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1. Dislocation structure of deformed olivine single crystals from conventional EBSD maps

   https://doi.org/10.1007/s00269-021-01157-3  

   Faul, Ulrich (September 2021, Physics and Chemistry of Minerals)

   Abstract Dislocations, linear defects in a crystalline lattice characterized by their slip systems, can provide a record of grain internal deformation. Comprehensive examination of this record has been limited by intrinsic limitations of the observational methods. Transmission electron microscopy reveals individual dislocations, but images only a few square$$\upmu$$ $\mu$m of sample. Oxidative decoration requires involved sample preparation and has uncertainties in detection of all dislocations and their types. The possibility of mapping dislocation density and slip systems by conventional (Hough-transform based) EBSD is investigated here with naturally and experimentally deformed San Carlos olivine single crystals. Geometry and dislocation structures of crystals deformed in orientations designed to activate particular slip systems were previously analyzed by TEM and oxidative decoration. A curvature tensor is calculated from changes in orientation of the crystal lattice, which is inverted to calculate density of geometrically necessary dislocations with the Matlab Toolbox MTEX. Densities of individual dislocation types along with misorientation axes are compared to orientation change measured on the deformed crystals. After filtering (denoising), noise floor and calculated dislocation densities are comparable to those reported from high resolution EBSD mapping. For samples deformed in [110]c and [011]c orientations EBSD mapping confirms [100](010) and [001](010), respectively, as the dominant slip systems. EBSD mapping thus enables relatively efficient observation of dislocation structures associated with intracrystalline deformation, both distributed, and localized at sub-boundaries, over substantially larger areas than has previously been possible. This will enable mapping of dislocation structures in both naturally and experimentally deformed polycrystals, with potentially new insights into deformation processes in Earth’s upper mantle. 

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2. Strengthening in multi-principal element alloys with local-chemical-order roughened dislocation pathways

   https://doi.org/10.1038/s41467-019-11464-7  

   Li, Qing-Jie; Sheng, Howard; Ma, Evan (August 2019, Nature Communications)

   Abstract High-entropy and medium-entropy alloys are presumed to have a configurational entropy as high as that of an ideally mixed solid solution (SS) of multiple elements in near-equal proportions. However, enthalpic interactions inevitably render such chemically disordered SSs rare and metastable, except at very high temperatures. Here we highlight the wide variety of local chemical ordering (LCO) that sets these concentrated SSs apart from traditional solvent-solute ones. Using atomistic simulations, we reveal that the LCO of the multi-principal-element NiCoCr SS changes with alloy processing conditions, producing a wide range of generalized planar fault energies. We show that the LCO heightens the ruggedness of the energy landscape and raises activation barriers governing dislocation activities. This influences the selection of dislocation pathways in slip, faulting, and twinning, and increases the lattice friction to dislocation motion via a nanoscale segment detrapping mechanism. In contrast, severe plastic deformation reduces the LCO towards random SS. 

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3. Deformation-induced crystalline-to-amorphous phase transformation in a CrMnFeCoNi high-entropy alloy

   https://doi.org/10.1126/sciadv.abe3105  

   Wang, Hao; Chen, Dengke; An, Xianghai; Zhang, Yin; Sun, Shijie; Tian, Yanzhong; Zhang, Zhefeng; Wang, Anguo; Liu, Jinqiao; Song, Min; et al (March 2021, Science Advances)

   null (Ed.)

   The Cantor high-entropy alloy (HEA) of CrMnFeCoNi is a solid solution with a face-centered cubic structure. While plastic deformation in this alloy is usually dominated by dislocation slip and deformation twinning, our in situ straining transmission electron microscopy (TEM) experiments reveal a crystalline-to-amorphous phase transformation in an ultrafine-grained Cantor alloy. We find that the crack-tip structural evolution involves a sequence of formation of the crystalline, lamellar, spotted, and amorphous patterns, which represent different proportions and organizations of the crystalline and amorphous phases. Such solid-state amorphization stems from both the high lattice friction and high grain boundary resistance to dislocation glide in ultrafine-grained microstructures. The resulting increase of crack-tip dislocation densities promotes the buildup of high stresses for triggering the crystalline-to-amorphous transformation. We also observe the formation of amorphous nanobridges in the crack wake. These amorphization processes dissipate strain energies, thereby providing effective toughening mechanisms for HEAs. 

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4. Micropillar compression of single-crystal single-phase (Co, Cu, Mg, Ni, Zn)O

   https://doi.org/10.1063/5.0211263  

   Norman, Jacob_E; Wang, Xin; Dupuy, Alexander_D; Schoenung, Julie_M (July 2024, Applied Physics Letters)

   Bulk, polycrystalline (Co, Cu, Mg, Ni, Zn)O was synthesized using solid-state sintering. Micropillars were prepared and mechanically deformed along three crystallographic orientations: (001), (101), and (111). Pillars (001) and (111) cracked, while Pillar (101) remained intact. Pillars (001) and (101) exhibited activated slip systems, confirmed by a large stress drop, and the presence of slip bands, respectively. Schmid factor (SF) analysis was performed to examine the effect of grain orientations on dislocation activity and slip behavior. SF values range from 0 to 0.5, with non-zero values indicating potential for slip. Six slip systems exist in the (Co, Cu, Mg, Ni, Zn)O rock salt crystal structure: 1/2⟨110⟩11¯0. For the (001) orientation, four slip systems are potentially active (SF = 0.5). For the (101) orientation, there are four potentially active slip systems (SF = 0.25). For the (111) orientation, no potentially active slips systems exist (SF = 0). Dislocation structures, which were observed post-compression via transmission electron microscopy, demonstrated variations in size, number, and distribution across the pillar, depending on micropillar orientation. Entangled dislocations created misorientation in Pillar (001), which led to the possible formation of subgrains, while singular dislocations were observed in Pillar (101), and a lack of dislocations was observed in Pillar (111). Zener–Stroh type dislocation entanglement-mediated cracking is the proposed cause of the transgranular-type cracks in Pillar (001). The possible subgrain formation, or lack of formation, respectively, caused intergranular-type cracks to additionally form in Pillar (001), while Pillar (111) only exhibited transgranular-type brittle fracture. In combination, these findings highlight the importance of dislocation activity, without the need for elevated temperature, and grain orientation in controlling the mechanical deformation response in single-phase (Co, Cu, Mg, Ni, Zn)O. 

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5. Effects of initial {10-12} twins on cyclic deformation and fatigue of magnesium alloy at low strain amplitudes

   https://doi.org/https://doi.org/10.1016/j.matchar.2019.01.017  

   Wang, F.; Lui, M.; Sun, J.; Feng, M.; Jin, L.; Dong, J.; Jiang, Y. (March 2019, Materials characterization)

   An extruded AZ31B (Mg-3Al-1Zn-0.5Mn) magnesium alloy with a twin volume fraction of 60% was subjected to fully reversed strain-controlled tension-compression along the extrusion direction at strain amplitudes ranging from 0.23% to 0.45%. Dislocation slips were the dominant plastic deformation mechanisms without involving persistent twinning-detwinning. At an identical strain amplitude, the fatigue life of the pre-twinned alloy was much lower than that of the as-extruded alloy. Fatigue cracks were mainly initiated on the prismatic or prismatic-basal slip bands in the parent grains. The material volume reduction of the parent grains in the pre-twinned alloy enhanced fatigue damage. Twin cracks were not observed. 

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