Nucleation properties of isolated shear bands.

Shear banding, or localization of intense strains along narrow bands, is a plastic instability in solids with important implications for material failure in a wide range of materials and across length scales. In this article, we report on a series of experiments on the nucleation of single isolated shear bands in three model alloys. Nucleation kinetics of isolated bands and characteristic stresses are studied using high-speed in situ imaging and parallel force measurements. The results demonstrate the existence of a critical shear stress required for band nucleation. The nucleation stress bears little dependence on the normal stress and is proportional to the shear modulus. These properties are quite akin to those governing the onset of dislocation slip in crystalline solids. A change in the flow mode from shear banding to homogeneous plastic flow occurs at stress levels below the nucleation stress. Phase diagrams delineating the strain, strain rate and temperature domains where these two contrasting flow modes occur are presented. Our work enables interpretation of shear band nucleation as a crystal lattice instability due to (stress-assisted) breakdown of dislocation barriers, with quantitative experimental support in terms of stresses and the activation energy.

In situ analysis of shear bands and boundary layer formation in metals.

Shear banding is a plastic instability in large deformation of solids where the flow becomes concentrated in narrow layers, with broad implications in materials processing applications and dynamic failure of metals. Given the extremely small length and time scales involved, several challenges persist in studying the development of shear bands. Here, we present a new approach to study shear bands at low speeds using low melting point alloys. We use in situ imaging to directly capture the essential features of shear banding, including transition from homogeneous to shear banded flow, band nucleation and propagation dynamics, and temporal evolution of the flow around a developing band. High-resolution, time-resolved measurements of the local displacement and velocity profiles during shear band growth are presented. The experiments are complemented by an analysis of the shear band growth as a Bingham fluid flow. It is shown that shear banding occurs only beyond a critical shear stress and is accompanied by a sharp drop in the viscosity by several orders of magnitude, analogous to the yielding transition in yield-stress fluids. Likewise, the displacement field around a nucleated band evolves in a manner that resembles boundary layer formation, with the band thickness scaling with time as a power law.

Surface plastic flow in polishing of rough surfaces.

We present experimental evidence for a new mechanism for how smooth surfaces emerge during repetitive sliding contacts, as in polishing. Electron microscopy observations of Ti-6Al-4V surface with a spherical asperity structure-realized via additive manufacturing-during successive polishing stages suggest that asperity-abrasive contacts exhibit viscous behavior, where the asperity material flows in the form of thin (1-10 µm) fluid-like layers. Subsequent bridging of these layers among neighboring asperities results in progressive surface smoothening. Using analytical asperity-abrasive contact temperature modeling and microstructural characterization, we show that the sliding contacts encounter flash temperatures of the order of 700-900 K which is in the range of the dynamic recrystallization temperature of the material considered, thus supporting the experimental observations. Besides providing a new perspective on the long-held mechanism of polishing, our observations provide a novel approach based on graph theory to quantitatively characterize the evolution of surface morphology. Results suggest that the graph representation offers a more efficient measure to characterize the surface morphology emerging at various stages of polishing. The research findings and observations are of broad relevance to the understanding of plastic flow behavior of sliding contacts ubiquitous in materials processing, tribology, and natural geological processes as well as present unique opportunities to tailor the microstructures by controlling the thermomechanics of the asperity-abrasive contacts.

Geometric flow control of shear bands by suppression of viscous sliding.

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.