Micropillar Research Papers - Academia.edu (original) (raw)

The study presents crystal plasticity finite element simulations of cylindrical Cu single crystal micropillar compression tests. The aim is to study the influence of the stability of the initial crystal orientation, sample geometry (diameter-to-length ratio) and friction on the anisotropy and crystallographic orientation changes during such tests. Initial anisotropy (initial orientation) has a strong influence on the evolution of crystallographic orientation changes and also, to a minor extent, on the sample shape during compression. Pronounced orientation changes occur at an early stage of compression (at engineering strains of 0.2), entailing as a rule a large orientation spread within the initially uniformly oriented sample. A non-zero friction has a stabilizing effect on the course of the compression test even in cases where strong orientation changes occur. The evolution of orientation changes during compression is in part due to rigid body rotations (shape inclination due to buckling) rather than exclusively to crystallographic reorientation. Orientations that are crystallographically unstable and non-symmetric during compression tend to entail shape instability of the pillars at an earlier stage than observed for more stable cases.

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- Crystal Plasticity, Micropillar, Buckling, Texture

The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of ~10 nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal system, with a CRSS of 196 ± 14.7 MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects.

It is known that cooling rate can affect the atomic structure and thus may possibly affect the mechanical properties of metallic glasses (MGs). In spite of the considerable efforts on the cooling rate, its effect on the mechanical properties is controversial at the present time. In this study, we present a micromechanical
study of the cooling-rate effect on Young’s moduli and hardness of the cast bulks and melt-spun ribbons for a Zr55Pd10Cu20Ni5Al10 metallic glass. Using the classic nanoindentation method, the Young’s moduli of the ribbon samples obtained at higher cooling rates were measured which appeared to be much lower than those of the bulk samples. However, through further experiments on slice samples cut from
the as-cast bulks and finite-element (FE) analyses, we have clearly demonstrated that the measured difference in elastic moduli was mainly caused by the sample thickness effect in nanoindentation tests. To overcome such a confounding effect, microcompression experiments were performed on the as-cast
and as-spun MG samples, respectively. Being consistent with the findings from nanoindentation, the microcompression results showed that the cooling rate, as ranging from ∼102 to ∼106 K/s, essentially has no influence on the Young’s modulus and hardness of the metallic glasses.

- by Sheng Guo
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- Nanoindentation, Metallic Glass, Cooling Rate, Micropillar

We report a study on the growth kinetics and resultant structures of arrays of pillars in photo-cross-linkable films during irradiation with a periodic array of microscale optical beams under ambient conditions. The optical beams experience a self-focusing nonlinearity owing to the photopolymerization-induced changes in refractive index, thereby concentrating light and driving the concurrent, parallel growth of microscale pillars along their path length. We demonstrate control over the pillar spacing and pillar height with the irradiation intensity, film thickness, and the size and spacing of the optical beams. The growth of individual pillars in a periodic array arises from the combination of intense irradiation in the beam regions and oxygen inhibition afforded by the open, ambient conditions under which growth is carried out. We propose a kinetic model for pillar growth that includes free-radical generation and oxygen inhibition in thick films of photoinitiated media in order to interpret the experimental results. The model effectively correlates micropillar array structure to the oxygen inhibition effects. This approach of growing micropillar arrays through photopolymerization is straightforward and scalable and opens opportunities for the design of textured surfaces for applications.

- by Ian D Hosein
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- Photopolymerization, Self-Writing, Micropillar, Photocuring Resins

Self-assembled catalyst-free GaN micropillars grown on (0001) sapphire substrates by metal organic vapor phase epitaxy are investigated. Transmission electron microscopy, as well as KOH etching, shows the systematic presence of two domains of opposite polarity within each single micropillar. The analysis of the initial growth stages indicates that such double polarity originates at the micropillar/substrate interface, i.e., during the micropillar nucleation, and it propagates along the micropillar. Furthermore, dislocations are also generated at the wire/substrate interface, but bend after several hundreds of nanometers. This leads to micropillars several tens of micrometers in length that are dislocation-free. Spatially resolved cathodoluminescence and microphotoluminescence show large differences in the optical properties of each polarity domain, suggesting unequal impurity/dopant/vacancy incorporation depending on the polarity.

- by Meletis Mexis and +3
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- Transmission Electron Microscopy, GaN, Cathodoluminescence, Micropillar

The depth-sensing indentation method has been applied for almost 30 years. In this review, a survey of several extended applications developed during the last three decades is provided. In depth-sensing indentation measurements, the load and penetration depth data are detected as a function of time, in most cases at controlled loading rates. Therefore, beside the determination of hardness and Young's modulus, different deformation mechanisms and many other dynamic characteristics and phenomena, such as the dynamic elastic modulus, load-induced phase transition, strain rate sensitivity, etc. can be studied. These extended applications of depth-sensing indentation measurements are briefly described and reviewed.

- by Dániel Olasz
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- Micropillar, Nano-indentation, Grain Boundary, Indentation

The microstructural features and micromechanical behavior of individual phases in a cast Al0.8CoCrCuFeNi high-entropy alloy
(HEA) were characterized by high-resolution scanning electron microscopy and micro-compression tests. Use of neutron diffraction enabled the detection of a new phase which was otherwise unobservable by conventional X-ray diffraction. The identified phase constitution agreed well with the compositional analysis and the micro-compression results. The delicate microscale characterization of individual phase provides new insights for the design of novel HEAs with desirable mechanical properties.

- by Sheng Guo
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- Nanoindentation, High Entropy Alloys, Micropillar

Laser surface texturing and through-mask electrochemical
micromachining (TMECMM) are some of the commonly
used methods which include multiple steps to achieve
micro-textures. However, for large-area applications, it is desirable
to have an economical single-step process. In this regard,
ECMM is expected to be a promising and economically
viable micro-texturing process for micro-manufacturing industries.
This paper proposes a novel maskless EC microtexturing
process using tool sinking technique with low voltage
and short pulses. Methodology of using edges (∼30 μm
each) of printed circuit boards (PCBs) for ECMM of microchannels
and micro-pillars is a unique one. Micro-pillars,
micro-dimples and micro-channels are produced through a
direct electrochemical cathode sinking process. Analysis of
current density and prediction of width and depth of microdimples
on a flat stainless steel surface are studied through 2D
numerical simulation carried out on COMSOL 4.3a. The proposed
method implies the use of less toxic electrolyte, low
voltage (1–6 V), short pulses (5 to 50 μs) and selective polymer
coating on the tool (cathode). A series of experiments of
EC sinking for creating various micro-patterns and microstructures
has been carried out on the flat as well as curved
metallic surfaces. This paper reports machining of microdimples
of 200–300 μm diameter, micro-channels of 150–
250 μm and square micro-pillars of 300–350 μm.
Comparison of the predicted geometrical dimensions of the
micro-dimples through the simulation shows reasonable
agreement with the experimental results for the given process
parameters of ECMM. The paper also reports a brief comparison
of laser surface texturing (LSTex) and electrochemical
surface texturing (ECSTex).

- by Hassan Ghassemi-armaki
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- Thermodynamics, Microstructure, Finite Element Modeling, Deformation