Surface Step Formation Research Articles

Recrystallization of deformed metal nanoparticles

Despite great advances in the fabrication methods of metal nanoparticles with various sizes and shapes, the available tools for manipulating their microstructure and morphology still remain limited. In the present work we introduce the age-old metallurgical technique of recrystallization to the synthesis of metallic nanoparticles. We uniaxially compressed a large number of single crystalline and defect free Pt nanoparticles obtained by solid-state dewetting and annealed them at the temperature of 1000 °C. Our findings reveal that, while the as-dewetted particles exhibited similar shapes and crystallographic orientations, the plastic deformation and annealing led to a diverse range of shapes, microstructures, and orientations among the particles. We categorized the particles into four distinct types based on their appearance: slanted, terraced, “Danish pastry”, and broken particles. Finally, we propose a differentiation mechanism responsible for this diversification, which relies on a combination of recrystallization, surface step formation, and solid-state dewetting processes.

Mechanical and helium swelling responses of K-RAFM steels to Fe ion irradiation and helium implantation

This study explores the effects of Fe ion irradiation and helium implantation on the mechanical and microstructural behaviors of K-RAFM steels. Through micropillar compression tests, we observed irradiation hardening, which manifested as an increase in strength and a decrease in strain bursts in the irradiated specimens. This change in deformation behavior indicates the impact of radiation-induced defects on dislocation dynamics. Helium implantation resulted in the formation of surface steps and, as shown by nanoindentation mapping, led to a localized increase in hardness, highlighting the significant influence of helium on mechanical properties. Microstructural examination revealed the steel's tempered martensitic structure and pointed to the importance of M23C6 and MX precipitates in modulating the effects of irradiation. Notably, K-RAFM steels showed a diminished extent of irradiation hardening, which can be attributed to the higher fraction of MX precipitates serving as effective trapping sites for point defects. Understanding how microstructural features influence mechanical performance in K-RAFM steels assists in steering the creation of materials tailored to improve radiation resistance in nuclear environments.

Synergistic Effect of He and H on the Surface Swelling of Ion-implanted Ti/Ta-added Reduced Activation Ferritic/Martensitic Steel for Fusion Reactors

Surface swelling caused by He implantation was studied for newly-developed Ti/Ta-added reduced-activation ferritic/martensitic (RAFM) steels. Post-implantation annealing at 623 K for 2 h of He-implanted RAFM-steel samples led to surface-step formation because of surface swelling induced by He bubbles formed inside the samples. Pre-implantation of a small number of H ions was found to cause an increase in the surface-step height, which was not expected. A possible mechanism for this synergistic effect of pre-implanted H is proposed.

Towards an understanding of grain boundary step in diamond cutting of polycrystalline copper

Abstract 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.

Dependence of optimum V/III ratio on substrate orientation, and influence of buffer layer on MOVPE grown InSb/GaSb quantum dots

The dependence of optimum V/III ratio on substrate orientation and the influence of buffer layer on the growth of InSb/GaSb quantum dots (QDs) using atmospheric pressure Metalorganic Vapor Phase Epitaxy (MOVPE) are investigated in this study. The misorientation on the (1 0 0) substrate is suggested to enhance the formation of surface steps, which provides low energy growth sites and hence promotes island nucleation. An increase in V/III ratio from 1.0 to 3.0 resulted in a significant reduction in dot density on both (1 0 0) and (1 1 1) substrates, a phenomenon attributed to the reduced number of indium adatoms available on the growth surface, which simultaneously stimulates a decrease in the migration length of indium species. The shape and size of the dots were observed to be sensitive to the thickness of the buffer layer. Likewise, an increase in buffer layer thickness from 50 nm to ∼300 nm shows the onset of surface corrugation for buffer layer thickness of ∼150 nm, which persisted up to a buffer thickness of ∼250 nm. The corrugated buffer surfaces were observed to introduce order in the arrangement of the dots, which formed preferentially inside the troughs. A reduction in growth rate (from 0.21 to 0.17 nm/s) is presumed to induce sufficient adatom migration and strong bonding between the adsorbate atoms which results in an even buffer layer surface due to an enhancement in surface coverage of adatoms.

Modeling dislocation absorption by surfaces within the framework of strain gradient crystal plasticity

Surfaces play important roles in plastic deformation of crystals at submicron scale by emitting/absorbing dislocations and hence may contribute to the complicated size effect. To take into account the dislocation absorption on the continuum level, an energetic surface model has been proposed by considering the change of surface energy contributed by the formation of surface steps via dislocation absorption. In combination with the conventional higher-order strain gradient theory for grain interior, a new model for single crystal plasticity has been developed. Within the model, there exists a critical threshold for the onset of dislocation absorption by surfaces, alternatively, an independent yield criterion for surfaces and the microscopic boundary conditions can be obtained naturally. As an example of application, plastic behavior of a thin film under plane constrained shear has been studied. Results demonstrate that surface yield strength strongly depends on the film thickness and the orientation of the activated slip system. Comparison with the strain gradient plasticity models under the conventional microscopic boundary conditions, i.e., the microhard and microfree conditions has also been made. It has been found that the plastic behavior is sensitive to the surface boundary conditions.

Onset of ductility and brittleness in silicon nanowires mediated by dislocation nucleation

Most studies show that materials at the nano-scale have different mechanical properties than in the bulk state. Semiconductors like silicon and germanium are brittle in the bulk state, but when their size is reduced to the nano-scale they appear to be ductile. Under tensile loading, we performed molecular dynamics simulations on silicon crystalline nanowires of different lengths. We present the details of the obtained mechanisms that led to ductility and brittleness. In the case of ductility, dislocation nucleation was observed with a signature of surface step formation on the surface and in the case of brittleness a cavity was formed after the distinct formation of a wedge-like shape on the surface. Interestingly, a common mechanism taking place behind ductility and brittleness is dislocation nucleation. We believe that the observed mechanisms reveal interesting information for understanding and explaining the size dependent brittle to ductile transition.

Shock-induced plasticity and the Hugoniot elastic limit in copper nano films and rods

Shock deformation of copper nano-films and nano-rods is examined with Molecular Dynamics (MD) simulations. The influence of the small system size on the onset of plasticity, its origin resulting from the nucleation of dislocation loops, and its reversible nature are determined. While simulations of large systems with periodic boundary conditions indicate that tremendous axial stresses are needed to induce plastic deformation in perfect copper crystals, the present results suggest that the stress levels needed to initiate irreversible plasticity in nano-rods are more than one order of magnitude smaller than what has been reported for bulk single crystals. MD studies of nano-films show that shock waves are purely elastic up until the Hugoniot elastic limit of PHEL ≈ 30–40 GPa, at which point Shockley partial dislocations are internally nucleated at the shock front. However, our recent experiments on shocked nano-rods show that plasticity is evident at much lower axial stress levels, on the order of 1–2 GPa. The present MD simulations of shocked nano-rods show that Shockley partial dislocations prefer to nucleate at lower stresses from the rod surface, at PHEL ≈ 1–2 GPa, consistent with our concurrent experimental observations, leading to surface step formation and mechanical damage. Nucleated dislocations are found to be Shockley partials in the [100] and [111] oriented nano-rods, with the additional presence of perfect dislocations in the latter. MD simulations of rarefaction shock waves in nano-films indicate that they can be spalled via a mechanism of nano-void nucleation, growth and coalescence at the spall plane. The origin of these nano-voids is shown to be at the intersection of stacking faults on conjugate slip {111} planes. Spallation by void nucleation and coalescence is found not to be achievable in nano-rods. Rarefaction shocks with high stresses were found to either severely deform or melt the nano-rod before it can be spalled.

The influence of laser-induced nanosecond rise-time stress waves on the microstructure and surface chemical activity of single crystal Cu nanopillars

An apparatus and test procedure for fabrication and loading of single crystal metal nanopillars under extremely high pressures (>1 GPa) and strain rates (>107 s-1), using laser-generated stress waves, are presented. Single-crystalline Cu pillars (∼1.20 μm in tall and ∼0.45 μm in diameter) prepared via focused ion beam milling of Cu(001) substrates are shock-loaded using this approach with the dilatational stress waves propagating along the [001] axis of the pillars. Transmission electron microscopy observations of shock-loaded pillars show that dislocation density decreases and that their orientation changes with increasing stress wave amplitude, indicative of dislocation motion. The shock-loaded pillars exhibit enhanced chemical reactivity when submerged in oil and isopropyl alcohol solutions, due likely to the exposure of clean surfaces via surface spallation and formation of surface steps and nanoscale facets through dislocation motion to the surface of the pillars, resulting in growth of thin oxide films on the surfaces of the pillars.

Surface effects and the size-dependent hardening and strengthening of nickel micropillars

We evaluate the extent to which two mechanisms contribute to the observed size effect of the ultimate yield strength of micropillars of diameters in the range of 1–30μm: dislocation pile-ups, modeled by means of a physically based non-local single-crystal plasticity model; and the short-range interaction of dislocations with the free surface of the micropillars, e.g., through the formation of surface steps. To this end, we formulate a crystal-plasticity model that accounts for the self-energy of geometrically necessary dislocations and the formation energy of dislocation steps at the boundary of the solid. These two additional sources of energy have the effect of rendering the internal energy of the solid non-local, thereby introducing the possibility of size effects. By way of validation of the model, we simulate the uniaxial compression tests on [269] nickel micropillars of Dimiduk et al. (2005). The calculated dependence of the ultimate strength of the micropillars exhibits strong power-law behavior, and is in good agreement with observation. Our analysis suggests that non-local hardening due to the self-energy of geometrically necessary dislocations does not suffice to account for the observed size effect of the ultimate yield strength of micropillars, and that surface effects, such as resulting from the formation energy of dislocation steps, contribute significantly to that size effect.

Relationship between deformation twinning and surface step formation in AZ31 magnesium alloys

Rolled sheets of AZ31 Mg alloys were deformed at room temperature under tension along the rolling direction. A number of large surface steps were observed in the region near the fractured edge. Transmission electron microscopy showed the presence of twins under these steps. Crystallographic analysis indicated that the twins are of the { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twin type with their basal planes tilted by 37° with respect to the basal planes of the matrix, which is considered to make basal dislocation slip highly active within the twin. The localized deformation within the twins may lead to the formation of crack-like faults and to failure in a macroscopically brittle manner.

Surface treatment for forming unit-cell steps on the (0 0 1) KTaO3 substrate surface

A method for the preparation of unit-cell-high steps on the (100) surface of KTaO3 has been developed. The effects of various surface treatments on the (100) KTaO3 surface, specifically the chemical etching and annealing characteristics, are discussed. Surface step formation was observed for KTaO3 single-crystals that were subjected to buffered HF etching followed by annealing in air. The resulting surface morphology of (001) KTaO3 was examined using atomic force microscopy (AFM). The role of etching time and annealing temperatures in determining the resulting step structure, roughness, and particulate formation on the KTaO3 surface is discussed.

Role of surface step on misfit dislocation nucleation and critical thickness in semiconductor heterostructures

We discuss the role of surface step formation on misfit dislocation nucleation and critical thickness in thin film semiconductor heterostructures. On the basis of an atomistic model, it is shown that the energy change due to step formation is negative or positive depending upon the sign of the misfit. The step formation energy can even be negative for compressive misfit stress in the heterolayer, while it is definitely positive for tensile misfit stress. This conclusion is in contrast to the classical model where the step energy is always positive and independent of the sign of the misfit. The step formation energy influences the critical thickness and the energy barrier for dislocation nucleation. Using a simple atomistic simulation, we show that the critical thickness depends upon the sign of the misfit; for example, it changes from 4 nm fe films on Si(100) substrates to 6 nm for Si films on Ge(100) substrates having the same misfit.

Influence of environment on the mobility of near-surface dislocations in ionic crystals

The environment-sensitive mobility of near-surface edge and screw dislocations around indentations in LiF, MgO, and NaCl has been studied. It is demonstrated that, contrary to recent reports, adsorbed species can markedly affect dislocation mobility in the absence of either surface step formation or significant reductions in surface free energy. The mechanistic significance of this is discussed.