Rodney J. McCabe

Statistical analyses of deformation twinning in magnesium

To extract quantitative and meaningful relationships between material microstructure and deformation twinning in magnesium, we conduct a statistical analysis on large data sets generated by electron backscattering diffraction (EBSD). The analyses show that not all grains of similar orientation and grain size form twins, and twinning does not occur exclusively in grains with high twin Schmid factors or in the relatively large grains of the sample. The number of twins per twinned grain increases with grain area, but twin thickness and the fraction of grains with at least one visible twin are independent of grain area. On the other hand, an analysis of twin pairs joined at a boundary indicates that grain boundary misorientation angle strongly influences twin nucleation and growth. These results question the use of deterministic rules for twin nucleation and Hall–Petch laws for size effects on twinning. Instead, they encourage an examination of the defect structures of grain boundaries and their role in twin nucleation and growth.

Effects of texture, temperature and strain on the deformation modes of zirconium

Clock-rolled, high-purity, textured polycrystalline zirconium exhibits significant plastic anisotropy for compression along the through-thickness and in-plane directions and strong temperature dependence of flow stress for both orientations. Orientation imaging microscopy in a scanning electron microscope and defect analysis via transmission electron microscopy are used to characterize the defect microstructures as a function of initial texture, deformation temperature and plastic strain. The observed deformation mechanisms are correlated with the measured mechanical response.

In-Situ TEM Observation of Twinning and Detwinning During Cyclic Loading in Mg

In-situ transmission electron microscopy (TEM) is used to directly observe twin evolution in Mg under tension and compression. Twins grow during tensile loading. Upon load reversal, the first-generation twin detwins by nucleation and growth of a second-generation twin within its volume. This mechanism for detwinning is different from the more traditional mechanism of detwinning by reverse motion of a twin boundary. Reloading in tension causes the second-generation twin to recede, leaving behind residual features. In compression, the second-generation twin re-nucleates in the area of this debris, and grows. Interactions between dislocations and twin boundaries change the character of the observed dislocations. Direct observation of such behavior aids in clearer understanding of the observed microstructures from post-mortem TEM.

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.

Co-zone {1¯012} Twin Interaction in Magnesium Single Crystal

Using in situ optical microscopy, electron backscatter diffraction analysis, and atomistic simulations, we studied co-zone {1¯012} twin interaction in magnesium single crystal under compression–tension along the [101¯0] direction. Two co-zone twins are activated and interact with each other, resulting in two types of tilt boundaries that have habit planes (0001) and (101¯0) and prevent twin-across-twin transmission but facilitate the propagation of a basal slip band. Upon strain reversal, the unfavorable dissociation of dislocations in the formed tilt boundaries hinder de-twinning.

A wedge-mounting technique for nanoscale electron backscatter diffraction

The practical spatial resolution of electron backscatter diffraction (EBSD) is around 100 nm, which limits the length scales from which phase and orientation relationship characterization can be accomplished. This precludes collection of statistically relevant data on the crystallography of interfaces within nanomaterials where such information is essential for understanding the unique properties of these materials. In this work, we present a wedge-mounting technique that enables EBSD data to be collected for sub-100 nm thick layers of Cu-Nb bimetallic multilayers fabricated via accumulative roll bonding. We present statistics on layer thickness distributions, grain morphology, orientation distributions, twin volume fraction, and interface character for material with an averaged layer thickness of 86 and 56 nm.

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.

Grain neighbour effects on twin transmission in hexagonal close-packed materials.

Materials with a hexagonal close-packed (hcp) crystal structure such as Mg, Ti and Zr are being used in the transportation, aerospace and nuclear industry, respectively. Material strength and formability are critical qualities for shaping these materials into parts and a pervasive deformation mechanism that significantly affects their formability is deformation twinning. The interaction between grain boundaries and twins has an important influence on the deformation behaviour and fracture of hcp metals. Here, statistical analysis of large data sets reveals that whether twins transmit across grain boundaries depends not only on crystallography but also strongly on the anisotropy in crystallographic slip. We show that increases in crystal plastic anisotropy enhance the probability of twin transmission by comparing the relative ease of twin transmission in hcp materials such as Mg, Zr and Ti.

Observations of the Atomic Structure of Tensile and Compressive Twin Boundaries and Twin–Twin Interactions in Zirconium

The boundary structures of twins in the hexagonal close-packed metal zirconium were studied. High-resolution transmission electron microscopy was used to characterize the boundary structure of