Dislocation Mechanics of Extremely High Rate Deformations in Iron and Tantalum

High strain rate simulations were performed using the multiscale dislocation dynamic plasticity (MDDP) method to calculate different rise times and load durations in mimicking high deformation rate shock or isentropic (ramp) testing of a-iron and tantalum crystals. Focus for both types of loading on both materials was on the inter-relationship between the (dislocation-velocity-related) strain rate sensitivity and the (time-dependent) evolution of dislocation density. The computations are compared with model thermal activation strain rate analysis (TASRA), phonon drag and dislocation generation predictions. The overall comparison of simulated tests and previous experimental measurements shows that the imposition of a rise time even as small as 0.2 ns preceding plastic relaxation via the MDDP method is indicative of relatively weak shock behavior.

Formation of a stable defects structure in silicon noise diodes

vThe possibilities and methods of creating a stable defective structure, including dislocation structure near the zones of p–n-transitions of silicon diodes of noise generators on plates with crystallographic orientations (111) and (001) have been investigated. The effective distribution control of uncontrolled impurities in monocrystalline silicon is achieved by forming a stable dislocation structure in its volume. In order to obtain the reproducible characteristics of noise generator diodes, it is necessary that the dislocation density be homogeneous throughout the plate area. Since the density of dislocations is slightly lower at the edge of the dislocation trail than in the middle, this means that the dislocation traces formed by the adjacent melting zones with the help of a laser beam should overlap. On the basis of experimental studies, it has been established that the necessary degree of uniformity of the density of defects generated is achieved by compliance with the condition of a = (1.5–5.0)d, where a is a step, d is a width of the laser spot on the wafer. The melting process was carried out in a nitrogen environment using a laser hettering unit. The real width of the melting zone turns out to be slightly larger than the diameter of the laser spot due to the thermal conductivity of the silicon and is about 10 μm. Increased dislocation generation on the Si3N4 inclusions, as opposed to dislocations on the Si–SiO2 border, leads to an additional expansion of the dislocation track at the work surface of the plate of noise diodes. The presence of the stable dislocation structure, as well as the presence of impurities and secondary metal atoms in the noise diodes ND 103L structure are confirmed by the secondary ion mass spectroscopy (SIMS) method. The results of the study have been tested at Corporation “INTEGRAL” (Belarus) and can be used in the manufacture of silicon noise diodes.

Revealing thermally-activated nucleation pathways of diffusionless solid-to-solid transition

Solid-to-solid transitions usually occur via athermal nucleation pathways on pre-existing defects due to immense strain energy. However, the extent to which athermal nucleation persists under low strain energy comparable to the interface energy, and whether thermally-activated nucleation is still possible are mostly unknown. To address these questions, the microscopic observation of the transformation dynamics is a prerequisite. Using a charged colloidal system that allows the triggering of an fcc-to-bcc transition while enabling in-situ single-particle-level observation, we experimentally find both athermal and thermally-activated pathways controlled by the softness of the parent crystal. In particular, we reveal three new transition pathways: ingrain homogeneous nucleation driven by spontaneous dislocation generation, heterogeneous nucleation assisted by premelting grain boundaries, and wall-assisted growth. Our findings reveal the physical principles behind the system-dependent pathway selection and shed light on the control of solid-to-solid transitions through the parent phase’s softness and defect landscape.

Rapid fabrication of complex nanostructures using room-temperature ultrasonic nanoimprinting

Despite its advantages of scalable process and cost-effectiveness, nanoimprinting faces challenges with imprinting hard materials (e.g., crystalline metals) at low/room temperatures, and with fabricating complex nanostructures rapidly (e.g., heterojunctions of metal and oxide). Herein, we report a room temperature ultrasonic nanoimprinting technique (named nanojackhammer) to address these challenges. Nanojackhammer capitalizes on the concentration of ultrasonic energy flow at nanoscale to shape bulk materials into nanostructures. Working at room temperature, nanojackhammer allows rapid fabrication of complex multi-compositional nanostructures made of virtually all solid materials regardless of their ductility, hardness, reactivity and melting points. Atomistic simulations reveal a unique alternating dislocation generation and recovery mechanism that significantly reduces the imprinting force under ultrasonic cyclic loading. As a proof-of-concept, a metal-oxide-metal plasmonic nanostructure with built-in nanogap is rapidly fabricated and employed for biosensing. As a fast, scalable, and cost-effective nanotechnology, nanojackhammer will enable various unique applications of complex nanostructures in optoelectronics, biosensing, catalysis and beyond.

Thermal Activation and Ductile vs. Brittle Behavior of Microcracks in 3D BCC Iron Crystals under Biaxial Loading by Atomistic Simulations

We present the results of free 3D molecular dynamics (MD) simulations, focused on the influence of temperature on the ductile-brittle behavior of a pre-existing central Griffith through microcrack (1¯10)[110] (crack plane/crack front) under biaxial loading σA and σB in tension mode I. At temperatures of 300 K and 600 K, the MD results provide new information on the threshold values of the stress intensity factor K and the energy release rate G, needed for the emission of <111>{112} blunting dislocations that support crack stability. A simple procedure for the evaluation of thermal activation from MD results is proposed in the paper. 3D atomistic results are compared with continuum predictions on thermal activation of the crack induced dislocation generation. At elevated temperature T and biaxiality ratios σB/σA ≤ 0.8 dislocation emission in MD is observed, supported by thermal activation energy of about ~30 kBT. With increasing temperature, the ductile-brittle transition moves to a higher biaxiality ratios in comparison with the situation at temperature of ~0 K. Near the transition, dislocation emission occurs at lower loadings than expected by continuum predictions. For the ratios σB/σA ≥ 1, the elevated temperature facilitates (surprisingly) the microcrack growth below Griffith level.