Dynamic interactions between dislocations and twin in Mg: experiments, simulations and theory

In sharply textured hexagonal metals and alloys, especially in Mg and traditional Mg alloys, the {10-12} twins can be profusely activated early during deformation under appropriate loading and eventually consume a large volume fraction of the polycrystals. Understanding the dynamic interactions between gliding dislocations and mobile twin boundary (TB) is essential in accurately describing the plasticity related to twinning. This talk presents comprehensive dislocation characterizations using transmission electron microscopy (TEM) and in situ straining in TEM, and highlights a characteristic dislocation substructure inside the {10-12} twins in hexagonal metals. Several microstructure evolutions directly related to the slip-twin interactions were observed: dislocation-induced twin growth, formation of sessile I_1 stacking faults (SFs) and glissile dislocations inside the twin. Aided by molecular dynamics simulations, it is concluded that the slip-twin interaction is not a slip transfer process. When basal dislocations from the matrix are swept by an advancing TB, they are converted into sessile partial dislocations within the twin. Using continuum dislocation theory, it is further shown that the dislocation configuration near TB renders it energetically favorable for a pair of partial dislocations to constrict into a dislocation within the twin. Rather than requiring a high energy dislocation pile-up in front of a TB, it is demonstrated that dislocations naturally form, within {10-12} twins in Mg, as TB migrates into a matrix. The study helps to explain how the twins may subsequently deform and why they may be particularly vulnerable to plastic localization.