In this paper, the mechanical response of ultra fine grained metallic materials under high strain rate impact conditions is investigated by means of a finite element based numerical framework. A dislocation based viscoplastic model is used to predict the evolution of the initial fine grain microstructure (average grain size of 203 nm or 238 run, depending on the material history) with impact deformation. A Taylor impact test is simulated in order to assess the validity of a numerical solution through comparison with experiment. It is shown that our model captures the essential features of the mechanical behaviour. A further grain refinement down to the average grain size of 140-160 nm is predicted by the simulations.