Crystal Pillars Research Articles

Effect of specimen size and crystallographic orientation on the nano/microscale mechanical properties and deformation behavior of CrCoNi medium-entropy alloy

CrCoNi medium-entropy alloy (MEA) shows an exceptional combination of macroscale mechanical properties, however, knowledge of its mechanical properties at nano/microscale is lacking. In this paper, nano/microscale single crystal MEA pillars with orientation of 〈116〉, 〈014〉, 〈102〉 were fabricated using focused ion beam milling, and the compressive strength was determined with micropillar compression testing using a flat-end tip in an indentation machine. Results showed that the yield and flow strength of these MEA pillars were orientation-dependent, i.e., the lower the Schmid factor, the higher the strength. At the same time the strength of the pillars was dependent on the pillar’s size, the strength increasing as the pillar diameter reduced. From the results of the micropillar compression testing, the bulk critical resolved shear stress (CRSS) was determined to be 92 MPa, which is the highest value of the group of CrCoNi-based medium and high-entropy alloys. Analysis of transmission electron microscope images and large-scale molecular dynamics simulations revealed a group of complimentary deformation mechanisms, including dislocation slides, dislocation annihilation, dislocation-slip band interaction, formation of nanoscale stacking-faulting networks, and deformation nanotwins.

Materials in the CaO-K2O-SO3-H2O System Based on Powder Mixtures including Calciolangbeinite K2Ca2(SO4)3 and Calcium Sulfate Anhydrite CaSO4

Materials (cement stone samples) in the CaO-K2O-SO3-H2O system with the target phase compositions, including syngenite K2Ca(SO4)2·H2O and calcium sulfate dihydrate CaSO4·2H2O, were prepared from powder mixtures of calcium sulfate anhydrite CaSO4, and/or calciolangbeinite K2Ca2(SO4)3, and potassium sulfate K2SO4 via hydration reactions at a water/powder ratio within an interval of 0.5–0.9. It was revealed that samples with contents of 25, 50, 75 and 100 mol% of syngenite K2Ca(SO4)2·H2O demonstrated a nonlinear dependence of their respective microstructures on their phase compositions. The microstructures of samples with phase compositions of 25 and 75 mol% of syngenite K2Ca(SO4)2·H2O consisted of pillar crystals. The microstructures of samples with phase compositions of 50 and 100 mol% of syngenite K2Ca(SO4)2·H2O consisted of plate crystals. An explanation of microstructure formation was set forth, taking into account equilibria of the dissolution–crystallization processes during cement stone formation. Materials obtained in the CaO-K2O-SO3-H2O system consisting of biocompatible and resorbable (soluble in water) phases can be recommended for testing as potential substances for bone defect treatments.

Unconventional dislocation starvation behavior of medium-entropy alloy single crystal pillars containing pre-existing dislocations

The excellent dislocation storage ability of bulk multi-principal element alloys (MPEAs) has been widely reported. To date, however, the underlying mechanisms of dislocation escape behavior in small-size face-centered cubic (FCC) MPEAs have rarely been studied. Here, we quantitatively control the initial dislocation densities (∼1015 m–2 and ∼1016 m–2) by large-scale molecular dynamics (MD) simulations and perform uniaxial compression simulations to compare the dislocation starvation behavior of CrCoNi with pure Cu single crystal pillars (SCPs). The analysis reveals that the CrCoNi SCPs with low initial dislocation density (∼1015 m–2) can continuously accommodate mobile dislocations, and the critical dimension for dislocation starvation is about 30 nm. In particular, the CrCoNi SCPs with chemical short-range ordering (SRO) exhibit better dislocation storage and multiplication abilities than the random solid solution (RSS) samples even when the initial dislocation density is low. However, the presence of a large number of pre-existing dislocation locks governs the strong dislocation multiplication ability of the small-size RSS CrCoNi SCPs, in obvious contrast to the deformation of all pure Cu SCPs which is completely dominated by intermittent mobile dislocation starvation. Most importantly, we reveal the fundamental physics for the good dislocation storage of CrCoNi SCPs at small sizes from the perspective of chemical heterogeneity. The new phenomena reported in this work provide a unique atomic-scale perspective for understanding the microscopic physical origin of the mechanical behavior of MPEAs and the discovery of extremely slow dislocation escape behavior in small-scaled pillars, providing a reliable basis for the development of the dislocation starvation model.

Electro-Acoustic Properties of Scandium-Doped Aluminum Nitride (ScxAl1-xN) Material and its Application to Phononic Crystal-Coupled SAW Devices

Within the framework of the Density Functional Theory, the elastic, dielectric, and piezoelectric coefficients of w-ScxAl1−xN material were investigated for scandium (Sc) concentrations x = 0 to 0.375. The electro-acoustic properties are used to investigate the frequency response of the SAW delay line, based on the tilt θ° of the normal c-axis of the w-ScxAl1−xN piezoelectric thin film. We found that the piezoelectric response is improved as the Sc concentration increases, which is consistent with existing works in the literature. A 2D-phononic crystal pillars was then grafted on top of the surface, and the dependence of the acoustic band gaps is investigated with the help of the finite element method as a function of the Sc concentration and the tilted angle of w-Sc0.375Al0.625N. It was found that the two first band gaps exhibit a shift toward low frequencies with increasing Sc concentration. Moreover, the second acoustic bandgap is more sensitive to the inclination angle than the first. Furthermore, the insertion loss (S21) of w-Sc0.375Al0.625N is improved by 22 dB at θ° = 60°. The c-axis tilted Sc0.375Al0.625N-SAW delay line coupled with 2D-phononic crystals is a promising structure for low-loss and high-frequency SAW devices.

In situ formation of composite thin film with (111) oriented Ni0.5Zn0.5Fe2O4 pillar array surrounded by BaTiO3 for ferroelectric-ferromagnetic coupling

For a new breakthrough of highly efficient magnetoelectric (ME) coupling of multiferroic composite thin film in application of next generation multifunctional devices, it is important to control constituent magnetic phase pillar array independently embedded inside piezoelectric phase. In this work, in situ spin-coated xNi0.5Zn0.5Fe2O4/(1-x)BaTiO3 composite thin films on Si substrate with high magnetoelectric coupling are successfully obtained via phase separation. Results show that hexagonal Ni0.5Zn0.5Fe2O4 (NZFO) grain grows (111)-oriented as single crystal pillars arrayed independently uniformly and surrounded by BaTiO3 (BTO) phase. Ti and Fe ions are doped into the NZFO and BTO phases respectively and decreased with the decrease in corresponding constituent phases. Grain size and lattice constant of the cubic NZFO are from ~1 to ~0.5 μm and from 0.8383 nm to 0.8369 nm respectively in terms of x. The orientation degree of thin films is> 56% when x is above 0.8. The 0.8NZFO/0.2BTO thin film with and without pure BTO after-spincoated shows giant ME coupling effect, about 40% change of magnetization of NZFO in terms of phase transition of ferroelectric BTO at Curie temperature ~398 K. It is much larger than others on Si substrate ever reported and may be applied cost-effectively, coupling-efficiently and widely in Si-based chips area.

Tempering the mechanical response of FCC micro-pillars: An Eulerian plasticity approach

The mechanical response of almost pure single-crystal micro-pillars under compression exhibits a highly localized behavior that can endanger the structural stability of a sample. Recent experiments revealed that the mechanical response of a crystal is very sensitive to both the presence of a quenched disorder in the sample, and the orientation of the crystal. In this work, we study the influence of disorder and crystal orientation on the large 2-D plane strain response of a FCC crystal with three active glide planes using a very simple Eulerian plasticity model in the FE framework. Our numerical and theoretical results on clean crystal pillars suggest that a single plane or many gliding planes can be activated depending on the crystal orientation. While in the former case, the deformation is localized, leading to ductile rupture, in the latter, a complex interplay between active planes takes place, resulting in a more uniform deformation. The strain-localization can be avoided when inhomogeneities are engineered inside the crystal, or the crystal orientation is altered because of the activation of multiple slip systems, resulting in a ”patchwork” of the distribution of the slip systems.

Mechanics of bacteria-assisted extrinsic healing

Self-healing materials can typically be divided into two types: intrinsic healing that harnesses dynamic bones to autonomously repair fractures, and extrinsic healing that uses the externally added components to enable the bonding of fractured interfaces. Although the theoretical modeling of intrinsic self-healing materials has been recently studied by Wang et al., the fundamental understanding and theoretical modeling of the extrinsic self-healing materials remain elusive. Without a deep understanding of the extrinsic healing mechanics, the design of extrinsic-healing materials and corresponding applications are still at the trial-and-error stage. Here, taking bacterial-precipitation-enabled healing as an example, we construct a modeling framework to explain the bacteria-assisted extrinsic healing mechanics. A model for the growth of crystal pillars is developed to explain the bacteria-assisted growth of the calcium carbonate (CaCO3) crystal forest within the fracture interface, and a cohesive zone model is built to explain the interfacial bonding. Our modeling framework can explain the evolution of the interfacial healing strength over the healing time and reveal the effects of interface distance and concentrations of bacteria and calcium ions on the healing performance. The modeling results are consistent with the bacteria-assisted healing experiments of ceramics and cement.

A Micromachined Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystal Composite Circular Array for Intravascular Ultrasound Imaging.

This paper describes the design, fabrication, and characterization of a micromachined high-frequency Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystal/epoxy 1-3 composite ultrasound circular array. The 1-3 composites were fabricated by deep reactive ion etching (DRIE) of PMN-PT single crystal. The feature size of single crystal pillars was 18 μm in diameter. The kerf between pillars was less than 4 μm. A 50-element circular array transducer (radially outward) with the pitch of 100 μm was wrapped around a needle resulting in an outer diameter of 1.7 mm. The array test showed that the center frequency reached 39±2 MHz and -6-dB fractional bandwidth was 82±6%. The insertion loss was -41 dB, and crosstalk between adjacent elements was -24 dB. A radial outward imaging testing with phantom wires (D = 50 μm) was conducted. The image was in a dynamic range of 30 dB to show a penetration depth of 6 mm by using the synthetic aperture method. The -6-dB beam width was estimated to be 60 μm in the axial direction at 3.1 mm distance away from the probe. The results suggest that the 40 MHz micromachined 1-3 composite circular array is promising for intravascular ultrasound (IVUS) imaging applications.

Plastic deformation behavior and microscopic mechanism of metastable Ti-10V-2Fe-3Al alloy single crystal pillars orientated to β in submicron scales Part I: Double size effects and martensitic transformation prediction

The effects of ω precipitates and orthorhombic (α″) martensitic transformation on stress-strain behavior were studied in metastable β Ti-10V-2Fe-3Al alloy single-crystal in submicron scales. We successfully distinguished the size effects induced by ω precipitates and α″ phase transformation through compressing the metastable β Ti-10V-2Fe-3Al along the orientation of <011>β, which is unfavorable for α″ phase transformation. Two individual exponential curves express well the relationship between the strengthening exponents, which were regressed using the flow stresses at different ranges of plastic strains of 0.2–13% and the pillar widths, and the corresponding plastic strain. It is worth noting that the strengthening exponent jumps from 0.256 to 0.293 at 8% plastic strain. Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars. The first (ω precipitate-related) size effect has been predicted with the proposed models, and the second (martensitic transformation-induced) size effect is interpreted with the modified Liu's equation.

Plastic deformation behavior and microscopic mechanism of metastable Ti-10V-2Fe-3Al alloy single crystal pillars orientated to β in submicron scales Part II: Phase transformation dependence of size effect and deformation mechanism

Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along <011>β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity.

Phase Transition of Graphene-Templated Vertical Zinc Phthalocyanine Nanopillars.

We report on the graphene-assisted growth, crystallization, and phase transition of zinc phthalocyanine (ZnPc) vertically oriented single crystal nanopillars. Postcrystallization thermal annealing of the nanostructures results in a molecular packing change while maintaining the vertical orientation of the single crystals orthogonal to the underlying substrate. Grazing incidence X-ray diffraction and high-resolution TEM studies characterized this phase transition from a metastable crystal phase to the more stable β-phase commonly observed in bulk crystals. These vertical arrays of crystalline nanopillars exhibit a high-surface-to-volume ratio, which is advantageous for applications such as gas sensors. We fabricated chemiresistor sensors with ZnPc nanopillars grown on graphene and demonstrated its selectivity for ammonia vapors, and improvement in sensitivity in the β-phase crystal packing pillars due to their molecular orientation increasing the exposure of the Zn2+ ion to the ammonia analyte. This work highlights the first morphology-retentive phase transition in organic single crystal nanopillars through simple postprocessing thermal annealing. This study opens up the possibility of molecular packing control without large variations in morphology, a necessity for high-performance devices and establishing structure-property relations.

Ferroelectric Phase Transition Induced a Large FMR Tuning in Self-Assembled BaTiO3:Y3Fe5O12 Multiferroic Composites.

Yttrium iron garnet (YIG) is of great importance in RF/microwave devices for its low loss, low intrinsic damping, and high permeability. Nevertheless, tuning of YIG-based multiferroics is still a challenge due to its near-zero magnetostriction and the difficulty of building epitaxial interface between ferromagnetic garnet and ferroelectric perovskite phases. In this work, the vertically aligned heterostructure of YIG:BTO/STO(001) with local epitaxial interface between BTO and YIG is well-constructed, where the single crystal BTO pillars are embedded in YIG matrix. A large magnetoelectric coupling effect that drives YIG's FMR shift up to 512 and 333 Oe (1-2 order greater than those of all state-of-the-art progresses) is obtained through BTO ferroelectric phase changes induced by temperature variation at 295 and 193 K, correspondingly. This record high magnetoelectric tunability of YIG paves a way toward thermal/electrical tunable YIG devices.

Crystal orientation dependence of the stress-induced martensitic transformation in zirconia-based shape memory ceramics

Small volume samples of zirconia can survive stress-induced martensitic transformation without cracking, which enables in-depth explorations of martensite mechanics using micro-scale specimens. Here we present a systematic investigation of the orientation dependence of tetragonal crystals undergoing a uniaxial stress-driven martensitic transformation to the monoclinic phase, in single crystal zirconia pillars doped with yttria and titania. The Young’s modulus, martensitic transformation stress and transformation strain are highly dependent on the crystallographic orientation, and generally align with expectations based on known tensor properties and transformation crystallography. However, in some orientations, fracture or plastic slip are apparently preferred to martensitic transformation, and thus crystallography favors certain orientations if superelasticity or shape memory properties are specifically desired in zirconia ceramics.

Temperature-dependent size effects on the strength of Ta and W micropillars

The strength of metals increases with decreasing sample size, a trend known as the size effect. In particular, focused ion beam-milled body-centered cubic (BCC) micropillars exhibit a size effect known to scale with the ratio of the test temperature to the critical temperature (Tc) of the BCC metal, a measure of how much the yield stress is governed by the lattice resistance. In this paper, this effect is systematically studied by performing high-temperature compression tests on focused ion beam-manufactured Ta and W single crystal pillars ranging in diameter from 500 nm to 5 μm at temperatures up to 400 °C, and discussed in the context of bulk strength and size dependent stresses. Both metals show larger size effects at higher temperatures, reaching values that are in the range of FCC metals at temperatures near Tc. However, it is demonstrated that size effects can be considerably affected by material parameters such as dislocation density and lattice friction, as well as by the yield criterion used. Furthermore, for W, a change from uniform wavy deformation to localized deformation is observed with increasing temperature and pillar size, further indicating that the temperature ratio strongly influences the relative motion of screw and edge dislocations.

Deformation twinning in small-sized face-centred cubic single crystals: Experiments and modelling

Small-sized crystals generally show deformation behaviour distinct from their bulk counterparts. In addition to dislocation slip, deformation twinning in small-sized face-centred cubic (FCC) single crystals has been reported to follow a different mechanism which involves coherent emission of partial dislocations on successive {111} planes from free surface. The present work employed a twinning-induced plasticity (TWIP) steel with a low stacking fault energy to systematically investigate the twin evolution in small-sized FCC single crystals. Micrometre-sized single crystal pillars of TWIP steel were fabricated by focus ion beam and then strained to different levels by compression experiments. Detailed transmission electron microscopy characterization was carried out to obtain a quantitative evaluation of the deformation twins, which contribute to most of the plastic strain. Emissions of partial dislocations from free surface (surface sources) and pre-existing perfect dislocations inside the pillar (inner sources) are found as the essential processes for the formation of deformation twins. Accordingly, a physically-based model, which integrates source introduction methods and source activation criterions for partial dislocation emission, is developed to quantitatively predict the twin evolution. The model is able to reproduce the experimental twin evolution, in terms of the total twin formation, the twin morphology and the occurrence of twinning burst.

Effect of a high angle grain boundary on deformation behavior of Al nanopillars

The effect of inclusion of a grain boundary in nanopillar was studied using Al bicrystal with [3 4 3]/[ 13 17 ‾ 14 ] orientation. Ex situ compression results showed higher strengths in bicrystal pillars than in single crystal pillars, which is explained with respect to the reduced single-armed source length that was confirmed in the in situ TEM tests. Molecular dynamics simulations indicated that the grain boundary can assist in dislocation nucleations without resulting in dislocation pile-up at the boundary.

Deep reactive ion etching of PZT ceramics and PMN-PT single crystals for high frequency ultrasound transducers

Micromachining fabrication technique based on thick photoresist lithography and ICP enhanced deep reactive ion etching of PZT ceramics and PMN-PT single crystals has been investigated to fabricate 1–3 composite piezoelectric materials for high frequency ultrasound transducer applications. The experiment procedure includes thick photoresist patterned lithography, Ni hard mask electroplating and ICP enhanced deep reactive ion etching using chlorine gas. PZT ceramic pillars with a height of 43.78μm and a sidewall angle of 78.9° were obtained. The optimized etching parameters for PZT ceramics were etching rate of 6.25μm/h, and PZT/Ni etching selection ratio of 6.9. PMN-PT single crystal pillars with a height of 30.74μm and a sidewall angle of 84.3° were obtained. The optimized etching parameters for PMN-PT were etching rate of 10.25μm/h and PMN-PT/Ni etching selection ratio of 14. The 1–3 piezoelectric composites fabricated by PZT and PMN-PT pillars are promising materials for fabrication of high frequency ultrasound transducers.

The effect of size on dislocation cell formation and strain hardening in aluminium

The formation of dislocation cells has a significant impact on the strain hardening behaviour of metals. Dislocation cells can form in metals with a characteristic size defined by three-dimensional tangles of dislocations that serve as “walls” and less dense internal regions. It has been proposed that inhibiting the formation of dislocation cells could improve the strain hardening behaviour of metals such as Al. Here we employ in situ scanning electron microscope compression testing of pure Al single crystal pillars with physical dimensions larger, close to and smaller than the reported cell size in Al, respectively, to investigate the possible size effect on the formation of dislocation cell and the consequent change of mechanical properties. We observed that the formation of dislocation cells is inhibited as the pillar size decreases to a critical value and simultaneously both the strength and the strain hardening behaviour become strongly enhanced. This phenomenon is discussed in terms of the effect of dimensional restriction on the formation of dislocation cells. The reported mechanism could be applied in polycrystalline Al where the tunable physical dimension could be grain size instead of sample size, providing insight into Al alloy design.

Approaching theoretical strengths by synergistic internal and external size refinement

Refinement of the internal grain size leads to strengthening by retarding dislocation motion. There have also been recent reports that a reduction in external diameter enhances the strength of single crystal pillars. Here we show, in a hitherto unexplored domain, a synergistic increase in strength by a combined reduction in internal (0.5 μm) and external (20–50 μm) dimensions, with strengths at failure approaching the theoretical value.

Superelastic response of [111] and [101] oriented NiTi micropillars

A combination of microcompression experiments on single crystal micron-scaled pillars of NiTi of known orientations and in situ neutron diffraction during loading of the same NiTi but in bulk, polycrystalline form are carried out to understand the stress-induced transformation associated with superelasticity at reduced length scales. At the length scales investigated, there is evidence through this work of a fully reversible stress-induced transformation from B2 to B19′ NiTi that does not involve additional dislocation activity or irrecoverable strains. The orientation dependence of the elastic deformation of the B2 phase, the onset of its transformation to the B19′ phase, the gradient and the hysteresis in the stress-strain response during transformation, the elastic modulus of the stress-induced B19′ phase and the onset of plasticity are quantified and analyzed in these experiments by examining the crystallography of the B2 to B19′ transformation.