Non-equilibrium basal stacking faults in hexagonal close-packed metals

Transmission electron microscopy (TEM) studies were performed to investigate non-equilibrium basal stacking faults (SFs) in deformed hexagonal, close-packed (HCP) metals, i.e., magnesium (Mg), cobalt (Co), titanium (Ti) and a Mg AZ31 alloy. These SFs present a width that is two to three orders of magnitude wider than the equilibrium width of the basal SFs created by partial dislocations. The non-equilibrium basal SFs are often generated inside deformation twins, and in conventional TEM the SFs present a morphology of straight lines well aligned with the trace of the basal planes, and may cross a whole twin. To investigate the mechanism of the formation of the SFs, we performed atomistic simulations. The simulation results show that the formation of the non-equilibrium basal SFs is closely associated with incoherent twin boundary (TB) migration. When the structure of the initial TB is incoherent, basal SFs are nucleated with one end being anchored at the moving TB. SFs subsequently grow along with the moving TB, resulting in SFs with a large width that may cross the whole twin. No Shockley or Frank partial dislocations are involved in the nucleation and the growth of SFs. When the structure of the initial TB is coherent or nearly coherent, no SFs are generated. The non-equilibrium basal SFs interact with prismatic and pyramidal slips, and impede the dislocation slip.