Twin nucleation, twin growth and their effects on annealing strengths of Mg–Al–Zn–Mn sheets experienced different pre-compressive strains

Abstract We combine pre-compressive test, annealing, re-compressive test, quasi-in-situ optical microscopy (OM), electron backscattered diffraction (EBSD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to systematically investigate the macroscopic yield strength differences ( Δ ASs ) of Mg-1.5Al-1.0Zn-0.3Mn (AZM110, wt.%) sheets with different pre-compressive strains 2%, 4% and 6% after re-compression with annealing, respectively. We observe that with increasing the annealing time, Δ AS firstly increased and then decreased. Moreover, with the increase of pre-compressive strains, Δ AS peak value gradually decreased. Quasi-in-situ OM images reveal that the contribution of new twin nucleation increased and initial twin growth decreased for pre-compressed AZM110 sheets after re-compression with annealing. With the pre-compressive strains, the contribution of new twin nucleation gradually is weakened and initial twin growth is enhanced, which leads to the alteration of Δ AS . By the observation of HAADF-STEM, twin boundaries are stabilized by the segregation of Al and Zn solutes and nano-Al8Mn5 phases pinning, creating pre-compressive annealing strengthening (PCAS). With the annealing time, the disappearance of back stress and dislocation tangle decreases the PCAS effect. Pre-compressive strain also strongly influences the PCAS effect. The increasing volume fraction and width of twins observed from EBSD accelerate twin boundary motion (TBM) with pre-compressive strains from 2% to 4%, decreasing the PCAS effect after re-compression with annealing. Although increasing pre-compressive strain to 6% can put off TBM, low new twin nucleation rate still gives rise to the decreasing PCAS effect.