  Open cluster members are coeval and share the same initial bulk chemical
compositions. Consequently, differences in surface abundances between members
of a cluster that are at different evolutionary stages can be used to study the
effects of mixing and internal chemical processing. We carry out an abundance
analysis of seven elements (Li, O, Na, Mg, Al, Si, Fe) in 66 stars belonging to
the open cluster M67, based on high resolution GALAH spectra, 1D MARCS model
atmospheres, and, for the first time for a large sample of M67 stars, non-local
thermodynamic equilibrium (non-LTE) radiative transfer. From the non-LTE
analysis, we find a typical star-to-star scatter in the abundance ratios of
around 0.05 dex; this scatter is slightly but systematically larger when LTE is
assumed instead. We find trends in the abundance ratios with effective
temperature, indicating systematic differences in the surface abundances
between turn-off and giant stars; these trends are more pronounced when LTE is
assumed. However, in the non-LTE analysis, most of the element trends have been
flattened. Two species are exceptions to this behaviour, namely Al and Si,
which both clearly display remaining trends in the non-LTE analysis. We comment
on the possible origin of these trends, by comparing them with recent stellar
models that include atomic diffusion.
