Distortion of a poly-crystalline Al bar in a vice fixture: Molecular dynamics analysis of grain movement and rotation

The deformation of a nano-sized polycrystalline Al bar under the action of vice plates is studied using molec- ular dynamics simulation. Two grain sizes are considered; deformation in the fine-grained sample is mainly caused by grain-boundary processes (sliding and rotation), while dislocation plasticity dominates in the coarse- grained sample. We show that the sample distortion is reflected by the center-of-mass motion of the grains. Grain rotation is responsible for surface roughening after the loading process. While the plastic deformation is caused by the loading process, grain rearrangements under load release also contribute considerably to the final sample distortion.

A low-to-high friction transition in gradient nano-grained Cu and Cu-Ag alloys

A unique low-to-high friction transition is observed during unlubricated sliding in metals with a gradient nano-grained (GNG) surface layer. After persisting in the low-friction state (0.2–0.4) for tens of thousands of cycles, the coefficients of friction in the GNG copper (Cu) and copper-silver (Cu-5Ag) alloy start to increase, eventually reaching a high level (0.6–0.8). By monitoring the worn surface morphology evolution, wear-induced damage accumulation, and worn subsurface structure evolution during sliding, we found that the low-to-high friction transition is strongly correlated with distinct microstructural instabilities induced by vertical plastic deformation and wear-off of the stable nanograins in the subsurface layer. A very low wear loss of the GNG samples was achieved compared with the coarse-grained sample, especially during the low friction stage. Our results suggest that it is possible to postpone the initiation of low-to-high friction transitions and enhance the wear resistance in GNG metals by increasing the GNG structural stability against grain coarsening under high loading.

RESEARCH OF THE PROPERTIES OF ZINC OXIDE NANOSTRUCTURES OBTAINED BY PULSED ELECTROCHEMICAL DEPOSITION

The properties of zinc oxide nanostructures allow it to be used in various fields of science and technology. The increased interest in this compound is caused by a rare combination of optical and electrophysical properties. The films of this compound have good piezoelectric and electroluminescent properties, due to which zinc oxide can be used as functional layers in surface acoustic waves, elements of nonlinear optics. This allows the oxide to pass visible radiation. In this paper, we study the influence of the conditions for obtaining a coating by pulsed electrochemical deposition on the forbidden zones and the power of a coating based on zinc oxide. It was shown that the temperature and duty cycle of pulses are of great importance in the formation of the coating, which is explained by the kinetics of the electrochemical reaction. The Urbach energy increases with a decrease in crystalline size and increased by almost 3 times, compared with a coarse-grained sample.

Tribological Behavior of the 316L Stainless Steel with Heterogeneous Lamella Structure

In this paper, the tribological behavior of 316L stainless steel with heterogeneous lamella structure (HLS), prepared through 85% cold rolling technology and subsequent annealing treatment (750 °C, 10 min), were conducted on a ball-on-disc tribometer under different normal loads in dry ambient air conditions. The morphologies, structures, and compositions of the raw and worn surfaces were analyzed by 3D surface profilometer, XRD, SEM, EDS and TEM. Based on this, the results showed that the HLS 316L stainless steel samples exhibited lower and more steady friction coefficients than coarse-grained samples, especially under higher loads, which can be attributed to the existence of numerous oxidative particles across sliding interfaces. However, the wear resistance of HLS 316L stainless steel sample was a little weakened compared to that of the coarse-grained sample under a normal load of 5 N. When the load increases up to 15 N, an obviously decreased wear resistance was found for the HLS of the 316L stainless steel sample, which was 50% lower than that of coarse-grained sample. This can be ascribed to the more severe oxidative and abrasive wear performance of HLS 316L stainless steel sample under dry sliding conditions.

Texture and Boundary Characteristics of Severely Deformed and Recrystallized Copper

Oxygen-free high conductivity copper was subjected to room temperature equal channel angular extrusion of 8 passes using route Bc. The resulting ultra-fine grain copper was then rolled to thickness reductions of up to 96.5% at liquid nitrogen temperatures. Annealed coarse grained copper was rolled to the same strain at room temperature for comparison. Samples from the two routes were isochronally and isothermally annealed, and the microstructure and texture evolution studied by electron back scattered diffraction and x-ray diffraction. Annealing of the ultrafine grained copper led to the development of a strong rotated cube texture from a texture in the rolled material dominated by the Brass component. In contrast the more commonly observed cube texture was found after annealing of the coarse-grained sample. Accompanying the rotated cube texture was the development of a large fraction of boundaries with rotation angle/axis close to 60° <111>.

Damping Characterization of Bulk Nanostructured Nickel Using an Innovative Circle-Fit Approach: Effect of Frequency

In the present study, elemental Ni powder was mechanically milled (MMed) for 10 hours to reduce the grain (crystalline) size in the nano-range (<100nm). The mechanically milled powder (10h-MMed) was consolidated by die-cold compaction and was further hot extruded at high temperatures to maintain a crystallite size within the nano range. Further, the specimen was tested by a novel free-free type suspended beam arrangement, coupled with circle-fit approach to determine damping characteristics. To vary the resonant frequency of the suspended beam, end masses with different weights were added. The characterization results revealed that the nano-size grains exhibit increased damping compared to a coarse-grained sample, under similar vibration frequency. Results also show that the damping capacity of both nano and coarse grained samples decreases with an increase in frequency of vibration. Particular emphasis was placed to correlate the damping capacity with the process induced residual stresses present in the samples.