  Atomic diffusion and mixing processes in stellar interiors influence the
structure and the surface composition of stars. Some of these processes cannot
yet be modelled from the first principles. This limits their applicability in
stellar models used for studies of stellar populations and Galactic evolution.
Our main goal is to put constrains on the stellar structure and evolution
models using new refined measurements of chemical composition in stars of
Galactic open cluster. We use medium-resolution, 19 200 <= R <= 21 500, optical
spectra of the stars in the open cluster NGC 2420 obtained within the Gaia-ESO
survey. The sample covers all evolutionary stages from the main-sequence to red
giant branch. Stellar parameters are derived using a combined Bayesian analysis
of spectra, 2MASS photometry, and astrometric data from Gaia DR2. The
abundances of Mg, Ca, Fe, and Li are determined from non-local thermodynamic
equilibrium (NLTE) synthetic spectra, computed using one-dimensional (1D) and
averaged three-dimensional (3D) model atmospheres. We compare our results with
a grid of Code d'Evolution Stellaire Adaptatif et Modulaire (CESTAM) stellar
evolution models, which include atomic diffusion, turbulent and rotational
mixing. We find prominent evolutionary trends in the abundances of Fe, Ca, Mg,
and Li with the mass of the stars in the cluster. Fe, Mg, and Ca show a
depletion at the cluster turn-off, but the abundances gradually increase and
flatten near the base of the RGB. The abundance trend for Li displays a
signature of rotational mixing on the main-sequence and abrupt depletion on the
subgiant branch, which is caused by advection of Li-poor material to the
surface. The analysis of abundances combined with the CESTAM model predictions
allows us to place limits on the parameter space of the models and to constrain
the zone in the stellar interior where turbulent mixing takes place.
