Microband formation in shock-loaded and quasi-statically deformed metals

A systematic examination of microbands developed in various materials, including pure Al, Cu, Ag, Nb metals and Al-Mg, 6061 Al and Al-Li-Cu alloys, deformed dynamically or quasi-statically to intermediate strains has been conducted. Based on extensive characterization of microbands using transmission electron microscopy, the characteristics of microbands do not appear to be strongly dependent on crystal structure, material properties, strain level or deformation path. This finding suggests that the formation mechanism of microbands may be similar in a variety of f.c.c. and b.c.c. metals and alloys. A possible microband formation mechanism is proposed, based on a concept of the further development of coarse slip bands (or dislocation tangles on glide planes), which is consistent with experimental observations. The model involves first the generation of polarized dislocations on primary slip systems, followed by an annihilation process for the primary dislocations in the central portion of a band structure, forming double dislocation walls parallel to the primary slip planes. Misorientation inside the double walls is believed to be caused by the directional shear of primary dislocations. Secondary slip is then induced by the internal stresses in the region between the double walls. Finally a stable dislocation configuration is created as a result of the interaction between the primary and secondary dislocations. The proposed model is in agreement with several observed phenomena, including the constancy of microband thickness, tilt axis, shear sense, and uniform shear strain over the cross-section of a microband. The roles of stacking fault energy, solute atoms and precipitates on microband formation are also discussed.