Macrodeformation Twins in Single-Crystal Aluminum

Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum at scales beyond nanotwins. Here, we present the first experimental demonstration of macrodeformation twins in single-crystal aluminum formed under an ultrahigh strain rate (∼10^{6}  s^{-1}) and large shear strain (200%) via dynamic equal channel angular pressing. Large-scale molecular dynamics simulations suggest that the frustration of subsonic dislocation motion leads to transonic deformation twinning. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates.

Note: Dynamic strain field mapping with synchrotron X-ray digital image correlation

We present a dynamic strain field mapping method based on synchrotron X-ray digital image correlation (XDIC). Synchrotron X-ray sources are advantageous for imaging with exceptional spatial and temporal resolutions, and X-ray speckles can be produced either from surface roughness or internal inhomogeneities. Combining speckled X-ray imaging with DIC allows one to map strain fields with high resolutions. Based on experiments on void growth in Al and deformation of a granular material during Kolsky bar/gas gun loading at the Advanced Photon Source beamline 32ID, we demonstrate the feasibility of dynamic XDIC. XDIC is particularly useful for dynamic, in-volume, measurements on opaque materials under high strain-rate, large, deformation.

Note: phase retrieval method for analyzing single-phase displacement interferometry data

We present a phase retrieval method (PRM) for analyzing single-phase displacement interferometry measurements on rapidly changing velocity histories, including photon Doppler velocimetry (PDV). PRM identifies the peaks and valleys as well as zero-crossing points in a PDV time series, performs normalization and extracts point-by-point phase and thus velocity information. PRM does not require a wide time window as in sliding window Fourier transformation, and thus improves the effective temporal resolution. This method is implemented in analyzing PDV data obtained from gas gun experiments, and validated against simultaneous measurements with velocity interferometer system for any reflector.