Characteristic boundaries associated with three-dimensional twins in hexagonal metals.

Twinning is a critically important deformation mode in hexagonal close-packed metals. Twins are three-dimensional (3D) domains, whose growth is mediated by the motion of facets bounding the 3D twin domains and influences work hardening in metals. An understanding of twin transformations therefore necessitates that the atomic-scale structure and intrinsic mobilities of facets be known and characterized. The present work addresses the former point by systematically characterizing the boundary structures of 3D { 1 ¯ 012 } twins in magnesium using high-resolution transmission electron microscopy (HRTEM). Eight characteristic facets associated with twin boundaries are reported, five of which have never been experimentally observed before. Further, molecular dynamics simulations suggest that the formation and motion of these facets is associated with the accumulation of twinning dislocations. This work provides insights into understanding the structural character of 3D twins and serves to develop strategies for modulating twin kinetics by modifying twin boundaries, such as solute segregation.

The critical role of grain orientation and applied stress in nanoscale twinning.

Numerous recent studies have focused on the effects of grain size on deformation twinning in nanocrystalline fcc metals. However, grain size alone cannot explain many observed twinning characteristics. Here we show that the propensity for twinning is dependent on the applied stress, grain orientation and stacking fault energy. The lone factor for twinning dependent on grain size is the stress necessary to nucleate partial dislocations from a boundary. We use bulk processing of controlled nanostructures coupled with unique orientation mapping at the nanoscale to show the profound effect of crystal orientation on deformation twinning. Our theoretical model reveals an orientation-dependent critical threshold stress for twinning, which is presented in the form of a generalized twinnability map. Our findings provide a newfound orientation-based explanation for the grain size effect: as grain size decreases the applied stress needed for further deformation increases, thereby allowing more orientations to reach the threshold stress for twinning.

Engineering interface structures and thermal stabilities via SPD processing in bulk nanostructured metals.

Nanostructured metals achieve extraordinary strength but suffer from low thermal stability, both a consequence of a high fraction of interfaces. Overcoming this tradeoff relies on making the interfaces themselves thermally stable. Here we show that the atomic structures of bi-metal interfaces in macroscale nanomaterials suitable for engineering structures can be significantly altered via changing the severe plastic deformation (SPD) processing pathway. Two types of interfaces are formed, both exhibiting a regular atomic structure and providing for excellent thermal stability, up to more than half the melting temperature of one of the constituents. Most importantly, the thermal stability of one is found to be significantly better than the other, indicating the exciting potential to control and optimize macroscale robustness via atomic-scale bimetal interface tuning. Taken together, these results demonstrate an innovative way to engineer pristine bimetal interfaces for a new class of simultaneously strong and thermally stable materials.

Analysis of recrystallized volume fractions in uranium using electron backscatter diffraction.

Electron backscatter diffraction was used to examine the recrystallization behaviour of warm, clock-rolled uranium. A new uranium preparation method was developed, resulting in acceptable specimen surface finishes nearly every time, even for as-rolled specimens. Recrystallized fractions were differentiated from unrecrystallized fractions using differences in the grain average misorientation, a measure of the internal level of misorientation within a grain. This new approach better estimates the recrystallized fraction than hardness measurements, and has the advantage over standard metallographic techniques of providing texture information.

A single-tilt TEM stereomicroscopy technique for crystalline materials.

A new single-tilt technique for performing TEM stereomicroscopy of strain fields in crystalline materials has been developed. The technique is a weak beam technique that involves changing the value of g and/or s g while tilting across a set of Kikuchi bands. The primary benefit of the technique is it can be used with single-tilt TEM specimen holders including many specialty holders such as in situ straining, heating, and cooling holders. Standard stereo-TEM techniques are almost always limited to holders allowing two degrees of rotational freedom (i.e., double-tilt or tilt/rotation holders). An additional benefit of the new technique is that it eliminates the need to focus with the specimen height control. These advantages make it useful for stereo viewing or for quantitative stereomicroscopy provided necessary consideration is given to errors that may result from the technique.