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The manufacturing process for wrought Ti alloys with the hexagonal close-packed (HCP) structure introduces a complicated microstructure with abundant intra- and inter-grain boundaries, which greatly influence performance. In the hexagonal close-packed (HCP) structure, two types of grain boundaries are commonly observed between grains with ~90° misorientation: the basal/prismatic boundary (BPB) and the coherent twin boundary (CTB). The mechanical response of the BPB and CTB under external loading was studied through molecular dynamic simulations of HCP-Ti. The results revealed that CTB undergoes transformation into BPB through the accumulation of twin boundary (TB) steps and subsequent emission of Shockley partial dislocations. When the total mismatch vector is close to the Burgers vector of a Shockley partial dislocation, BPB emits partial dislocations and further grows along the stacking faults. When a pair of CTBs are close to each other, severe boundary distortion occurs, facilitating the emission and absorption of partial dislocations, which further assists the CTB-BPB transformation. The present results thus help to explain the frequent observation of coexisting CTB and BPB in HCP alloys and further contribute to the understanding of their microstructure and property regulation. [ABSTRACT FROM AUTHOR]

Ti65 alloy has extensive potential applications in manufacturing high-temperature components in the aerospace industry. During thermomechanical processing in the α+β phase region, complicated microstructure and texture evolution occurs. In this work, isothermal hot compression experiments were conducted to comprehensively investigate the substructure and texture evolution of Ti65 alloy with bimodal microstructure via the electron backscatter diffraction (EBSD) technique, and the influence of processing parameters on the evolution of substructure and texture was systematically analyzed. The experimental results showed that the distribution of grain boundaries and the transformation of low-angle boundaries (LABs) to high-angle boundaries (HABs) exhibited sensitivity to temperature and strain rate. The fraction of sub-grain boundaries decreased, while the fraction of HABs increased with the increasing of strain rate or temperature due to the spheroidization of the lamellar α phase accompanied by the wedging of the β phase during deformation and the relative misorientation angle distribution between different α variants precipitated from β phase. Temperature rising at a high strain rate resulting from adiabatic heat promoted atomic activity, caused dislocations to be absorbed by sub-grain boundaries or HABs, and finally reduced the dislocation density in both α p and α s phases. Dynamic spheroidization was the main mechanism with increasing temperature, and coarsening of the lamellar α phase was the main mechanism with increasing strain rate. With the increase of temperature or decrease of strain rate, the texture of the α p phase became stronger and the basal and pyramidal slip could be activated. For the α s phase, spheroidization and variant selection together influence the orientation distribution. As a result, the texture evolution showed a complicated tendency. • Substructure evolution of a novel near-α Ti65 alloy were studied. • Texture evolution of α p and α s under different conditions was discussed. • Deformation mechanisms were discussed based on the experiments results. • The slip systems activated under different parameters were analyzed. [ABSTRACT FROM AUTHOR]

Polycrystalline La 0.85 Sr 0.15 MnO 3 :Ag x (LSMO:Ag x , 0 ≤ x ≤ 0.5) ceramics were synthesized using sol-gel procedure. All specimens showed rhombohedral perovskite structures in R 3 ‾ c space group. As the amount of Ag increased, grain size of ceramics increased, and resistivity accordingly decreased. Large temperature coefficient of resistivity (TCR) of 22.9% K−1 was obtained at room temperature (289.2 K) for x = 0.4. Interestingly, analyses of resistivity-temperature (ρ - T) experimental data showed that conduction mechanism over entire temperature region perfectly matched theoretical percolation model. This reveals that electrical transport properties of LSMO:Ag x ceramics are mainly affected by domains, grain boundaries (GBs), and Kondo-like spin-dependent scattering. It also demonstrates the coexistence of paramagnetic (FM) and ferromagnetic (PM) phases in LSMO:Ag x ceramics. The ρ-T and scanning electron microscopy (SEM) results confirmed that the introduction of Ag weakened these scattering effects. [ABSTRACT FROM AUTHOR]

Using homemade device, in situ atomic-scale bending experiments were conducted on twinned Ni nanowires. Many in situ atomic-scale observations revealed that the plastic behaviours of twinned Ni nanowires can be significantly affected by the magnitude of the bent strain. In the early stages of bending, their plastic deformation was controlled by dislocations. With increasing bending, fcc-bct phase transitions were observed. When the bent strain increased to a high level, grain boundary resulting from lattice distortion/collapse were detected. This study advances our understanding of the plasticity mechanisms of twinned Ni. Unlabelled Image [ABSTRACT FROM AUTHOR]

Polycrystalline La 0.67 (Ca 0.27 Sr 0.06 )MnO 3 :mol%Ag x (LCSMO:Ag x , x  = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by conventional sol-gel method. Effects of Ag-doping ( x ) on structures, surface morphologies, electrical and magnetic transport properties of the as-prepared LCSMO:Ag x samples were evaluated. X-ray diffraction (XRD) showed that all samples crystallized in orthorhombic structures with Pnma space group. As x content increased, main diffraction peak ( 121 ) shifted downwards lower angles, giving rise to A site ions (Ca 2+ or Sr 2+ ) substitution by Ag + . This, in turn, caused slight expansion of the lattice and improved Mn 4+ ion concentration. Meanwhile, grain size gradually increased and grain boundaries (GBs) reduced, the resistivity decreased and metal-insulator transition temperature ( T MI ) almost kept constant at room temperature. On the other hand, the magnetoresistance ( MR ) and temperature coefficient of resistance ( TCR ) increased during first stages and then decreased as x further rose. An optimum doping molar ratio of x  = 0.2 was obtained, with room temperature MR and TCR reaching up to 44.5% and 22.5% K −1 , respectively. Curve fitting results indicated that in low temperature ferromagnetic metallic (FM) region, conductive behavior of LCSMO:Ag x was mainly related to electron-electron, electron-phonon, and electron-magnon scattering. In high temperature paramagnetic insulating (PI) region, data followed the small-polaron-hopping model. Conductive behavior in the entire temperature region was fitted with phenomenological equation under percolation approach, and found to depend on segregation of FM and PI phases. These findings suggest promising applications of LCSMO:Ag x materials in photoelectric (uncooled bolometer or infrared detectors) or magnetic devices (uncooled magnetic sensors) at room temperature. [ABSTRACT FROM AUTHOR]

Synchrotron X-rays are integrated with a modified Kolsky tension bar to conduct in situ characterization of the grain refinement mechanism operating during the dynamic deformation of metals. Copper with an initial average grain size of 36 μm is refined to 6.3 μm when loaded at a constant high strain rate of 1200 s −1 . Synchrotron measurements revealed the temporal evolution of the grain refinement mechanism in terms of the initiation and rate of refinement throughout the loading test. A multiscale finite element based recrystallization model has been developed to predict the grain size evolution occurring during the dynamic deformation process. The model accurately predicts the initiation and temporal evolution of the refinement phenomenon with a predicted final average grain size of 2.4 μm. [ABSTRACT FROM AUTHOR]

Revealing the ductile deformation mechanisms of ultra-hard brittle cubic silicon carbide (3C-SiC), as well as their correlations with microstructure evolution, are crucial for facilitating the ductile machinability of the ceramic material. In the present work, we report the formation of highly oriented high density stacking faults accompanied with suppressed amorphization and cracking in polycrystalline 3C-SiC in ultrasonic elliptical vibration-assisted diamond cutting, which contributes to significantly enhanced ductile material removal of the ceramic material compared to ordinary cutting. Specifically, characterizations of Raman spectroscopy on machined surface and cross-sectional transmission electron microscopy on subsurface, as well as molecular dynamics simulations of the two kinds of cutting processes, are jointly performed to elucidate the mechanisms of phase transformation and microstructure evolution that govern the ductile material removal of polycrystalline 3C-SiC under the vibration assistance. In particular, the formation mechanisms of highly oriented high density stacking faults emitted from grain boundaries are revealed. Current findings provide insights into the ductile deformation behavior of hard brittle ceramics enhanced by field-assisted deformation process. [ABSTRACT FROM AUTHOR]

The machinability of hard brittle polycrystalline ceramic has a strong correlation with internal microstructures and their accommodated deformation behavior. In the present work, we investigate the mechanisms governing the brittle-to-ductile transition behavior of polycrystalline 3C–SiC in diamond cutting by means of molecular dynamics simulations. Simulation results reveal the co-existence of dislocation slip and amorphization-dominated ductile deformation and cracking along grain boundaries-mediated brittle fracture, as well as the correlation of individual deformation modes with machining force variation and machined surface morphology. In addition, inter-granular fracture, grain boundary sliding and grain pull-up are also operating brittle deformation modes of polycrystalline 3C–SiC. The strong competition between above heterogeneous deformation modes determines the brittle-to-ductile transition behavior in grooving of polycrystalline 3C–SiC. Simulation results also demonstrate that grain size has a strong impact on the brittle-to-ductile transition and material deformation behavior of polycrystalline 3C–SiC under diamond cutting. [ABSTRACT FROM AUTHOR]

The transition of material removal mode of polycrystalline 3C-SiC cutting with temperature, i.e., from brittle mode cutting that is dominated by inter-granular fracture and grain pulling-out at low temperature to ductile mode cutting that is governed by intra-granular cracking and dislocation slip. [Display omitted] • Temperature-dependent mechanical properties of both grains and GBs in polycrystalline 3C-SiC at elevated temperatures are derived. • Temperature-dependent properties of 3C-SiC GBs are bridged between MD simulation and FE simulation with cohesive zone model. • There is transition from brittle mode by inter-granular fracture toductile mode by intra-granular cracking with temperature. While the deformation behavior of grain boundaries has a strong impact on the mechanical response of polycrystalline materials, investigating the coupled thermal–mechanical properties of grain boundaries in their real formats is crucial for enhancing the ductile machinability of hard brittle polycrystalline ceramic materials. In the present work, we report the thermal softening-suppressed inter-granular fracture, accompanied with enhanced stacking fault formation, increased healing ability of grain boundaries and promoted ductile material removal, in diamond cutting of polycrystalline 3C-SiC at elevated temperatures by multi-scale simulations. Molecular dynamics simulations and experiments of high temperature nanoindentations are performed to derive the temperature-dependent mechanical properties of bulk polycrystalline 3C-SiC. Furthermore, molecular dynamics simulations of Mode I (tension) and Mode II (shearing) loading of a Σ9<1 1 0>{1 2 2} symmetric tilt grain boundary at elevated temperatures are performed to extract the temperature-dependent mechanical properties of grain boundaries in polycrystalline 3C-SiC. A novel finite element model of heat treatment-assisted diamond cutting of polycrystalline 3C-SiC with the comprehensive consideration of thermal properties of both grains and grain boundaries at elevated temperatures is established to reveal the transition of material removal mode, grain boundary failure behavior, cutting force characteristics and chip profile with temperature. [ABSTRACT FROM AUTHOR]

Microstructural deformation at the grain level has an inherent impact on the achievable ultimate machining accuracy of polycrystalline materials. In the present work, numerical simulations and experiments of diamond cutting of polycrystalline copper are carried out to investigate the formation of surface step at grain boundaries on machined surface. Single crystal diamond cutting tool with straight cutting edge is chosen for experiments to mimic the tool geometry utilized in 2D crystal plasticity finite element simulations. Moreover, the same crystallography configuration of bi-crystal Cu is employed between experiments and simulations. Formation mechanisms of surface steps at grain boundaries are revealed by finite element simulations and corresponding experimental validation, as well as cross-sectional transmission electron microscope characterization. Finally, finite element simulations of orthogonal cutting of bi-crystal Cu are carried out to examine effects of both extrinsic cutting edge radius of diamond cutting tool and intrinsic misorientation angle of grain boundary on the propensity of grain boundary surface step formation. The present work provides theoretical guidelines on the strategy of suppressing grain boundary surface step formation for achieving superior surface finish of polycrystalline materials by diamond cutting. [ABSTRACT FROM AUTHOR]