Highlights

Using Electron Tomography to Characterize Twin Nuclei in a High Strain Rate Deformed Nitrogen-strengthened Austenitic Steel

Scientific Achievement

The focus of this work was to compare the capabilities of the Philips CM30 and the Hitachi H9000 to determine which instrument could be used to obtain a series of weak-beam dark-field (WBDF) images for use in modeling the three-dimensional (3D) structure of a deformation twin nucleus. This type of modeling requires a large number of images to reconstruct the sample volume with high spatial resolution and thus a microscope with a high tilt capacity is required.  It was found the CM30 was unsuitable because of the inability to maintain WBDF imaging conditions using a tilt-rotate holder.  Further, this holder exhibited a significant amount of backlash in the motor responsible for specimen rotation which made maintaining sample position difficult over the required range of tilt.  Attempts to collect the required images using a standard double-tilt holder were unsuccessful due to limitations in range of the secondary tilt axis. This prevented collecting a sufficient number of WBDF images for a 3D reconstruction with suitable resolution.

The H9000, in combination with a double-tilt/rotate holder, was found suitable for obtaining WBDF images over a ±30° range of tilt.  The one difficulty which needs to be addressed involves imaging under WBDF conditions at tilt angles in excess of 30°.  Above this level of tilt, extensive inelastic scattering due to specimen thickness in the region of interest was found to inhibit the formation of WBDF images.  Because of the subsequent inability to access the maximum ±45° tilt range of the H9000, the number of images which could be obtained was effectively reduced by one-third.  In the future, the effects of inelastic scattering will be mitigated by increasing the accelerating voltage of the H9000 to 300 keV. 

Significance

The current work represents our initial effort to establish the feasibility of collecting a sufficient number of images over a large tilt angle to enable reconstruction of the 3D structure of a deformation twin nucleus.  Identification of the twin nucleation mechanisms remains one of the key challenges in understanding this deformation process.  The successful identification of the source and of the mechanism under which it operates will have a significant impact on our ability to model this response mechanism.

Performers

B. D. Miller, I. M. Robertson (U. Illinois Urbana-Champaign); M. A. Kirk (Argonne-MSD)