Abstract
We observe abundance anomalies at the surface of many field and cluster A stars. Theseabundance anomalies are generated by microscopic diffusion acting within the stable envelopes and may be modulated by the competition between many other mixing processes such as convection, rotational mixing and mass loss. The typical patterns encountered are underabundances of Ca and/or Sc as well as overabundances of iron peek elements and rare earths. Non magnetic A stars with such patterns are classified as Am. By observing the surface of A and F stars from open clusters in particular, we can infer additional stellar parameters such as age and initial chemical composition which are invaluable for the comparison with models.<br />Therefore, by determining the surface composition of A and F stars from many clusters of different ages we can follow the evolution of chemical composition and constrain the processes considered in evolutionary models.<br />In this thesis, the surface abundances of 22 elements are presented for 21 A/F stars of the Pleiades open cluster (100 Myr) and for 22 A/F stars of the Coma Berenices open cluster (450 Myr) which were observed with the three spectrographs AURELIE, ELODIE and SOPHIE at the Observatoire de Haute-Provence (OHP). The synthetic spectra method, which is based on ATLAS9-12 atmospheric models, is used in order to determine the abundances. These observations are then compared with evolutionary models from the Montreal group.<br />The results show that there are hydrodynamical processes acting within the radiative zone of these stars and hindering the effects of microscopic diffusion.