Nanoscale Plastic Deformation Research Articles

Molecular dynamics simulations of ultrasonic vibration-assisted grinding of polycrystalline iron: Nanoscale plastic deformation mechanism and microstructural evolution

Ultrasonic vibration-assisted grinding (UVAG) has attracted plenty of attention to significantly improve surface integrity. However, existing research has not systematically investigated the effect of vibration directions during UVAG at atomic and nanoscales, limiting our understanding of the plastic deformation mechanisms and microstructural evolution of ultra-precision machining. To address this issue, we proposed an MD model for multi-abrasive UVAG of polycrystalline iron considering various vibration directions. We comprehensively analyzed atomic flow fields, surface atom distributions, temperature fields, grinding forces, stress distributions, crystal orientations, and dislocation distributions. The simulation results demonstrate that UVAG effectively reduces grinding force and results in instantaneous grinding forces greater than conventional grinding (CG), and the instantaneous load impact phenomenon of radial vibration is more obvious. In surface morphology, axial vibration contributes to reducing surface roughness, while radial vibration is detrimental to surface smoothness. Microstructural evolution is mainly induced by stress. The primary plastic deformation mechanisms in UVAG of polycrystalline iron involve lattice rotation and dislocation mediation. Compared to axial vibration-assisted grinding (AVAG), radial vibration-assisted grinding (RVAG) and elliptical vibration-assisted grinding (EVAG) exhibit finer grain refinement, with RVAG leading to the highest dislocation density.

Nanoindentation plasticity and loading rate sensitivity of laser additive manufactured functionally graded 316L and 410L stainless steels

The present work is the first to show the effect of processing cooling rate on phase structure and the resulting nano-scale deformation mechanisms in functionally graded stainless steels produced by directed energy deposition. To this end, fabrication by laser additive manufacturing involved implementing a closed-loop feedback control approach to monitor and control the peak temperature of the molten pool and the corresponding cooling rate. Single-track wall structures were produced from 316-L austenitic and 410-L martensitic stainless steels deposited using open-loop conditions with varying processing parameters (laser power and beam scanning speed) to produce graded microstructures. These were then compared to uniformly deposited layers where a constant cooling rate was maintained to generate homogenous microstructures using these two alloys, containing well-aligned columnar austenitic grains and random martensitic packets. Microstructural aspects (grains and crystallographic texture) across different layers (bottom, middle, and top) of the consolidated stainless steel walls (austenitic and martensitic) were compared under various processing conditions (without and with closed-loop or feedback control) and characterized using electron backscattering diffraction analysis. The nano-scale plastic deformation of these austenitic and martensitic stainless steel structures were assessed using nanoindentation testing over a range loading rates from 1 to 50 mN/s. The indentation response using varying strain rates was investigated based on the plastic flow indenting transition behavior at a high loading rate in order to correlate their dependence on microstructural features (predominantly for the martensitic phase). According to the rate sensitivity analyses, the gradient in grain structure (size and morphology) in the case of additively manufactured austenitic microstructure was not sensitive to loading rate, while the martensitic grains had a high local loading rate sensitivity.

Nano-Indentation Hardness and Strain Hardening of Silicon, Sodium Chloride and Tungsten Crystals

BackgroundA previous review of micro- and nano-indentation hardness tests and their analyses gave emphasis to obtaining measurements of continuous nano-indentation load, P, versus, depth, h, recordings that monitor the full elastic–plastic deformation behavior of a localized crystal volume [1].ObjectiveAttention is given to determining the complete, indentation-based, elastic–plastic deformation properties of the local volume, including the initial crystal elastic deformation behavior and, especially to evaluation of post pop-in plastic strain hardening.MethodStress–strain calculations are presented for an initial Hertzian elastic loading and follow-on crystal micro- and nano-scale plastic deformation responses [2].ResultsApplied load, P, dependencies on contact diameters, di, of silicon crystals are compiled on the basis of elastic, plastic and cracking predictions, giving indication at the lowest P values of an indentation size effect (ISE) for the crystal hardness. Elastic–plastic stress–strain curves are presented for sodium chloride and tungsten crystals. The hardness and strain hardening calculations also demonstrate an influence of the ISE.ConclusionsThe exceptional plastic strain hardening behaviors scale dimensionally with corresponding dislocation interactions and sessile reactions within the very localized plastic indentation zones. There is usefulness in determining elastic modulus values from the initial loading record. Micro- and nano-scale dislocation interactions/reactions account for the high stress and strain hardening levels as well as the occurrence of an ISE.

Understanding specimen- and grain-size effects on nanoscale plastic deformation mechanisms and mechanical properties of polycrystalline yttria-stabilized tetragonal zirconia nanopillars

Specimen- and grain-size effects on nanoscale plastic deformation mechanisms and mechanical properties of polycrystalline yttria-stabilized tetragonal zirconia (YSTZ) nanopillars are studied by molecular dynamics simulations. Through uniaxial compression of YSTZ columnar nanopillars, intergranular and transgranular deformation mechanisms are investigated. Cooperative intergranular deformations including grain boundary sliding and migration, grain rotation, and amorphous phase formation at grain boundaries are revealed. Results also reveal formation of partial dislocations, which act as splitters of large grains and play a significant role in facilitating the rotation of grains, and consequently promote amorphous-to-crystalline phase transition in-between neighboring grains. An increase in free surface-to-volume ratio is found to be responsible for specimen size-induced softening phenomenon, where a decrease in Young's modulus and strength is observed when the specimen width decreases from 30 nm to 10 nm. Also, a decrease in Young's modulus and strength is revealed with the decrease of average grain size from 15 nm to 5 nm. Grain boundary density is identified to be responsible for the observed grain size-induced softening behavior in polycrystalline YSTZ nanopillars. A transition in dominant deformation mechanism is observed from amorphous phase formation at grain boundaries to a competition between intergranular grain boundary sliding and transgranular phase transformation. Furthermore, an inverse Hall-Petch relationship is revealed describing the correlation between grain size and strength for polycrystalline YSTZ nanopillars with grain sizes below 15 nm.

Learning local, quenched disorder in plasticity and other crackling noise phenomena

When far from equilibrium, many-body systems display behavior that strongly depends on the initial conditions. A characteristic such example is the phenomenon of plasticity of crystalline and amorphous materials that strongly depends on the material history. In plasticity modeling, the history is captured by a quenched, local and disordered flow stress distribution. While it is this disorder that causes avalanches that are commonly observed during nanoscale plastic deformation, the functional form and scaling properties have remained elusive. In this paper, a generic formalism is developed for deriving local disorder distributions from field-response (e.g., stress/strain) timeseries in models of crackling noise. We demonstrate the efficiency of the method in the hysteretic random-field Ising model and also, models of elastic interface depinning that have been used to model crystalline and amorphous plasticity. We show that the capacity to resolve the quenched disorder distribution improves with the temporal resolution and number of samples.

Nanoscale serration and creep characteristics of Al0.5CoCrCuFeNi high-entropy alloys

Nanoindentation tests were performed to investigate the nano-scale plastic deformation in the Al0.5CoCrCuFeNi high entropy alloys at room temperature (RT) and 200 °C, respectively. Serrated plastic flow, manifested as discrete bursts of plasticity on the load-displacement curves, was observed for both temperatures during the loading period, and its behavior and dependence on the temperature was analyzed from the experimental perspective. Creep occurred during the nanoindentation holding period and exhibited very large stress exponent, implying that the dislocation glide-climb is the dominant mechanism. The creep simulations with a two-layer viscoplastic model further revealed that the deformation at a higher temperature (e.g., 200 °C) featured a greater and faster-growing plastic zone underneath the indenter, implying more pronounced dislocation activities.

Effects of specimen size and yttria concentration on mechanical properties of single crystalline yttria-stabilized tetragonal zirconia nanopillars

The nanoscale plastic deformation of yttria-stabilized tetragonal zirconia (YSTZ) is highly dependent on the crystallographic orientations, i.e., dislocation is induced when the loading direction is 45° tilted to {111} and {101} slip planes, while tetragonal to monoclinic phase transformation dominates the plastic deformation when loading direction is perpendicular to the slip planes. This study investigates the effects of specimen size and yttria concentration on the mechanical response of single crystalline YSTZ nanopillars. Through uniaxial compression test, the smaller-is-stronger phenomenon is revealed in nanopillars deformed through a dislocation motion mechanism. Serrated stacking faults are observed in the smallest nanopillar, while neat primary slip plane forms in the largest nanopillar. In contrast, the larger-is-stronger relation is observed in nanopillars in which deformation is mediated by tetragonal to monoclinic phase transformation. It is noted that the ratio of transformed monoclinic phase to the remaining tetragonal phase is the highest in the smallest nanopillar. The strength of nanopillars is identified to decrease by increasing the amount of yttria due to the creation of more oxygen vacancies that act as weak points to facilitate dislocation motion and accelerate phase transformation.

Nanoscale plastic deformation mechanisms of single crystalline silicon under compression, tension and indentation

Mechanical behavior and underlying nanoscale plastic deformation mechanisms of single crystalline silicon under compression, tension and indentation are investigated through molecular dynamics in this work. Simulation results show that phase transformation from diamond cubic Si-I to [Formula: see text]-Sn is responsible for the plastic deformation behavior of Si both under compression and nanoindentation. A stress plateau is observed when the specimen is compressed uniaxially. Si-I to [Formula: see text]-Sn phase transformation has been demonstrated to be responsible for such stress plateau. Periodic boundary condition is found not suitable to study the tensile strength of silicon pillars. A pop-in behavior is observed in the force–displacement curve of nanoindentation. It has been proved that this pop-in region is induced by Si-I to [Formula: see text]-Sn phase transformation. Through tracking the atom stress, shear stress rather than normal stress is revealed to dominate the phase transformation process. During nanoindentation, to exclude size effect the substrate should be larger enough than the indenter.

Competing mechanisms between dislocation and phase transformation in plastic deformation of single crystalline yttria-stabilized tetragonal zirconia nanopillars

Molecular dynamics (MD) is employed to investigate the plastic deformation mechanisms of single crystalline yttria-stabilized tetragonal zirconia (YSTZ) nanopillars under uniaxial compression. Simulation results show that the nanoscale plastic deformation of YSTZ is strongly dependent on the crystallographic orientation of zirconia nanopillars. For the first time, the experimental explored tetragonal to monoclinic phase transformation is reproduced by MD simulations in some particular loading directions. Three distinct mechanisms of dislocation, phase transformation, and a combination of dislocation and phase transformation are identified when applying compressive loading along different directions. The strength of zirconia nanopillars exhibits a sensitive behavior depending on the failure mechanisms, such that the dislocation-mediated deformation leads to the lowest strength, while the phase transformation-dominated deformation results in the highest strength.

Control of γ lamella precipitation in Ti–39 at.% Al single crystals by nanogroove-induced dislocation bands

We attempted to control γ lamellae precipitation in Ti–39at.% Al single crystals by nanogrooving process (NGP) intending to introduce periodic dislocation bands as preferential nucleation sites of γ-TiAl lamellae precipitates. In NGP, grooves were formed along the trace of (0001) basal plane on the surfaces of single crystal of α2-phase supersaturated with Al-atoms by nano-scale plastic deformation using diamond knives with the edge angle of 60°. The widths of grooves ranged from 180nm to 1860nm for the load per knife-edge length in the range from 0.3N/mm to 8.3N/mm. Beneath the grooves, dislocation bands consisting of both basal dislocations and non-basal dislocations were formed. After aging at 1073K for 1×104s, pairs of γ lamellae were found to form only beneath the grooves to depths greater than 30μm, meaning that the location of γ lamellae could be controlled by NGP. Increasing the aging temperature to 1173K provided even greater control, with arrays of γ lamella bundles exhibiting a periodicity identical to that of the nanogrooves, and even near the surface no γ lamellae were formed in un-deformed regions. No concentration gradient was found within individual γ lamellae, which had a near-equilibrium Al concentration; thinner γ lamellae tended to have a slightly lower Al concentration. These results indicate that the precipitation of γ lamellae can be effectively controlled by introducing periodic dislocation bands using NGP, which opens new avenue for the manufacturing of near- or sub-wavelength grating structures with ultra-high aspect ratios through the selective dissolution of these γ lamellae.

Nanoplastic deformation on Ti–39 at.% Al single crystals for manipulation of every single γ lamella

The effects of nanoscale plastic deformation, produced by nanoimprinting, nanoindentation and nanogrooving, on the formation of periodic dislocation bands in Ti–39 Al(at.%) single crystals were investigated, with a focus on the formation of periodic, parallel basal dislocations. Slip traces of both basal and prism slips were produced on the nanoindented crystal’s surface, while only basal slip traces were formed on the surfaces of the nanoimprinted and nanogrooved crystals. A dislocation band, consisting of basal dislocations with relatively high density and non-basal dislocation tangles with exceedingly high density, was produced beneath the nanogrooved trench. However, low-density basal and non-basal dislocations were produced beneath the nanoimprinted indentations. The plastic zones formed were relatively narrow; moreover, non-basal slip traces or dislocations were connected to the basal slip traces and dislocations. The reasons for these formations were examined based on the motion of dislocations affected by antiphase domain boundaries as well as the cross-slip of prism dislocations onto the (0001) basal planes. According to the dislocation band characteristics, nanogrooving is the most appropriate of the three processes for producing periodic basal dislocation bands with tunable intervals. Furthermore, a pair of long and thin γ lamellae was formed along the basal plane after annealing of the nanogrooved crystals; they were ∼45μm deep from the groove with a 500nm interval, and the thickness of each γ lamella was <100nm. The results suggest that γ precipitation as a laminated periodic structure can be controlled by nanogrooving-induced plastic deformation.

Nanoscale plastic deformation mechanism in single crystal aragonite

Molecular dynamics simulations have been performed to study the dynamic behaviors of single crystal aragonite under indentation, tension, and compression. The elastic modulus and hardness of single crystalline aragonite measured in our simulations are found in good agreement with experimentally measured values. Our simulation results show that the mechanical properties of aragonite crystal, including the elastic modulus, hardness, strength, and toughness, strongly depend on the crystallographic orientations and loading conditions. We have identified that this dependence is resulted from different deformation mechanisms, i.e., phase transformation, amorphous phase formation, dislocation, and twining. This work is an attempt to identify the deformation mechanisms in aragonite and to establish the relationship between the dominant deformation mechanisms and its crystallographic orientations and loading conditions.

Deformation-induced change in the structure of metallic glasses during multistep indentation

The perplexing controversy of whether deformation-induced hardening/softening is intrinsic or extrinsic remains an outstanding fundamental problem in the low-temperature rheology and dynamics of metallic glasses. In this Brief Report, by manipulating nanoscale plastic deformation in a multistep instrumented indentation process, we in situ characterize and definitively elucidate the deformation-driven structural changes in metallic glasses via their strengthening or softening effect on consecutive mechanical behavior.

Characterization of Nanometer Step Structure Formation During the Fabrication of Large-Scale Superconducting-Tunnel-Junction Array Detectors

Superconducting-tunnel-junction (STJ) array detectors with an effective sensitive area of 4 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were fabricated for mass spectrometry. An array detector has one hundred Nb/Al/AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Al/Nb junctions with a 200 mum-square sensitive area. The junctions are quasi-horizontally arranged on a 10 mm-square chip. In the course of fabrication processes, we have found that an anomalous nanometer-step structure appears within the junctions located in the outer region of the array detectors at a specific stage of the fabrication processes. The anomalous steps cause an increase of the leakage current from ~5 nA to a few 100 nA. In order to characterize and prevent the step structure formation, we performed structural analyses with atomic force microscopy, interference microscopy, differential interference contrast microscopy, and transmission electron microscopy. Vertical fault and blister formations were identified in the junctions around the periphery of the array arrangement even on a low stress sputtering condition of 40 MPa. In addition, it is reasonable that a horizontal slip in the AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> barrier layer is induced by a nanoscale plastic deformation, which causes the large increase of the leakage current.

Nanoscale Shape-Memory Function in Highly Cross-Linked Polymers

Topographic engraving of structures in polymer surfaces attracts widespread interest for application in imprint lithography and data storage. We study the nonlinear interaction of nanoindents written in close proximity, 20-100 nm, to one another in a highly cross-linked polystyrene matrix. The indents are created thermomechanically by applying heat and force stimuli of 10 micros duration to a tip, thereby raising the polymer temperature to 250 degrees C and exerting contact pressures of up to 1 GPa. We show that on the nanoscale plastic deformation is highly reversible providing outstanding shape-memory functionality of the material.

Nanoscale plastic deformation and fracture of polymers studied by in situ nanoindentation in a transmission electron microscope

Nanoscale plasticity and fracture of polymers were studied by in situ nanoindentation in a transmission electron microscope. The indentation-induced plasticity during loading and the significant recovery of the deformed polymer upon unloading provide insight into the nanoscale viscoelastic and plastic behavior. Crack initiation and growth from a preexisting surface nanodefect due to adjacent nanoindentation was found to lead to localized delamination. The in situ observations of this study provide information for the evolution of nanoscale viscoelastic-plastic deformation and fracture (delamination) processes at polymer surfaces during the loading and unloading phases of a nanoindentation cycle.

Atomistic Analysis of Nano-Scale Crystal Plasticity in Thin Metal Films

ABSTRACTNumerical simulations based on molecular statics are carried out to study nano-scale plastic deformation behavior in thin metal films. Particular attention is devoted to correlating the overall mechanical response and the underlying crystal defect mechanisms during mechanical loading. The simulations are within the two-dimensional framework involving pair molecular interactions in singlecrystal materials. Special modeling features are utilized for studying the formation of dislocations, interface characteristics, and defect interactions. Specific problems investigated in this work include: plastic deformation and tensile fracture in a free-standing film, interface-constrained plasticity in substrate-bonded films, and homogeneous nucleation of dislocations during nanoindentation.