Browsing by Author "Lo, Yu-Chieh"

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An optimized random structures generator governed by chemical short-range order for multi-component solid solutions
(2019-12-01)
Yu, Peijun
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Chou, Jyh-Pin
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Lo, Yu-Chieh
;
Hu, Alice
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材料科學與工程學系
Multi-component random solid solutions have caused much interest for research because they often exhibit excellent mechanical properties. Many computational efforts have been made through various length scales, by first principles, molecular dynamics or finite elements. Here, a quasi-simulated-annealing (Markov Chain Monte Carlo algorithm) with prior burn-in procedure using chemical short-range order parameters as input values is developed for searching the global optimal distribution of species with regard to the phase stability of face-centered cubic CoCrNi alloys. The results show that this new approach could effectively generate the stable structures with a desired concentration and moreover, the stacking fault energies of these configurations have less uncertainties statistically.
Corrosion mechanism of annealed equiatomic AlCoCrFeNi tri-phase high-entropy alloy in 0.5 M H2SO4 aerated aqueous solution
(2019-08-15)
Yen, Chao-Chun
;
Lu, Hsueh-Ning
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Tsai, Ming-Hung
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Wu, Bo-Wei
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Lo, Yu-Chieh
This work investigates the corrosion mechanism of annealed equiatomic AlCoCrFeNi tri-phase alloy in 0.5 M H2SO4 aerated aqueous solution. Experimental results indicate that the B2 matrix is preferentially corroded away while grain-boundary FCC and labyrinth-like BCC endure, consistent with the tendency of formation energy for monolayer hydroxide on the (001) plane of three phases calculated by the first principle based on density function theory. Furthermore, the stable passive current density is related to lower k(sp) values of Co(OH)(3), Fe (OH)(3), and Cr(OH)(3), while the annealing effect on enhancing corrosion resistance is owing to the more uniformity of passive hydroxide film.
Development of Novel Lightweight Dual-Phase Al-Ti-Cr-Mn-V Medium-Entropy Alloys with High Strength and Ductility
(2020-01-01)
Liao, Yu-Chin
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Chen, Po-Sung
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Li, Chao-Hsiu
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Tsai, Pei-Hua
;
Jang, Jason S. C.
A novel lightweight Al-Ti-Cr-Mn-V medium-entropy alloy (MEA) system was developed using a nonequiatiomic approach and alloys were produced through arc melting and drop casting. These alloys comprised a body-centered cubic (BCC) and face-centered cubic (FCC) dual phase with a density of approximately 4.5 g/cm(3). However, the fraction of the BCC phase and morphology of the FCC phase can be controlled by incorporating other elements. The results of compression tests indicated that these Al-Ti-Cr-Mn-V alloys exhibited a prominent compression strength (1940 MPa) and ductility (30%). Moreover, homogenized samples maintained a high compression strength of 1900 MPa and similar ductility (30%). Due to the high specific compressive strength (0.433 GPag/cm(3)) and excellent combination of strength and ductility, the cast lightweight Al-Ti-Cr-Mn-V MEAs are a promising alloy system for application in transportation and energy industries.
Differences in texture evolution from low-entropy to high-entropy face-centered cubic alloys during tension test
(2020-03-01)
Lin, Kuan-Hao
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Chang, Shou-Yi
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Lo, Yu-Chieh
;
Wang, Chun-Chieh
;
Lin, Su-Jien
Texture evolution during tension test was investigated on three face-centered cubic (FCC) metals, namely, Ni (unary), equiatomic CrFeNi (ternary), and equiatomic CoCrFeMnNi (quinary) with small and large grain sizes of similar to 20 and similar to 100 mu m, respectively. The microtextures for 18%, 36% and 54% engineering strains of a tension-interrupted specimen were characterized on the same area. The three metals exhibited a similar texture evolution, which the fraction of {001}//ND remains constant However, the fraction of {011}//ND decreases, whereas that of {111}//ND increases. On the other hand, the fraction of <001>//RD also remains constant, that of <011>//RD decreases, and that of <111>//RD increases. However, the several differences still were found during plastic deformation. Fine-grained Ni initiated a significantly crystallographic rotation (CRo) at the lower strain, increased with strain and then remained nearly unchanged as strain was higher than 18%. Coarse-grained Ni delayed this rotation and slowly developed the trend at large strains. As for CrFeNi, the comparison between fine-grained and coarse-grained structure does not have obvious difference. As for fine-grained CoCrFeMnNi, it can be seen that an obvious CRo develops under 36% strain, and maintains constant over 36% strain. But the coarse-grained CoCrFeMnNi reveals that the significant CRo develops continuously from 0% to 54% strain. The mechanisms for the effects of number of element and grain size in the present study are elucidated.
Electron Beam Irradiation-Induced Deoxidation and Atomic Flattening on the Copper Surface
(2019-10-30)
Lin, Ting-Yi
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Lee, Shih-Kuang
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Huang, Guan-Min
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Huang, Chun-Wei
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Tai, Kuo-Lun
Electron beam (e-beam) has been developed for nanomaterial observation and moreover to induce structural evolutions in atomic scale. In this work, we demonstrate the deoxidation of cuprous oxide (Cu2O) and the formation of an atomically flat surface on a Cu nanowire by e-beam irradiation. To develop e-beam irradiation applications, the relation between e-beam radiation and the atomic surface is significant. Through the density functional theory simulation of atomic sputtering, an obvious disparity in the sputtering threshold has been found under different structural conditions, which leads to different structural evolutions. Both surface deoxidation and atomic surface flattening reactions have been identified as self-limiting and irreversible processes via in situ transmission electron microscope observation. Under e-beam irradiation, the dynamic mechanism of atomic surface flattening is driven by the convergence of total surface energy and confirmed by climbing-image nudged elastic band (ci-NEB) calculations. With precise control, e-beam irradiation reveals enormous potentials in atomic surface engineering.
Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment
(2019-01-23)
Naresh, Gollapally
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Hsieh, Pei-Lun
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Meena, Vandana
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Lee, Shih-Kuang
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Chiu, Yi-Hsuan
ZnS particles were grown over Cu2O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu2O cubes stay photocatalytically inactive. ZnS-decorated Cu2O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photo excitation. Surprisingly, Cu2O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu2O(110)/ZnS(200) plane interface. A modified Cu2O-ZnS band diagram is presented. Density functional theory calculations generating plane -specific band energy diagrams of Cu2O and ZnS match well with the experimental results, showing that charge transfer across the Cu2O(110)/ZnS(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.
Facet-Specific Photocatalytic Activity Enhancement of Cu2O Polyhedra Functionalized with 4-Ethynylanaline Resulting from Band Structure Tuning
(2020-06-24)
Chen, Tzu-Ning
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Kao, Jui-Cheng
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Zhong, Xin-Yan
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Chan, Shang-Ju
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Patra, Anindya S.
Cu2O rhombic dodecahedra, octahedra, and cubes were densely modified with conjugated 4-ethynylaniline (4-EA) for facet-dependent photocatalytic activity examination. Infrared spectroscopy affirms bonding of the acetylenic group of 4-EA onto the surface copper atoms. The photocatalytically inactive Cu2O cubes showed surprisingly high activity toward methyl orange photodegradation after 4-EA modification, while the already active Cu2O rhombic dodecahedra and octahedra exhibited a photocatalytic activity enhancement. Electron, hole, and radical scavenger experiments prove that the photocatalytic charge transport processes have occurred in the functionalized Cu2O cubes. Electrochemical impedance spectroscopy also indicates reduced charge transfer resistance of the functionalized Cu2O crystals. A band diagram constructed from UV-vis spectral and Mott-Schottky measurements reveals significant band energy shifts in all Cu2O samples after decorating with 4-EA. From density functional theory (DFT) calculations, a new band has emerged slightly above the valence band maximum within the band gap of Cu2O, which has been found to originate from 4-EA through band-decomposed charge density analysis. The increased charge density localized on the 4-EA molecule and the smallest electron transition energy to reach the 4-EA-generated band are factors making {100}-bound Cu2O cubes photocatalytically active. Proper molecular decoration represents a powerful approach to improving the photocatalytic efficiency of semiconductors.
In Situ Investigation of Defect-Free Copper Nanowire Growth
(2018-02-01)
Lin, Ting-Yi
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Chen, Yong-Long
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Chang, Chia-Fu
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Huang, Guan-Min
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Huang, Chun-Wei
The fabrication and placement of high purity nanometals, such as one-dimensional copper (Cu) nanowires, for interconnection in integrated devices have been among the most important technological developments in recent years. Structural stability and oxidation prevention have been the key issues, and the defect control in Cu nanowire growth has been found to be important. Here, we report the synthesis of defect-free single-crystalline Cu nanowires by controlling the surface-assisted heterogeneous nucleation of Cu atomic layering on the graphite-like loop of an amorphous carbon (a-C) lacey film surface. Without a metal-catalyst or induced defects, the high quality Cu nanowires formed with high aspect ratio and high growth rate of 578 nm/s. The dynamic study of the growth of heterogeneous nanowires was conducted in situ with a high-resolution transmission electron microscope. The study illuminates the new mechanism by heterogeneous nucleation control and laying the groundwork for better understanding of heterosurface-assisted nucleation of defect-free Cu nanowire on a-C lacey film.
Interplay between potassium doping and bandgap profiling in selenized Cu(In,Ga)Se-2 solar cells: A functional CuGa: KF surface precursor layer
(2018-05-01)
Cai, Chung-Hao
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Chen, Rong-Zhi
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Chan, Ting-Shan
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Lu, Ying-Rui
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Huang, Wei-Chih
The progress of selenized Cu(In,Ga)Se-2 (CIGSe) solar cells is limited by low open-circuit voltage (Voc), which results from the Ga-deficient surface and undesirable bandgap profile after selenization. Controlling the Ga grading, especially on the CIGSe surface, is challenging but critical for further efficiency improvement. Here, the simple sputtering route with K incorporation is presented to engineer single-or double-graded bandgap. The K incorporation through sputtered precursors can considerably affect the Ga profile in CIGSe during selenization, essentially distinct from reported KF post-deposition treatment, in which the Ga profile keeps unchanged. Using synchrotron-based X-ray absorption spectroscopy and first-principle calculations, we verify that Ga diffusion via Cu vacancies is restrained due to the presence of K-Cu defects. Therefore, by introducing a CuGa:KF surface layer on the bi-layer precursors, the surface Ga content of CIGSe is increased effectively, achieving a notch-like Ga distribution after selenization. This unique CuGa: KF layer significantly boosts the Voc and yields over 15% efficiency, even with a low reactive Se vapor for selenization. Our approach, completely compatible with the existing fabrication process, offers a new direction for engineering band structure without sulfurization.
Lattice distortion effect on elastic anisotropy of high entropy alloys
(2020-03-25)
Yen, Chao-Chun
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Huang, Guan-Rong
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Tan, Yun-Cheng
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Yeh, Han-Wen
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Luo, Da-Ji
The superior mechanical properties of high-entropy alloys (HEAs) have made an outstanding success in materials science and engineering. Studies to date have been devoted to what the severe lattice distortion induces. However, most researchers focus on its stimulation to plastic deformation instead of scrutinizing the variations on elasticity. Compared with conventional alloys, HEAs may perform disproportionate elasticity with uneven local lattice strain resulting from the severe lattice distortion. Therefore, it is necessary to survey its influence on the mechanical properties of HEAs systematically. In this study, the Lennard-Jones (LJ) potential, the embedded atom method (EAM) potential, and the modified embedded atom method (MEAM), are respectively conducted to investigate the lattice distortion effect on Young's modulus E (hkl) and Poisson's ratio nu (hkl, theta) along [100], [110], and [111] loading directions for several fcc metals composed of 1 similar to 5 atomic types, including Ni, Ni98W2, Ni96W4, FeCrNi, and CoNiCrFeMn HEAs. Also, a method is used to analyze the performance of the individual element on the elastic properties in the HEA environment. As a result, it can be unveiled that the effect of electron density inconsistency is more dominant than the effect of lattice distortion associated with the atomic size difference. The electronic configuration in the HEA environment plays a major role in elastic anisotropy while the difference of the atomic radii does the minor one. The anisotropy of CoNiCrFeMn HEA analyzed by this work is also consistent with in-situ neutron diffraction measurements. (C) 2019 Elsevier B.V. All rights reserved.
Mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries: An accelerated molecular dynamics study
(2016-09-14)
Du, Jun-Ping
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Wang, Yun-Jiang
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Lo, Yu-Chieh
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Wan, Liang
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Ogata, Shigenobu
Accelerated molecular dynamics reveals a mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries (GBs) in Cu. At stress levels up to similar to 90% of the ideal dislocationnucleation stress, atomic shuffling at the E structural unit in a GB acts as a precursor to dislocation nucleation, and eventually a single dislocation is nucleated. At very high stress levels near the ideal dislocation-nucleation stress, a multiple dislocation is collectively nucleated. In these processes, the activation free energy and activation volume depend strongly on temperature. The strain-rate dependence of the critical nucleation stress is studied and the result shows that the mechanism transition from the shuffling-assisted dislocation-nucleation mechanism to the collective dislocation-nucleationmechanism occurs during the strain rate increasing from 10(-4) s(-1) to 10(10) s(-1).
Observing phase transformation in CVD-grown MoS2 via atomic resolution TEM
(2018-09-14)
Tai, Kuo-Lun
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Huang, Guan-Min
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Huang, Chun-Wei
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Tsai, Tsung-Chun
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Lee, Shih-Kuang
We utilized in situ transmission electron microscopy to observe phase transformation in CVD-grown MoS2. Significantly, the reaction was performed under electron irradiation through appropriate control of the electron dose and exposure time. Moreover, we proposed a new route between the 2H and 1T phases that involved the higher energy states TS1/TS2.
Origin of serrated flow in bulk metallic glasses
(2019-03-01)
Xie, Xie
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Lo, Yu-Chieh
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Tong, Yang
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Qiao, Junwei
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Wang, Gongyao
Bulk metallic glasses (BMGs) possess amorphous structure and show unique mechanical properties, such as extremely high strength and excellent damage tolerance, entitling them as potential structural materials. So far a great amount of work has been conducted to study BMGs' macroscopic mechanical properties and examine corresponding microscopic deformation defects. However, the connection between macroscopic inhomogeneous deformation at room temperature and microscopic deformation carriers is still poorly understood, due to the lack of an appropriate experimental technique to directly probe the inhomogeneous deformation process on the proper spatial and temporal scales. Here we present the deformation details via in situ thermal imaging about the evolution of heat bands associated with successive serration behavior. For the first time, our experimental work clarifies the coupling of serrated flows with shear band activities, especially the often omitted fine serrations induced by shear band nucleation or the early stage of propagation. Meanwhile, serration behavior of BMGs is simulated through the kinetic Monte Carlo (kMC) method by integrating local heating (thermal softening and beta-relaxation) effects, which exhibits good agreement with experimental results. These findings will advance our fundamental understanding of the shear band operation down to microscopic level, which may shed light on the control of shear banding for the application of BMGs. (C) 2018 Elsevier Ltd. All rights reserved.
Phase transformation assisted twinning in a face-centered-cubic FeCrNiCoAl0.36 high entropy alloy
(2019-12-01)
Yu, Peijun
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Feng, Rui
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Du, Junping
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Shinzato, Shuhei
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Chou, Jyh-Pin
The FeNiCoCr-based high entropy alloys (HEAs) exhibit excellent mechanical properties, such as twin-induced plasticity (TWIP) and phase transformation plasticity (TRIP) that can reach a remarkable combination of strength and ductility. In the present work, the face-centered-cubic (FCC) single-crystal FeNiCoCrAl0.36 HEAs were studied, using the density functional theory (DFT) combined with the phonon calculation to estimate the stacking fault energies, temperature-dependent phase stabilities of different structures. And the kinetic Monte Carlo (kMC) was used to predict the substructures evolution based on the transition state energies obtained from DFT calculations. We proposed two different formation paths of nano-twins in this Al-composited HEA and found that short-range hexagonal-close-packed (HCP)-stacking could occur in this HEA. The DFT calculations suggest that this HEA has negative stacking fault energy (SFE), HCP formation energy, and twin-formation energy at OK. Phonon calculations indicate that at the finite temperature, the competing FCC/HCP phase stability and propensity for twinning make it possible to form HCP-like twin boundaries. The kMC simulations suggest that under deformation, TWINs could form within the HCP laths which differs from the study of others. With the great agreement of results from kMC simulations and experiments, this twin-hcp laminated substructure formation path offers a new concept of designing TWIP HEAs containing tunable twin structures with HCP and TWIN lamellae structures, which could result in better mechanical properties of HEAs. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
The influence of dilute aluminum and molybdenum on stacking fault and twin formation in FeNiCoCr-based high entropy alloys based on density functional theory
(2019-07-29)
Yu, Peijun
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Zhuang, Yu
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Chou, Jyh-Pin
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Wei, Jie
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Lo, Yu-Chieh
Stacking faults, as defects of disordered crystallographic planes, are one of the most important slipping mechanisms in the commonly seen lattice, face-centered cubic (FCC). Such defects can initiate twinning which strengthens mechanical properties, e.g. twinning-induced plasticity (TWIP), of high entropy alloys (HEAs) at cryogenic temperatures. In this work, by using density functional theory (DFT), the twinning initiated from stacking faults is discussed with regard to two different solute elements, Al and Mo, in the FeNiCoCr HEAs. Our results show that adding aluminum (Al) has noticeable enhancement of twinnability while molybdenum (Mo) only induces more stacking faults in the FeNiCoCr-based HEAs.