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Shear banding, a type of inhomogeneous plastic flow involving very large local strains, occurs in a variety of material systems. We study dynamics of evolution of single shear bands at strain rates of up to per second in three different polycrystalline metal systems, using a special shear deformation framework and a micro-marker technique calibrated to track localised deformation fields at micrometer resolution. Once a band is nucleated as a weak interface, localised plastic flow occurs via Bingham-type viscous sliding between material segments on either side of the interface. As a result, the evolution and magnitude of strains and material displacements in the band vicinity are well-described by a model based on momentum diffusion. The viscosity at the band interface is very small, only a few mPa·sec, and is comparable to those of liquid metals at their melting point. Based on analysis of various contributions to band viscosity at the microscopic level, a plausible explanation based on phonon drag on dislocation motion is presented for the small viscosity. The accuracy of predictions made by the momentum diffusion model for different materials and deformation rates suggests that once nucleated, a shear band evolves by a common mechanism that is relatively insensitive to microstructure details. [ABSTRACT FROM AUTHOR]

Shear banding is a type of plastic flow instability with often adverse implications for cutting and deformation processing of metals. Here, we study the mechanics of plastic flow evolution within single shear bands in Ti- and Ni-based alloy systems. The local shear band displacement profiles are quantitatively mapped at high resolution using a special micromarker technique. The results show that shear bands, once nucleated, evolve by a universal viscous sliding mechanism that is independent of microstructural details. The evolution of local deformation around the band is accurately captured by a momentum diffusion equation based on a Bingham-type flow rule. The predicted band viscosity is very small, compared to those of liquid metals. A plausible explanation for this small viscosity and fluid-like behavior at the band, based on phonon drag, is presented. [ABSTRACT FROM AUTHOR]

Shear banding is a plastic flow instability with highly undesirable consequences for metals processing. While band characteristics have been well studied, general methods to control shear bands are presently lacking. Here, we use high-speed imaging and micro-marker analysis of flow in cutting to reveal the common fundamental mechanism underlying shear banding in metals. The flow unfolds in two distinct phases: an initiation phase followed by a viscous sliding phase in which most of the straining occurs. We show that the second sliding phase is well described by a simple model of two identical fluids being sheared across their interface. The equivalent shear band viscosity computed by fitting the model to experimental displacement profiles is very close in value to typical liquid metal viscosities. The observation of similar displacement profiles across different metals shows that specific microstructure details do not affect the second phase. This also suggests that the principal role of the initiation phase is to generate a weak interface that is susceptible to localized deformation. Importantly, by constraining the sliding phase, we demonstrate a material-agnostic method--passive geometric flow control--that effects complete band suppression in systems which otherwise fail via shear banding. [ABSTRACT FROM AUTHOR]

Constrained chip (sheet) formation by large strain extrusion machining is used to impose effective strains of ∼1 in Mg alloy AZ31B sheet in a single step of deformation. High-speed image analysis shows the deformation underlying sheet formation to be simple shear that is confined to a narrow zone. This confinement of the deformation limits the need for pre-heating of the workpiece to realize continuous sheet forms. Tilted-basal textures, wherein the basal poles are inclined from the sheet surface normal, are achieved by this processing. These textures are quite different from those prevalent in rolled sheet. By controlling the strain path, the basal pole inclination could be varied in the range 32–53°. The primary texture component is the B-fiber, indicating basal slip to be the main deformation mode over the temperature range 165–400°C. An additional C2-fiber component appears above 250°C due to the activation of pyramidal slip. In conjunction with these textures, microstructures ranging from ultrafine-grained (∼200nm) to fine-grained (∼2μm) could be obtained by controlling the deformation temperature. Implications of the results for production of Mg sheet are discussed. [ABSTRACT FROM AUTHOR]