Nanopillar Compression Research Articles

Dislocation plasticity in c$c$‐axis nanopillar compression of wurtzite ceramics: A study using neural network potentials

AbstractCeramics typically exhibit brittle characteristics at room temperature. However, under high compressive stress conditions, such as during nanoindentation or compression tests on nanopillars, these materials can reach the necessary shear stress for dislocation nucleation and glide before fracture occurs. This allows for the observation of plastic deformation through dislocations even at room temperature. Yet, detailed insights into their atomic‐scale dislocation plasticity remain scarce. This study employs atomistic simulations to explore the dislocation plasticity in ‐oriented nanopillars of wurtzite ceramics (i.e., GaN and ZnO) under uniaxial [0001] compression, utilizing a specially developed first‐principles neural network interatomic potential. We observed activation of and dislocations, corroborating experimental findings from in‐situ compression tests. Moreover, a wurtzite‐to‐hexagonal phase transformation begins at facet edges and extends inward, forming wedge‐shaped regions. As compression progresses, dislocations nucleate at the wurtzite‐hexagonal phase boundary and spread throughout the nanopillar, contributing to a rougher surface texture. These quantitative results offer new insights into the plastic deformation behaviors of nanopillars under compressive loading, highlighting the potential of dislocation engineering in these ceramics.

Neural network potential for dislocation plasticity in ceramics

Dislocations in ceramics are increasingly recognized for their promising potential in applications such as toughening intrinsically brittle ceramics and tailoring functional properties. However, the atomistic simulation of dislocation plasticity in ceramics remains challenging due to the complex interatomic interactions characteristic of ceramics, which include a mix of ionic and covalent bonds, and highly distorted and extensive dislocation core structures within complex crystal structures. These complexities exceed the capabilities of empirical interatomic potentials. Therefore, constructing neural network potentials (NNPs) emerges as the optimal solution. Yet, creating a training dataset that includes dislocation structures proves difficult due to the complexity of their core configurations in ceramics and the computational demands of density functional theory for large atomic models containing dislocation cores. In this work, we propose a training dataset from properties that are easier to compute via high-throughput calculation. Using this dataset, we have successfully developed NNPs for dislocation plasticity in ceramics, specifically for three typical functional ceramics: ZnO, GaN, and SrTiO3. These NNPs effectively capture the nonstoichiometric and charged core structures and slip barriers of dislocations, as well as the long-range electrostatic interactions between charged dislocations. The effectiveness of this dataset was further validated by measuring the similarity and uncertainty across snapshots derived from large-scale simulations, alongside extensive validation across various properties. Utilizing the constructed NNPs, we examined dislocation plasticity in ceramics through nanopillar compression and nanoindentation, which demonstrated excellent agreement with experimental observations. This study provides an effective framework for constructing NNPs that enable the detailed atomistic modeling of dislocation plasticity, opening new avenues for exploring the plastic behavior of ceramics.

Atomistic investigation into the formation of axial weak twins during the compression of single-crystal Mg nanopillars

Molecular dynamics simulations are performed to provide a detailed atomic-level understanding of the deformation and twinning behavior of single-crystal Mg nanopillars under [0001] and [011¯0] compressions. To that end, a new interatomic potential based on the second nearest-neighbor modified embedded-atom method is developed to improve the reproducibility of overall physical properties, particularly in relation to plastic deformation. Further nanopillar compression analysis reveals that the simulation based on the developed potential satisfactorily reproduces the experimentally observed slip and twinning phenomena, consistent with theoretical interpretations. The present simulation results provide visual evidence for differentiated deformation characteristics of single-crystal Mg in different loading orientations and for the detailed nucleation and growth mechanisms of the recently discovered unconventional twins known as “axial weak twins” that exhibit 90° and 62° orientation relationships with the parent matrix. Our investigation reveals that the formation of both weak twins is commonly associated with atomic shuffling in the high-stress state, and the nucleation of the 62° weak twin is facilitated by pyramidal I dislocations.

Crystal plasticity modeling for the strengthening effect of multilayered copper–graphene nanocomposites

The paper explores plastic deformation mechanisms in metal–graphene nanocomposites, illustrating their material strengthening effects through a crystal plasticity finite element (CPFE) model. This model is compared with published experimental results, which have previously shown that the two-dimensional shape of graphene effectively controls dislocation motion and significantly enhances the strength of metals. Simulated nanopillar compression tests, using a physics-based CP model that incorporates surface nucleation and single-arm source dislocation mechanisms, help us understand dislocation motions at submicron length scales. The crystal plasticity models are applied to nanolayered composites with copper grain layers and monolayer graphene, featuring repeat layer spacings of 200 nm, 125 nm, and 70 nm, respectively. This study quantifies the accumulation of dislocations at the graphene interfaces, contributing to the ultra-high strength of copper–graphene composites. Moreover, a Hall–Petch-like correlation is established between yield strength and the number of embedded graphene layers.

Transition in helium bubble strengthening of copper from quasi-static to dynamic deformation

Damage from low-temperature irradiation and the subsequent degradation of materials performance pose significant challenges for the storage of radioactive materials and for peripheral components in some nuclear reactor designs. Fully understanding the mechanical behavior of such materials requires test data for strain rates in both the quasi-static (< 10/s) and dynamic (≫ 10/s) regimes. While dynamic testing has generally been avoided in the past for neutron irradiated (contamination concerns) and ion irradiated (insufficient volume) materials, surface-sensitive Richtmyer-Meshkov instability (RMI) tests were used in the present work to overcome these limitations. Here, nanopillar compression, nanoindentation, and RMI testing data from a helium implanted surface layer (∼10 µm thick) were compiled to explore the effects of helium bubbles on the materials strength of high-purity copper at strain rates of 0.001/s – 108/s. While nano-mechanical testing revealed increases in yield strength and hardness with increasing helium dose from 1000 to 4000 appm He, RMI indicated no significant changes in strength as compared to unimplanted copper. This discrepancy in behavior was rationalized through a combination of recent literature and follow-on molecular dynamics (MD) simulations, leading to the conclusion that the nanoscale helium bubbles acting as dispersed barriers to dislocation motion at quasi-static strain rates collapse under shock loading and cease to be effective barriers at high strain rates.

Stability and strengthening effect of aging induced-nanofeatures in δ-ferrite in an austenitic stainless-steel weld

The deformation behavior of δ-ferrite in an aged austenitic stainless-steel weld was investigated to obtain insights on the contribution of aging-induced nanofeatures, such as spinodal decomposition and G-phase precipitation, to aging embrittlement. To evaluate the hardening effect of the G-phase, a reversion heat treatment was applied to the aged weld. The strengthening effect of spinodal decomposition and the G-phase was measured using nanopillar compression tests. The deformed microstructure exhibited the dissolution of G-phases but a slight change in the spinodal nanostructure, indicating a clear difference in the stability of the nanofeatures during deformation. The quantitative measurement of the G-phase and the application of the Orowan model showed that the measured strengthening effect from the G-phase was less than that of the estimation, implying that the G-phase was not as effective as an impenetrable obstacle. On the other hand, spinodal decomposition showed high stability after deformation, leading to a noticeable hardening effect.

Evaluation of thermal aging activation energies based on multi-scale mechanical property tests for an austenitic stainless steel weld beads

The long-term thermal aging behavior of a 316L austenitic stainless steel welds containing ∼12% of δ-ferrite was investigated using the nano-scale (nanopillar compression), micro-scale (micro-hardness and small punch), and macro-scale (tensile and J-R) mechanical property testing methods. Specimens were aged at 343, 375, and 400 °C for up to 20,000 h. The thermal aging activation energies were estimated based on the various mechanical test results using two fitting methods. The activation energies vary from 124 to 300 kJ/mol, depending on fitting and mechanical testing methods. Among the mechanical properties, the nanopillar and J-R test results showed similar activation energies lower than those from other mechanical properties. The similarities and discrepancies among the estimated activation energies were discussed in view of the contribution of embrittled δ-ferrite to the fracture and deformation of test specimens.

Evidence for a High Temperature Whisker Growth Mechanism Active in Tungsten during In Situ Nanopillar Compression.

A series of nanopillar compression tests were performed on tungsten as a function of temperature using in situ transmission electron microscopy with localized laser heating. Surface oxidation was observed to form on the pillars and grow in thickness with increasing temperature. Deformation between 850 °C and 1120 °C is facilitated by long-range diffusional transport from the tungsten pillar onto adjacent regions of the Y2O3-stabilized ZrO2 indenter. The constraint imposed by the surface oxidation is hypothesized to underly this mechanism for localized plasticity, which is generally the so-called whisker growth mechanism. The results are discussed in context of the tungsten fuzz growth mechanism in He plasma-facing environments. The two processes exhibit similar morphological features and the conditions under which fuzz evolves appear to satisfy the conditions necessary to induce whisker growth.

Improved mechanical properties of W-Zr-B coatings deposited by hybrid RF magnetron – PLD method

In this work, novel W-Zr-B coatings were developed by a hybrid process combining pulsed laser deposited ZrB2 and radio frequency magnetron sputtered W2B5. The influence of the laser power density on the structure and mechanical properties of the deposited films was studied. Addition of zirconium causes a change in the structure of the deposited films from columnar to mainly amorphous. The nanoindentation tests and compression of nanopillars showed that doped W-Zr-B layers are still super-hard and incompressible in comparison to WB2 films without doping, but they change their behaviour from brittle to ductile. Films obtained with a fluence of 1.06 J/cm2 are superhard (H = 40 ± 4 GPa) and incompressible (12 ± 1 GPa), but possess a relatively low Young’s modulus (E = 330 ± 32 GPa) and a high elastic recovery (We = 0.9). Further increase in the fluence causes films to consist of deeply embedded fragments of laser ablated ZrB2 target in the deposited layer. Taking into account that the particles are made of ZrB2 which possess extraordinary thermal properties, and the matrix is made of W-Zr-B, a super-hard material, such a composite can also be interesting for industrial applications.

Evaluating the effects of pillar shape and gallium ion beam damage on the mechanical properties of single crystal aluminum nanopillars

In situ TEM nanopillar compression experiments are widely used to study the mechanical behavior of nanoscale materials. Often, the pillars are fabricated using gallium-focused ion beam (FIB) milling from a bulk sample. During the FIB process, the choice of the pillar shape and the energy of the Ga ions can significantly impact the mechanical performance of samples with electron-transparent dimensions. Here, we systematically explore the effects of various pillar fabrication parameters in a single crystal aluminum (Al) system with a well-controlled crystal orientation. A novel method is proposed to fabricate square pillars to minimize FIB artifacts such as tapering, high pillar base compliance, and preferential deformation at the pillar tip. These square pillars enable more uniform deformation and accurate measurement of the engineering strain. Lastly, we show an intriguing in situ TEM laser irradiation experiment, which has enabled direct visualization of the surface oxide layer in FIB-fabricated Al pillars.

Effect of crystal orientation on the size effects of nano-scale fcc metals

The present work investigates the dominant mechanisms in the plasticity of nano-sized fcc metallic samples. Molecular dynamics simulations of nanopillar compression show that plasticity always starts with the nucleation of dislocations at the free surface, and the crystal orientation affects the subsequent microstructural evolution. The Schmid factor of leading and trailing partials plays a decisive role in leading to the twinning, or slip deformation. A significant difference is observed in the strength of pillars of the same size with different orientations. The power-law equation exponent is completely dependent on the crystal orientations, and a weak or no size effect is observed in the compression of [100]- and [110]-oriented nanopillars. The observed orientation based behaviour decreases by confining the free surface.

Dislocation-graphene interactions in Cu/graphene composites and the effect of boundary conditions: a molecular dynamics study

In this paper, systematic molecular dynamics simulations were employed to study dislocation-graphene interactions in Cu/graphene systems using two carefully selected configurations: (i) edge dislocations in the pileup interacting with graphene during nanoindentation and (ii) complex dislocations interacting with graphene in compression of layered nanopillars. The intrinsic and extrinsic size effects were investigated with respect to varying the Cu lamella thickness and pillar sizes, revealing an anomalous extrinsic size effect: the smaller, the weaker. To understand which boundary conditions are physically consistent, both free and periodic graphene were considered. It was seen that edge dislocations in the pileup continuously transmitted across the free graphene through Cu/graphene interfacial sliding and graphene reorientation, whereas transmission was very difficult for periodic graphene as its in-plane deformation was required. In compression of Cu/graphene nanopillars, the strengthening effect of free graphene was found to be less obvious than that of the periodic case, which could be attributed to free graphene edges acting as dislocation sources after the compressed Cu overflowed the graphene sheet from all directions. We therefore conclude that the strengthening effect of periodic graphene inclusions appears to be overestimated, such that free graphene should be more appropriate.

Breaking the elastic limit of piezoelectric ceramics using nanostructures: A case study using ZnO

Piezoelectric materials are suitable for haptic technology as they can convert mechanical stimuli into electrical signals and vice-versa. However, owing to their disadvantageous mechanical properties such as brittleness (in ceramics) and a low piezoelectric coefficient (in polymers), their application in haptic technology remains challenging. In this paper, we introduce a truss-like 3D hollow nanostructure using zinc oxide (ZnO) that exhibits a drastically improved elastic strain limit while maintaining a piezoelectric coefficient similar to that of single-crystal ZnO. The ZnO hollow nanostructure was fabricated using proximity field nanopatterning (PnP) and atomic layer deposition (ALD) at four different processing temperatures. The piezoelectric characteristics were analyzed through dual AC resonance tracking piezoresponse force microscopy (PFM), and the piezoelectric coefficient was measured to be up to 9.2 pm/V. The nanopillar compression test result showed that the measured elastic strain limit of approximately 10% was at least 3 times greater than the previously reported value. The extended elastic limit of the 3D hollow structure was further supported by finite element simulations. The ZnO hollow nanostructure shows excellent potential for its application to enhanced haptic devices, which mimic the human sense of touch.

Evaluation of thermal ageing activation energy of δ-ferrite in an austenitic stainless steel weld using nanopillar compression test

Nanopillar compression tests were applied on δ-ferrites in austenitic stainless steel welds to measure the thermal ageing activation energy caused by spinodal decomposition. Welds of an austenitic stainless steel were thermally aged at 343, 375, and 400 oC for up to 20,000 h. To avoid the interference of surrounding austenite matrix on embrittlement measurement, austenite matrix was selectively dissolved before the nanopillar fabrication. Using the yield stress changes measured by nanopillar compression tests, the thermal ageing activation energy of δ-ferrite was estimated as 154 KJ/mol, which was compared with literature results and the reasons of discrepancy were discussed.

Synthesis and Mechanical Characterization of a CuMoTaWV High-Entropy Film by Magnetron Sputtering.

Development of high-entropy alloy (HEA) films is a promising and cost-effective way to incorporate these materials of superior properties in harsh environments. In this work, a refractory high-entropy alloy (RHEA) film of equimolar CuMoTaWV was deposited on silicon and 304 stainless-steel substrates using DC-magnetron sputtering. A sputtering target was developed by partial sintering of an equimolar powder mixture of Cu, Mo, Ta, W, and V using spark plasma sintering. The target was used to sputter a nanocrystalline RHEA film with a thickness of ∼900 nm and an average grain size of 18 nm. X-ray diffraction of the film revealed a body-centered cubic solid solution with preferred orientation in the (110) directional plane. The nanocrystalline nature of the RHEA film resulted in a hardness of 19 ± 2.3 GPa and an elastic modulus of 259 ± 19.2 GPa. A high compressive strength of 10 ± 0.8 GPa was obtained in nanopillar compression due to solid solution hardening and grain boundary strengthening. The adhesion between the RHEA film and 304 stainless-steel substrates was increased on annealing. For the wear test against the E52100 alloy steel (Grade 25, 700–880 HV) at 1 N load, the RHEA film showed an average coefficient of friction (COF) and wear rate of 0.25 (RT) and 1.5 (300 °C), and 6.4 × 10–6 mm3/N m (RT) and 2.5 × 10–5 mm3/N m (300 °C), respectively. The COF was found to be 2 times lower at RT and wear rate 102 times lower at RT and 300 °C than those of 304 stainless steel. This study may lead to the processing of high-entropy alloy films for large-scale industrial applications.

Evaluation of Thermal Ageing Activation Energy of Δ-Ferrite in an Austenitic Stainless Steel Weld Using Nanopillar Compression Test

Nanopillar compression tests were applied on δ-ferrites in austenitic stainless steel welds to measure the thermal ageing activation energy caused by spinodal decomposition. Welds of an austenitic stainless steel were thermally aged at 343, 375, and 400 oC for up to 20,000 h. To avoid the interference of surrounding austenite matrix on embrittlement measurement, austenite matrix was selectively dissolved before the nanopillar fabrication. Using the yield stress changes measured by nanopillar compression tests, the thermal ageing activation energy of δ-ferrite was estimated as 154 KJ/mol, which was compared with literature results and the reasons of discrepancy were discussed.

Size-induced room temperature softening of nanocrystalline yttria stabilized zirconia

Nanocrystalline oxides exhibit exceptional resistance to plastic deformation, manifesting increased strength and hardness with reduced grain size that qualitatively follows the so-called Hall-Petch relationships. However, below a critical grain size, softening has been observed to occur, in the so-called inverse Hall-Petch regime. The mechanisms underlying these phenomena are still not well understood in oxides. Here we observe, using nanopillar compression, that the yield strength initially increases with decreasing grain size for yttria-stabilized zirconia ceramics produced by high-pressure spark plasma sintering. A hardening-to-softening transition occurs at grain sizes below ≈21 nm. The experiments indicate that this transition depends on strain rate, and the onset of the decrease in yield strength occurs before any shear fracture begins. Nanopillar compression combined with in situ electron diffraction demonstrates the onset of softening coincides with an increase in the amount of crystallographic rotation per unit strain, suggesting a change in deformation mechanism.

Surface flaws control strain localization in the deformation of Cu|Au nanolaminate pillars

The authors carried out matched experiments and molecular dynamics simulations of the compression of nanopillars prepared from Cu|Au nanolaminates with up to 25 nm layer thickness. The stress–strain behaviors obtained from both techniques are in excellent agreement. Variation in the layer thickness reveals an increase in the strength with a decreasing layer thickness. Pillars fail through the formation of shear bands whose nucleation they trace back to the existence of surface flaws. This combined approach demonstrates the crucial role of contact geometry in controlling the deformation mode and suggests that modulus-matched nanolaminates should be able to suppress strain localization while maintaining controllable strength.

Size Effect in the Uniaxial Compression of Polycrystalline Ni Nanopillars with Small Number of Grains

Molecular dynamics (MD) simulations are used to investigate the compression of nickel nanopillars in the ranges from 3 to 18 nm grain size (d) and from 12 to 30 nm specimen size (D). The results reveal that grain size play a more significant role than specimen size. There is a strong grain size effect—smaller is weaker—on the yield as well as the flow stress. The deformation is mainly governed by grain boundary (GB) motion for d < 12 nm, while for $$ d \ge 12\;{\text{nm}} $$ , it is governed by dislocation activity. When the grain size is small enough, the deformation of nanopillar is specimen size independent. For larger grain sizes, an irregular fluctuation in flow stress is observed resulting from sensitivity to grain orientation, fracture, and coalescence. Extended dislocations and twins play important roles in grain-shape evolution during compression. Due to the high fraction of surface grains, the “theory based on GB-shear and surface layer” is found inapplicable to explain the size effect in the plasticity of specimens with small number of grains across their diameter. Consequently, an expression of flow stress is proposed that includes effects of grain and specimen sizes, and the specimen-to-grain size ratios.

An overview of interface-dominated deformation mechanisms in metallic nanocomposites elucidated using in situ straining in a TEM

Abstract