Deformation twinning mechanism in magnesium at the microscale: strain rate, temperature and orientation effects

Della Ventura, Nicolò Maria

PhD Tesis (2022)

The project aims to validate some recent theoretical developments on the deformation twinning nucleation mechanism in HCP metals through small-scale mechanic experiments. To this end, a systematic investigation of the mechanical response of pure magnesium single crystals has been con-ducted by studying the singular and reciprocal effects of orientation, strain rate, and temperature on the accommodation mechanisms of deformation twinning at the microscale. Through in-depth analyses of different misorientation relationships between initial and reoriented crystals, as well as the characteristics of several three-dimensionally-reconstructed parent-twin interfaces, unconventional twin features that deviate from the classical shear-based model were identified, promoting a new interpretation of the deformation twinning mechanism. In particular, it is discussed how the formation of twins is accompanied by a mechanism for which the interface between the parent and twin is not a mirror plane: the deformation cannot be therefore described by a shear tensor, questioning the centenary shear theory of twinning. A full understanding of how changes in the twinning mechanism affect the stress/strain curves is al-so presented. The latter were used for the experimental determination of strain rate sensitivity, activation volume, and activation energy for twinning; parameters typically used to classify the nature of the deformation mechanisms. It is observed that, in contrast to what is being reported at the macroscale in polycrystalline magnesium, the critical stress for twinning nucleation and the post deformation crystallographic characteristics of the twin boundaries are sensitive to changes in temperature and strain-rate. Furthermore, the transition from a twinning-dominated to a slip-dominated plastic deformation is revealed to occur with increasing temperature, although twinning becomes again predominant by increasing strain rate. By loading the crystal from different crystallographic directions, the orientation dependent ductility of the material is also discussed. It is shown that basal slip activity favors the twin propagation with consequent higher probability of twin-twin interactions. An in situ uniaxial testing setup allowing high resolution Electron Backscatter Diffraction acquisitions during deformation was fine tuned to measure, in some cases, the sequence of events that lead to the formation and propagation of twins induced by microtensile testing.