  We investigate the radial metallicity gradients and azimuthal metallicity
distributions on the Galactocentric $X$--$Y$ plane using mono-temperature
stellar populations selected from LAMOST MRS young stellar sample. The
estimated radial metallicity gradient ranges from $-$0.015\,dex/kpc to
$-$0.07\,dex/kpc, which decreases as effective temperature decreases (or
stellar age increases) at $7500 < T_{\rm eff} < 12500$\,K ($\tau < $1.5 Gyr).
The azimuthal metallicity excess (metallicity after subtracting radial
metallicity gradient, $\Delta$\,[M/H]) distributions exhibit inhomogeneities
with dispersions of 0.04\,dex to 0.07\,dex, which decrease as effective
temperature decreases. We also identify five potential metal-poor substructures
with large metallicity excess dispersions. The metallicity excess distributions
of these five metal-poor substructures suggest that they contain a larger
fraction of metal-poor stars compared to other control samples. These
metal-poor substructures may be associated with high-velocity clouds that
infall into the Galactic disk from the Galactic halo, which are not quickly
well-mixed with the pre-existing ISM of the Galactic disk. As a result, these
high-velocity clouds produce some metal-poor stars and the observed metal-poor
substructures. The variations of metallicity inhomogeneities with different
stellar populations indicate that high-velocity clouds are not well mixed with
the pre-existing Galactic disk ISM within 0.3\,Gyr.
