Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.
On the intrusion-like co-zone twin-twin structure: an in situ observation
An in situ optical microscopy combined with ex situ electron backscatter diffraction testing was applied to a pristine single-crystal magnesium specimen under monotonic tension along the c-axis. An intrusion-like co-zone twin-twin structure is observed for the first time at the micron scale. In situ observation reveals that the intrusion-like twin-twin structure consists of multiple twin-twin boundaries (TTBs) and incoherent twin boundaries (I-CTBs) following energetically favorable formation sequences. The initial interaction results in the impinging TTBI and the acute-angle TTBA. In the local junction region on the obtuse angle side, the impinging twinning dislocations (TDs) further deposit near TTBI due to the preferred local twinning shear stress, leading to the incoherent curve of the impinging twin boundary adjacent to TTBI. Shortly after, the barrier twin boundary on the obtuse angle side migrates and encompasses the incoherent impinging twin boundary. The combination of sequential TTBI, TTBA, and I-CTBs formed locally on the obtuse angle side shapes the final configuration of the intrusion-like twin-twin structure at the micron scale.
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- Award ID(s):
- 1762312
- NSF-PAR ID:
- 10274269
- Date Published:
- Journal Name:
- Materials letters
- Volume:
- 286
- ISSN:
- 0167-577X
- Page Range / eLocation ID:
- 129140
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Twin–twin interactions (TTIs) take place when multiple twinning modes and/or twin variants are activated and interact with each other. Twin–twin junctions (TTJs) form and affect subsequent twinning/detwinning and dislocation slip, which is particularly important in determining mechanical behavior of hexagonal metals because twinning is one major deformation mode. Atomic-level study, including crystallographic analysis, transmission electronic microscopy (TEM), and molecular dynamics (MD) simulations, can provide insights into understanding the process of TTIs and structural characters associated with TTJs. Crystallographic analysis enables the classification of TTIs and the prediction of possible interfaces of twin–twin boundaries (TTBs), characters of boundary dislocations, and possible reactions of twinning dislocations and lattice dislocations at TTBs. MD simulations can explore the process of TTIs, microstructures of TTJs, atomic structures of TTBs, and stress fields associated with TTJs. The predictions based on crystallographic analysis and the findings from MD can be partially verified by TEM. More importantly, these results provide explanation for microstructural characters of TTJs and guidance for further TEM characterizations.more » « less
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Application of polycrystalline hexagonal close packed (HCP) metals in engineering designs has been constrained by their anisotropic responses due to twinning and limited plasticity. In deformation, twins most often initiate at grain boundaries (GBs), and thicken and propagate across the grain. In this work, the GB twin embryos in Mg and Mg alloys, and the conditions that influence their propagation are investigated. Using a micromechanical crystal plasticity model, the role of embryo shape on the driving forces prevailing at the embryo boundaries that could support its expansion is studied. The modeled embryos are either planar, extending more in the shear direction than normal to the twin plane, or equiaxed. Results show that the thinner the embryo, the greater the driving forces for both thickening and forward propagation. Alloys with low prismatic-to-basal critical resolved shear stress (CRSS) ratios promote embryo thickening and large CRSS values for the slip mode that primarily accommodates the twin shear encourage propagation. The neighboring grains with orientations that enable local accommodation of the embryo twin shear by pyramidal slip promote forward propagation but have little effect on thickening. When two like embryos lie along the same GB, their paired interaction promotes forward propagation but hinders thickening.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/https://doi.org/10.1016/j.matlet.2020.129140
