  The chemical evolution of the Milky Way is investigated using a dual-phase
metal-enriched infall model in which primordial gas fuels the earliest epoch of
star formation, followed by the ongoing formation of stars from newly accreted
gas. The latest metallicity distribution of local K-dwarfs is reproduced by
this model, which allows the Galactic thin disk to form from slightly
metal-enriched gas with alpha-element enhancement. Our model predicts ages for
the stellar halo and thin disk of 12.5 and 7.4 Gyr, respectively, in agreement
with empirically determined values. The model presented in this paper is
compared with a similar dual-phase infall model from Chiappini et al. (2001).
We discuss a degeneracy that enables both models to recover the K-dwarf
metallicity distribution while yielding different star formation histories.
  The metallicity distribution function (MDF) of K-dwarfs is proposed to be
more directly comparable to chemical evolution model results than the G-dwarf
distribution because lower mass K-dwarfs are less susceptible to stellar
evolutionary effects. The K-dwarf MDF should consequently be a better probe of
star formation history and provide a stronger constraint to chemical evolution
models than the widely used G-dwarf MDF. The corrections that should be applied
to a G-dwarf MDF are quantified for the case of the outer halo of NGC 5128.
