M.R. Shankar

Stabilizing nanostructured materials by coherent nanotwins and their grain boundary triple junction drag

The role of nanotwin lamellae in enhancing thermal stability of nanostructured materials is examined. Nanostructured copper with varying densities of twins was generated by controlling the deformation strain rate during severe plastic deformation at cryogenic temperatures. While the nanostructured materials produced under cryogenic conditions are characteristically unstable even at room temperatures, their stability is markedly improved when a dense dispersion of nanotwins is introduced. Observations of the role of nanotwins in pinning grain and subgrain structures suggest an interfacial engineering approach to enhancing the stability of nanostructured alloys.

Effect of Severe Plastic Deformation in Machining Elucidated via Rate-Strain-Microstructure Mappings

Machining induces severe plastic deformation (SPD) in the chip and on the surface to stimulate dramatic microstructural transformations which can often result in a manufactured component with a fine-grained surface. The aim of this paper is to study the one-to-one mappings between the thermomechanics of deformation during chip formation and an array of resulting microstructural characteristics in terms of central deformation parameters–strain, strain-rate, temperature, and the corresponding Zener–Hollomon (ZH) parameter. Here, we propose a generalizable rate-strain-microstructure (RSM) framework for relating the deformation parameters to the resulting deformed grain size and interface characteristics. We utilize Oxley’s model to calculate the strain and strain-rate for a given orthogonal machining condition which was also validated using digital imaging correlation-based deformation field characterization. Complementary infrared thermography in combination with a modified-Oxley’s analysis was utilized to characterize the temperature in the deformation zone where the SPD at high strain-rates is imposed. These characterizations were utilized to delineate a suitable RSM phase-space composed of the strain as one axis and the ZH parameter as the other. Distinctive one-to-one mappings of various microstructures corresponding to an array of grain sizes and grain boundary distributions onto unique subspaces of this RSM space are shown. Building on the realization that the microstructure on machined surfaces is closely related to the chip microstructure derived from the primary deformation zone, this elucidation is expected to offer a reliable approach for controlling surface microstructures from orthogonal machining.

Characterization of Deformation Mechanics and Microstructure Evolution During Indirect Extrusion in Small Length Scales

In situ characterization of mechanics of deformation including dynamic strain, strain-rate and rotation of material elements was performed in prototypical small length-scale forming operation- indirect extrusion (IE) of commercially pure Lead (Pb) and Aluminum (Al 1100) using Digital Image Correlation (DIC) technique. Effects of scaling and deformation rate over a range of dimensions and velocities on mechanical response of CP Pb and the resultant anomalies were studied. Additionally, previous and post-deformation characterization of microstructure in Al was accomplished by performing Orientation Imaging Microscopy (OIM) on workpiece materials with different grain sizes. Finally, In situ characterization aided by high speed imaging of indirect extrusion of Al coupled with a Visco-Plastic Self-Consistent (VPSC) model was used to predict the evolved textures in various regions of deformation zone and the results were compared to OIM observations.Copyright