Quantification of precipitate hardening of twin nucleation and growth in Mg and Mg-5Zn using micro-pillar compression

© 2018 Acta Materialia Inc. In polycrystalline materials, the stress corresponding to twin nucleation is difficult to separate from twin growth because these events occur concurrently during deformation. In this work, we separate the nucleation stress and growth stress of {101¯2} twinning by compression of micro-pillars containing pre-existing twins through their centre. Micro-pillar compression results showed a strong size effect on both twin nucleation and twin growth stresses for pure Mg and Mg-5Zn alloys. Taking this into account, the critical stress for growth of twins in pure magnesium is found to be ∼7 MPa which is consistent with previously published measurements on macroscopic single crystals. These experiments have been used to deduce the precipitate hardening of twin growth, and for the present precipitate dispersion this has been measured to be ∼30 MPa. Back-stress calculations based on elastic bending of the precipitates showed close agreement to the measured precipitate hardening, and this model therefore accounts well for the observed strengthening. Site-specific atom probe tomography of the twin boundaries showed that room temperature ageing is sufficient to produce segregation of zinc to the twin boundary. This was found to immobilise the twin, and is believed to be the first report of solute locking of twins from room temperature exposure.