  We present $\gtrsim 15,000$ metal-rich (${\rm [Fe/H]}>-0.2$dex) A and F stars
whose surface abundances deviate strongly from Solar abundance ratios and
cannot plausibly reflect their birth material composition. These stars are
identified by their high [Ba/Fe] abundance ratios (${\rm [Ba/Fe]}>1.0$dex) in
the LAMOST DR5 spectra analyzed by Xiang et al. (2019). They are almost
exclusively main sequence and subgiant stars with $T_{\rm eff}\gtrsim6300$K.
Their distribution in the Kiel diagram ($T_{\rm eff}$--$\log g$) traces a sharp
border at low temperatures along a roughly fixed-mass trajectory (around
$1.4M_\odot)$ that corresponds to an upper limit in convective envelope mass
fraction of around $10^{-4}$. Most of these stars exhibit distinctly enhanced
abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg
and Ca. Rotational velocity measurements from GALAH DR2 show that the majority
of these stars rotate slower than typical stars in an equivalent temperature
range. These characteristics suggest that they are related to the so-called
Am/Fm stars. Their abundance patterns are qualitatively consistent with the
predictions of stellar evolution models that incorporate radiative
acceleration, suggesting they are a consequence of stellar internal evolution
particularly involving the competition between gravitational settling and
radiative acceleration. These peculiar stars constitute 40% of the whole
population of stars with mass above 1.5$M_\odot$, affirming that "peculiar"
photospheric abundances due to stellar evolution effects are a ubiquitous
phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced
chemically peculiar A/F stars with individual element abundances provides the
statistics to test more stringently the mechanisms that alter the surface
abundances in stars with radiative envelopes.
