Kevin J. Hemker

Learn More
High-resolution electron microscope observations of shock-loaded boron carbide have revealed the formation of nanoscale intragranular amorphous bands that occur parallel to specific crystallographic planes and contiguously with apparent cleaved fracture surfaces. This damage mechanism explains the measured, but not previously understood, decrease in the(More)
We report transmission electron microscope observations that provide evidence of deformation twinning in plastically deformed nanocrystalline aluminum. The presence of these twins is directly related to the nanocrystalline structure, because they are not observed in coarse-grained pure aluminum. We propose a dislocation-based model to explain the preference(More)
In high-resolution electron microscopy (HREM), dislocation core structures are examined by tilting the dislocation end-on along the appropriate zone axis. For end-on screw dislocations diffraction contrast is largely due to surface relaxation in the form of the Eshelby twist. In this paper, simulated, many-beam images of end-on, 1/2<111> Mo screw(More)
In crystalline materials, plastic deformation occurs by the motion of dislocations, and the regions between individual crystallites, called grain boundaries, act as obstacles to dislocation motion. Grain boundaries are widely envisaged to be mechanically static structures, but this report outlines an experimental investigation of stress-driven grain(More)
Nanoporous gold (np-Au) is a macroscopically brittle material, which poses diffi culties for tensile testing of bulk specimens. By combining a fabrication approach that minimizes cracking in bulk np-Au and a microspecimen test technique that permits small testing volumes, both tension and compression tests were performed on sub-millimeter gage thicknesses(More)
The saying “a picture is worth a thousand words” is as true at the nanoscale as it is in everyday life. On page 115 of this issue, Zhiwei Shan and colleagues describe the in situ deformation of nanometresized columns in a transmission electron microscope (TEM) in a way that provides a unique and highly informative glimpse into the world of nanomechanics1.(More)
Silicon-based microelectromechanical systems (MEMS) sensors have become ubiquitous in consumer-based products, but realization of an interconnected network of MEMS devices that allows components to be remotely monitored and controlled, a concept often described as the "Internet of Things," will require a suite of MEMS materials and properties that are not(More)
transmission electron microscopy on commercial quality boron showing that ∼2=3 of the grains exhibit smooth microstructure, leading to an x-ray diffraction pattern of well-known beta boron [1]. The other 1=3 grains exhibit a uniform zigzag pattern that extends across the entire grain and exhibits a very regular twinlike symmetry on every other lattice(More)