of stable Ni, which limits their use in
distinguishing among MCh and sub-MCh progenitors. However, we argue that a
low-luminosity SN Ia displaying strong [Ni II] lines would most likely result
from a Chandrasekhar-mass progenitor. [Abridged]
