Micro- and mesomechanical aspects of deformation-induced surface roughening in polycrystalline titanium

This article addresses the problem of multiscale surface roughening in commercial purity titanium subjected to uniaxial tension. In situ investigations of the evolution of grain- and mesoscale roughness in selected regions of titanium specimens were performed. Based on the experimental data obtained, 3D polycrystalline models with explicit consideration of grain structure were generated and implemented in finite-element calculations. Constitutive models of grains were constructed, using crystal plasticity theory to take into account the elastic-plastic anisotropy on the grain scale. The experimental and numerical results obtained have shown that a series of multiscale surface undulations are formed on the free surface of the specimen subjected to tension. The smallest out-of-plane surface displacements are attributed to intragrain dislocation glide. Larger displacements are associated with relative grain motion. The latter give rise to the formation of an orange peel pattern. The displacements of the two types are a microscale phenomenon. The largest surface displacements formed by the grain groups involved in out-of-plane and in-plane cooperative motion are referred to as the mesoscale roughness. The latter is found to correlate well with local strains of the specimen regions under examination. The main conclusion drawn from the experimental and numerical results is that it is the mesoscale that will furnish a clue to prediction of plastic strain localization and fracture of materials far in advance of the macroscale manifestation of these processes.