Pillar Compression Research Articles

A New Insight into the Mechanical Behavior of Llzo through Experimental Pillar Compression and Molecular Dynamic Modeling

The transition to safe, high-energy-dense battery designs assumes the implementation of an all-solid-state lithium metal design. Solid electrolytes commonly short due to mechanical failure, where lithium dendrites will protrude into the surface until the anode and cathode are electrically connected. This phenomenon can be suppressed through materials engineering, i.e. designing solid electrolytes with specific mechanical properties. This work, for the first time, presents the measured compressive fracture strength of the oxide-based solid electrolyte. Fracture strength was measured using pillar compression with a flat punch tip and in-situ SEM imaging. Experimental validation of the measured fracture strength was completed using molecular dynamic modeling (MD), exploring the relationship of localized porosity to mechanical properties. Milled pillars (3 μm diameter) in the LLZO solid electrolyte show a linear relationship between the compressive yield strength and Young’s modulus, showing defects (porosity, cracks, voids, impurities, etc.) have a strong relationship on the ceramic’s physical response. MD modeling was used to introduce porosity to show the localized effect of porosity on the Young’s modulus and yield strength, providing strong evidence that localized porosity in the pillars may account for the substantial reduction in compressive yield strength. These insights shed light on the mechanical properties and electrochemical performance of LLZO, offering valuable guidance for the design of high-performance solid-state batteries. Figure 1

Micro-pillar compression of proton-irradiated chromium examined using cross-sectional site selection, electron microscopy, and molecular dynamics simulation

Deformation of proton-irradiated chromium was investigated using micro-pillar compression and electron microscopy. After 2 MeV proton irradiation at 350 °C, four micro-pillars were prepared from a single grain on the polished specimen cross section. Depending on the distance away from the irradiated surface, hardness as a function of local damage level was studied. All pillars developed a narrow deformation band on one set of near-adjacent {110} planes, arising from closely-positioned parallel gliding. The critical resolved shear stress for gliding along 〈111〉/{110} was measured to be 59.6 MPa in unirradiated material beyond the proton range. The critical stress increased by 20 % after 0.5 dpa, and by 58 % after 1 dpa, with saturation of hardening occurring by 0.7 dpa. Post-compression characterization using transmission electron microscopy showed extensive formation of nanometer size voids in a matrix dominated by tangled dislocations. No twinning was observed. The experimental observations are in good agreement with molecular dynamics simulation of pillar compression of chromium, showing dislocation gliding along 〈111〉/{110} and 〈111〉/{112}. The continued stability of chromium for LWR application requires extension of the exposure level from 1 dpa to ∼15 dpa expected for typical fuel pin exposure.

Unraveling factors affecting the reversibility of martensitic phase transformation in FeNiCoAlTi shape memory alloys: Insights from HR-EBSD and acoustic emission analysis

The reversibility of the martensitic phase transformation is crucial for the functionality of shape memory alloys. The easier the martensite is pinned, the faster occurs the degradation of the shape memory effect during cyclic loading. This study investigates various factors influencing the stabilization of martensite during deformation of single-crystalline states of the FeNiCoAlTi system. In order to investigate early pinning mechanisms in more detail a 〈011〉 single crystal with expected lower reversibility of the martensitic phase transformation and, consequently, lower superelasticity was chosen. Complementary in situ methods, such as digital image correlation and acoustic emission, were used to characterize the evolution of the martensitic phase transformation and the role of dislocations during deformation. Thereby, experimental evidence was elaborated pointing at mechanisms that so far have been underrated in terms of functional degradation. Beside interaction of martensitic variants and dislocation activity, detwinning, associated with high acoustic energy, was identified as an additional significant factor contributing to reduced reversibility and superelasticity. In addition, high-resolution electron backscatter diffraction measurements were applied for the first time to identify residual stresses in the austenitic matrix, which significantly contribute to the reverse transformation. Micro-mechanical experiments using pillar compression tests were carried out to study the influence of residual back stresses of the austenitic matrix on the reversibility of the martensitic phase transformation. A reduced reversibility was found in the case of the absence of back stresses. The findings from this study foster the understanding of pinning mechanisms during loading of the FeNiCoAlTi shape memory alloy eventually enabling targeted optimization for enhanced superelastic material behavior.

Observation of Grain Boundary Sliding in a Lamellar Ultrafine‐Grained Steel

The deformation behavior of a ferrite steel with ultrafine‐grained (UFG) lamellar microstructure generated by linear flow splitting is investigated and compared to a coarser cold‐worked reference state, using a set of complementary local deformation and microstructural characterizations methods. The pile‐up around indentations shows a pronounced anisotropy for the UFG lamellar microstructure indicating the relative motion of grains along their elongated boundaries. This observation is confirmed by stepwise compression testing of micropillars along the normal direction of lamellar‐shaped grains using a new faceted pillar geometry to image the initial microstructure and its evolution throughout the test. The surface roughening in pillar compression testing can be categorized into the formation of discrete steps at the surface along particular grain boundaries and a more gradual roughening that is attributed to intragranular dislocation slip. Potential mechanisms for the observed grain boundary sliding are discussed taking several factors such as the strain rate sensitivity and potential Coble creep rates into account. In conclusion, a grain boundary sliding process carried by grain boundary dislocations appears to be the most likely explanation for the observed behavior.

Micromechanical response of an electrodeposited NiP metallic glass by pillar compression under extreme conditions

In this work, the mechanical properties of pulse electrodeposited amorphous nickel phosphorous (NiP), with ∼16 wt% P, was investigated for an improved understanding of the deformation behavior of metallic glasses at extreme conditions, particularly at the microscale. Upon decreasing the temperature to sub-ambient, both the yield strength and the shear band density increase. While the change of yield strength with temperature can be well explained by a cooperative shear model, the increase in the number of shear bands can be related to an increase in potential sites for activation of shear transformation zones due to the enhanced structural heterogeneity at lower temperatures. This, however, hinders the percolation of deformation units and leads to a decrease in the shear band propagation speed, from ∼50 nm/s at 295 K to ∼20 nm/s at 195 K. By testing micropillars across seven orders of magnitude in strain rate, a two-stage strain rate sensitivity is observed, from nearly negligible at the quasi-static range (m1 = 0.001 ± 0.002 for 10−3 s−1 to 10−1 s−1) to highly positive at dynamic high strain rate range (m2 = 0.02 ± 0.004 for 1 s−1 to 1000 s−1). This transition implies a change in the yield-controlling process, from the diffusion-assisted relaxation at quasi-static conditions to the rate of energy barrier crossing of shear transformation zones at high strain rates. These findings highlight several distinctive features of microscale metallic glasses, including the reduced shear band velocity at sub-ambient temperature and the two-stage strain rate sensitivity.

Neural Network Supported Microscale In Situ Deformation Tracking: A Comparative Study of Testing Geometries

Micro- and nanomechanical testing techniques have become an integral part of today’s materials research portfolio. Contrary to well-studied and majorly standardized nanoindentation testing, in situ testing of various geometries, such as pillar compression, dog bone tension, or cantilever bending, remains rather unique given differences in experimental equipment and sample processing route. The quantification of such experiments is oftentimes limited to load-displacement data, while the gathered in situ images are considered a qualitative information channel only. However, by utilizing modern computer-aided support in the form of the recently developed Segment Anything Model (SAM), quantitative mechanical information from images can be evaluated in a high-throughput manner and adds to the data fidelity and accuracy of every individual experiment. In the present work, we showcase image-assisted mechanical evaluation of compression, tension and bending experiments on micron-scaled resin specimens, produced via two-photon lithography. The present framework allows for a determination of an accurate sample strain, which further enables determination of quantities such as the elastic modulus, Poisson’s ratio or viscoelastic relaxation after fracture.

Micro/nano mechanical properties differences of shallow irradiation damage layer revealed by a combined experimental and MD study

Micropillar compression and nanoindentation techniques are both adopted to measure the mechanical properties in multi-energy He ion-irradiated Ni-based samples to study their relationship, and a distinct variation is observed. TEM characterization of the microstructure evolution and calculations based on the DBH model indicate that irradiation defects are not responsible for the observed variation. Subsequently, a comparative analysis of these two methods is simulated and their mechanisms are investigated using MD. Several factors, including activation of various slip systems, plastic area, and surface absorption, account for the differences observed in indentation and pillar compression tests, leading to the distinct evolution of internal initial dislocation lines, which significantly contribute to additional hardening in indentation tests.

The strength of a constrained lithium layer

A constrained compression test is developed to replicate the mechanical state of a lithium filament within a solid state battery. Lithium microspheres are compressed between parallel quartz plates into a pancake shape of thickness on the order of 15 µm. Full adhesion with no slip exists between the lithium and platens, and the attendant mechanical constraint implies that the average pressure on the pancake-shaped specimens increases with increasing aspect ratio of radius to height. In addition to mechanical constraint, a thickness-dependant size effect is observed whereby the apparent flow strength of the lithium increases from 0.7 MPa in the bulk to 2.0 MPa at a thickness of 15 µm. The lithium deforms in a power-law creeping manner at room temperature, and to simplify interpretation of the results, the relative velocity of the loading platens is adjusted to ensure that the true compressive strain rate is held fixed at 10−3 s−1. Additional measurements of lithium flow strength are obtained by subjecting the pancake-shaped specimens to simple shear. The size effect under shear loading is comparable to that in constrained compression. The observed size effect for lithium is consistent with that reported in the literature for lithium in indentation tests and in single pillar compression tests. Finally, the size effect of lithium in the power law creep regime is compared with that for rate-independent plasticity (for copper).

Helium irradiation-induced segregation and mechanical response in China low activation martensitic (CLAM) steel

The China Low Activation Martensitic (CLAM) steel was irradiated by high energy helium ions under different ion doses, the microstructure evolution was studied by using a transmission electron microscope (TEM) and Energy Dispersive Spectrometer (EDS) mapping. The mechanical response of this material before and after irradiation was investigated under micropillar compression test. The results indicate that radiation leads to Cr segregation at the grain boundary and facilitates the growth of M23C6 precipitate. The pillar compression demonstrate that the strength of the CLAM steel increased due to irradiation-induced defects strengthening. The deformed micropillar is analyzed by TEM observation, indicating the cracks nucleated around the M23C6 precipitate. Furthermore, the effect of the M23C6 on mechanical behavior is discussed.

Residual Strength Estimation Method of Soft Coal Based on Equivalent Residual Strain Ratio

In underground mining engineering, the residual strength of surrounding rock has an important influence on the secondary stress distribution caused by excavation. Generally, the residual strength of coal can be measured by experiments, but for soft coal, due to its large postpeak deformation characteristics and the limitation of strain sensor range, it is difficult to measure residual strength. Therefore, it is of great practical significance to estimate the residual strength of soft coal with incomplete stress-strain curves. In this paper, a method for estimating residual strength of soft coal based on the ratio of equivalent residual strain to peak strain is proposed. Taking advantage of the characteristics that the strain-softening curve of soft coal decreases approximately linearly in the initial stage, keeping the drop modulus unchanged, the endpoint of the extension line can be determined based on the ratio of equivalent residual strain to peak strain, which is estimated residual strength. There are 342 groups of typical complete stress-strain curves analyzed to determine the value range of the ratio of equivalent residual strain to peak strain. It is concluded that for briquette composed of soft coal, the ratio of equivalent residual strain to peak strain is between 1 and 3.35 when the confining pressure is 0-6 MPa, and the maximum probability interval of the ratio value under different confining pressures is further calculated. When the confining pressure is higher than 6 MPa, the briquette composed of soft coal tends to ideal plastic state after peak. The feasibility of the residual strength estimation method is verified by comparing the numerical simulation results of triaxial compression of coal pillars with the experimental results.

Kink mechanism in Cu/Nb nanolaminates explored by in situ pillar compression

Nano metallic laminates (NMLs) exhibit different failure modes depending on the loading conditions due to their mechanical anisotropies. Kinking is a typical failure mode in many NMLs compressed along a layer-parallel direction. However, a detailed description of the microstructure evolution during kink band (KB) formation and an in-depth understanding of the formation mechanisms are lacking. In this work, the KB process is investigated in Cu/Nb NMLs by in situ micro pillar compression in the scanning electron microscope (SEM) along a layer-parallel direction. Post-mortem S/TEM and transmission Kikuchi diffraction (TKD) analyses show that kink banding leads to significant microstructure changes characterized by an accumulation of geometrically necessary dislocations (GNDs) and of tilt geometrically necessary boundaries (GNBs) near KB boundaries (KBBs). The distinct microstructure evolution implies that KB formation is facilitated by the inhomogeneous microstructures resulting in constrained deformation modes. Specifically, dislocations active on slip planes nearly parallel to the interfaces make a major contribution to kink evolution after the onset of kinking. Once layer-parallel slip systems are activated, preexisting lattice dislocations and dislocations nucleating from interfaces will accumulate as GNDs near KBBs via the stochastic storage of lattice dislocations that have certain Burgers vectors. GNDs can further transform into GNBs via cross-slip and climb driven processes near the KBB. Furthermore, GNBs near KBBs can grow by incorporating more GNDs or by coalescence to accommodate the KB evolution. We further hypothesize that microstructural perturbations and their ensuing stresses can initiate KB formation in Cu/Nb NMLs.

Micro/nano-mechanical behaviors of individual FCC, BCC and FCC/BCC interphase in a high-entropy alloy

• Micro-pillar compression tests were carried on a multiple high-entropy alloy. • Deformation behaviors of FCC, BCC and their interphase pillars were analyzed. • The strain bursts weakened with increasing strain in interphase pillar. • Interphase pillar exhibited better mechanical properties compared with FCC and BCC. • Revealed the relation between small scale deformation and interphase strengthening. Here, a systematic investigation was made on the interphase strengthening effects induced superior mechanical performances of multiphase high-entropy alloys (HEAs) at micro/nano-scale, compared with single phase HEAs. A pillar compression test under a scanning electron microscope (SEM) was performed on the individual face centered cubic (FCC), body centered cubic (BCC), and mixed-phases with different diameters in a Fe 24 Co 25 Ni 24 Cr 23 Al 4 HEA using focused ion beam (FIB) milling and a nanoindenter equipped with a flat punch. The stress-strain response of pillar underneath the indenter was selected to explore the diameter/phase-dependent size effect, the periodically fluctuation of local stress, and strain hardening. It was revealed that the pillars at the interphase exhibited significantly higher strength, compared with the FCC and BCC pillars. An experiment also verified the coincident mechanical size effects independent with the type of phases. The stress responses in the mixed-phase pillars manifested as a distinct transition from the dramatic drop to the minor fluctuation during the post-yield stages with the increasing strain, indicating the propagation of Al-Ni enriched solid solution phase (BCC1) under compression. Except the BCC1 phase, numerous dislocations were observed in the post-deformed pillars, particularly serving as the major source to enhance the strain hardening of BCC pillars.

Ultra-strong and thermally stable nanocrystalline CrCoNi alloy

Grain refinement to the nanocrystalline regime is the most effective way to strengthen materials but this often deteriorates the grain-size thermal stability and plasticity. Here we manufactured a nanocrystalline face centred cubic CrCoNi medium entropy alloy with columnar grains via magnetron sputtering. Compression of CrCoNi pillars with diameters of ∼ 1 µm revealed a record high yield strength of ∼5 GPa for pillars with face centred cubic structures and engineering plastic strain of > 30%. The alloy possessed an outstanding grain-size thermal stability even at 1073 K. Both nanocrystalline grain size and a high density of nanotwins/stacking faults are critical to the exceptional yield strength. Deformation twinning, grains refinement during deformation, grain boundary sliding and random grain orientation all contribute to the large plasticity. The outstanding thermal stability is attributed to the sluggish diffusion effect and the low energy of twin boundaries.

Ultra-low-density digitally architected carbon with a strutted tube-in-tube structure.

Porous materials with engineered stretching-dominated lattice designs, which offer attractive mechanical properties with ultra-light weight and large surface area for wide-ranging applications, have recently achieved near-ideal linear scaling between stiffness and density. Here, rather than optimizing the microlattice topology, we explore a different approach to strengthen low-density structural materials by designing tube-in-tube beam structures. We develop a process to transform fully dense, three-dimensional printed polymeric beams into graphitic carbon hollow tube-in-tube sandwich morphologies, where, similar to grass stems, the inner and outer tubes are connected through a network of struts. Compression tests and computational modelling show that this change in beam morphology dramatically slows down the decrease in stiffness with decreasing density. In situ pillar compression experiments further demonstrate large deformation recovery after 30-50% compression and high specific damping merit index. Our strutted tube-in-tube design opens up the space and realizes highly desirable high modulus-low density and high modulus-high damping material structures.

Room-temperature plasticity and size-dependent mechanical responses in small-scale B1-NbC(001) single-crystals

We report on the mechanical responses of NbC(001) single-crystals subjected to nanoindentation and in situ scanning electron microscopy based uniaxial microcompression tests at room-temperature. Using X-ray diffraction and X-ray photoelectron spectroscopy, we determine that the as-purchased bulk NbC is a 001-oriented, B1-structured, single-crystal with lattice parameter of 0.4446 ± 0.0002 nm and C/Nb ratio of ∼0.99. From nanoindentation experiments performed on the bulk crystal using a Berkovich diamond indenter, we measure an elastic modulus of 495 ± 14 GPa and an indentation-depth (h) dependent hardness decreasing from 31.0 ± 1.2 GPa at h = 0.2 µm to 26.4 ± 0.4 GPa at h = 0.9 µm. During uniaxial compression of cylindrical pillars with top diameters D between ∼0.25 µm and ∼0.75 µm, we observe strain hardening, increasing yield strengths with decreasing D, and surprisingly extensive plastic deformation with strains exceeding 2% and up to 30% in larger-size pillars. Electron microscopy characterization of the compressed pillars reveal slip traces characteristic of {110}〈11¯0〉 and {111}〈11¯0〉 slip systems. We suggest that the unexpected activation of {111}〈11¯0〉 facilitates plasticity in NbC(001).

Effects of radiation damage and precipitate distribution on micro-pillar compression testing of irradiated CuCrZr

The results of small-scale mechanical tests are convoluted by the so-called size effect, whereby materials appear stronger when the scale of the test is reduced to the order of microns or less. The dimensional range over which this occurs has been shown to be linked to a change in sample microstructure, such as the addition of defects induced by irradiation. To investigate this response, a CuCrZr alloy was subjected to proton irradiation and mechanically tested using micro compression of pillars with a range in size. It was found that irradiation defects dominate over the extrinsic size effect and the sensitivity to differences in precipitate microstructure was also somewhat reduced, suggesting that size-independent results could be obtained from much smaller test volumes in irradiated material compared to their non-irradiated counterparts. Finally, comparison was made between the increase in yield strength predicted by models and the experimentally measured values to establish the key parameters driving the strengthening behaviour.

Effect of co-deformation of ceramic layer in Ag/Al-doped ZnO nanolayered composites

Metal–ceramic nanolayered composites have been reported to have high strength because of their effective confinement of dislocations at interfaces; however, this unfortunately results in unstable deformation due to the brittleness of the ceramic layer. This study explores the deformation behavior of a metal–ceramic nanolayered composite in which the ceramic layer thickness is varied to examine the effect of brittle-to-ductile transition of the ceramic layer. The thickness of AZO, which is theoretically expected to be ductile, is first fixed at 9 nm while varying the metal thickness. For further comparison, 150 nm Ag/120 nm AZO is studied, where the AZO layer is expected to be brittle. In-situ SEM pillar compression show stable deformation for samples with 9 nm thick AZO, in which co-deformation of metal and ceramic is confirmed by TEM analysis. A systematic increase in strength is observed with reduction of Ag thickness with stable plastic deformation for nanolayered composite with AZO thickness of 9 nm. For the 150 nm Ag/120 nm AZO, unstable deformation is observed in which brittle fracturing of the AZO layer leads to reduction in flow stresses or strain softening. Finite element analysis shows that the stresses in the Ag and the AZO layers reach the stresses required for co-deformation.

Room-Temperature Plasticity and Size-Dependent Mechanical Responses in Small-Scale B1-NbC(001) Single-Crystals

We report on the mechanical responses of NbC(001) single-crystals subjected to nanoindentation and in situ scanning electron microscopy based uniaxial microcompression tests at room-temperature. Using X-ray diffraction and X-ray photoelectron spectroscopy, we determine that the as-purchased bulk NbC is a 001-oriented B1-structured single-crystal with lattice parameter of 0.4446 ± 0.0002 nm and C/Nb ratio of ~0.99. From nanoindentation experiments performed on the bulk crystal using a Berkovich diamond indenter, we measure an elastic modulus of 495 ± 14 GPa and an indentation-depth (h) dependent hardness decreasing from 31.0 ± 1.2 GPa at h = 0.2 µm to 26.4 ± 0.4 GPa at h = 0.9 µm. During uniaxial compression of cylindrical pillars with top diameters D between ~0.25 µm and ~0.75 µm, we observe strain hardening, increasing yield strengths with decreasing D, and surprisingly extensive plastic deformation with strains exceeding 2% and up to 30% in larger-size pillars. Electron microscopy characterization of the compressed pillars reveal slip traces characteristic of {110}〈1‾10〉 and {111}〈1‾10〉 slip systems. We suggest that the unexpected activation of {111}〈1‾10〉 facilitates plasticity in NbC(001).

Influence of strain rate on the activation of {110}, {112}, {123} slip in ferrite of DP800

We have performed micro pillar compression to investigate the influence of strain rate on the activation of three slip plane families, namely {110}, {112} and {123}, in ferrite of a dual phase steel. The critical resolved shear stress of all three slip plane families rises with increased strain rate. The strain rate sensitivity drops with increasing strain. Increasing strain rate does not reduce the number of activated slip systems, instead resulting in slip plane activation outside of that predicted by Schmid´s law. The activation volume of 13b³ to 16b³ suggests that the Peierl's process is the rate controlling mechanism in ferrite of DP800.

Nanotwin-induced strengthening in silicon: A molecular dynamics study

Mechanical performance of silicon nanopillars with homogeneous and gradient nanotwinned structures are investigated through a series of molecular dynamics simulations. The most observed Σ3 twin boundary (TB) with two preferable (lowest surface energy) planes of {111} and {001} are used to generate homogeneous and gradient nanotwinned structures. Simulations of compression and tension of nanotwinned pillars reveal an extra strengthening behavior due to the addition of Σ3 TBs when compared to the single crystalline nanopillar without any TBs. The increase in strength of Σ3 nanotwinned pillars is attributed to a high density of dislocations in grains caused by reducing the twin thickness or increasing the number of TBs in a homogenous nanotwinned structure. Moreover, Hall-Petch strengthening behavior is observed as the twin thickness decreases to a critical value in silicon nanopillars with homogenous Σ3 {111} or Σ3 {001} TBs. Below the critical twin thickness, an inverse Hall-Petch phenomenon is observed, which results in softening of nanotwinned pillars. The pile-up of dislocations and their interactions with TBs cause enhancement in strength, while the nucleation and movement of partial dislocations parallel to the TBs cause the softening. Furthermore, the nanotwinned gradient structures with different configurations are examined, and an optimal nanotwinned gradient structure is obtained for maximizing the strength. Our simulation results provide fundamental understanding of twinning effect on mechanical deformation of homogenous and gradient nanotwinned silicon pillars.