14 research outputs found
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Revealing Grain Boundary Sliding from Textures of a Deformed Nanocrystalline Pd–Au Alloy
Employing a recent modeling scheme for grain boundary sliding [Zhao et al. Adv. Eng. Mater. 2017, doi:10.1002/adem.201700212], crystallographic textures were simulated for nanocrystalline fcc metals deformed in shear compression. It is shown that, as grain boundary sliding increases, the texture strength decreases while the signature of the texture type remains the same. Grain boundary sliding affects the texture components differently with respect to intensity and angular position. A comparison of a simulation and an experiment on a Pd–10 atom % Au alloy with a 15 nm grain size reveals that, at room temperature, the predominant deformation mode is grain boundary sliding contributing to strain by about 60%
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Phase Transformation Induced by High Pressure Torsion in the High-Entropy Alloy CrMnFeCoNi
The forward and reverse phase transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) in the equiatomic high-entropy alloy (HEA) CrMnFeCoNi has been investigated with diffraction of high-energy synchrotron radiation. The forward transformation has been induced by high pressure torsion at room and liquid nitrogen temperature by applying different hydrostatic pressures and large shear strains. The volume fraction of hcp phase has been determined by Rietveld analysis after pressure release and heating-up to room temperature as a function of hydrostatic pressure. It increases with pressure and decreasing temperature. Depending on temperature, a certain pressure is necessary to induce the phase transformation. In addition, the onset pressure depends on hydrostaticity; it is lowered by shear stresses. The reverse transformation evolves over a long period of time at ambient conditions due to the destabilization of the hcp phase. The effect of the phase transformation on the microstructure and texture development and corresponding microhardness of the HEA at room temperature is demonstrated. The phase transformation leads to an inhomogeneous microstructure, weakening of the shear texture, and a surprising hardness anomaly. Reasons for the hardness anomaly are discussed in detail
Revealing Grain Boundary Sliding from Textures of a Deformed Nanocrystalline Pd–Au Alloy
Employing a recent modeling scheme for grain boundary sliding [Zhao et al. Adv. Eng. Mater. 2017, doi:10.1002/adem.201700212], crystallographic textures were simulated for nanocrystalline fcc metals deformed in shear compression. It is shown that, as grain boundary sliding increases, the texture strength decreases while the signature of the texture type remains the same. Grain boundary sliding affects the texture components differently with respect to intensity and angular position. A comparison of a simulation and an experiment on a Pd–10 atom % Au alloy with a 15 nm grain size reveals that, at room temperature, the predominant deformation mode is grain boundary sliding contributing to strain by about 60%
Hall-plot of the phase diagram for Ba(Fe1-xCox)2As2
The Hall effect is a powerful tool for investigating carrier type and
density. For single-band materials, the Hall coefficient is traditionally
expressed simply by , where is the charge of the carrier,
and is the concentration. However, it is well known that in the critical
region near a quantum phase transition, as it was demonstrated for cuprates and
heavy fermions, the Hall coefficient exhibits strong temperature and doping
dependencies, which can not be described by such a simple expression, and the
interpretation of the Hall coefficient for Fe-based superconductors is also
problematic. Here, we investigate thin films of Ba(FeCo)As
with compressive and tensile in-plane strain in a wide range of Co doping. Such
in-plane strain changes the band structure of the compounds, resulting in
various shifts of the whole phase diagram as a function of Co doping. We show
that the resultant phase diagrams for different strain states can be mapped
onto a single phase diagram with the Hall number. This universal plot is
attributed to the critical fluctuations in multiband systems near the
antiferromagnetic transition, which may suggest a direct link between magnetic
and superconducting properties in the BaFeAs system.Comment: Accepted for publication in Scientific Reports, 6 main figures plus
Supplemental Information (8 figures
The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films
Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on
(La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2
substrates, which exhibit an almost identical in-plane lattice parameter. The
composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy
dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the
lattice parameters of all templates have comparable values, the in-plane
lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that
the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is
strongly correlated with their a-axis lattice parameter. The highest Tc of over
19 K was observed for the film on bare CaF2 substrate, which is related to
unexpectedly large in-plane compressive strain originating mostly from the
thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.Comment: Accepted in AIP Advances, 4 figure
Universal scaling behavior of the upper critical field in strained FeSe0.7Te0.3 thin films
open15Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field H c2 at the superconducting transition temperature T c (i.e. -dH c2/dT) for FeSe0.7Te0.3 thin films. The in-plane strain tunes T c in a broad range, while the composition and disorder are almost unchanged. We show that -dH c2/dT scales linearly with T c, indicating that FeSe0.7Te0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s ± superconductivity.openYuan, Feifei; Grinenko, Vadim; Iida, Kazumasa; Richter, Stefan; Pukenas, Aurimas; Skrotzki, Werner; Sakoda, Masahito; Naito, Michio; Sala, Alberto; Putti, Marina; Yamashita, Aichi; Takano, Yoshihiko; Shi, Zhixiang; Nielsch, Kornelius; Hühne, RubenYuan, Feifei; Grinenko, Vadim; Iida, Kazumasa; Richter, Stefan; Pukenas, Aurimas; Skrotzki, Werner; Sakoda, Masahito; Naito, Michio; Sala, Alberto; Putti, Marina; Yamashita, Aichi; Takano, Yoshihiko; Shi, Zhixiang; Nielsch, Kornelius; Hühne, Rube
Phase Transformation Induced by High Pressure Torsion in the High-Entropy Alloy CrMnFeCoNi
The forward and reverse phase transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) in the equiatomic high-entropy alloy (HEA) CrMnFeCoNi has been investigated with diffraction of high-energy synchrotron radiation. The forward transformation has been induced by high pressure torsion at room and liquid nitrogen temperature by applying different hydrostatic pressures and large shear strains. The volume fraction of hcp phase has been determined by Rietveld analysis after pressure release and heating-up to room temperature as a function of hydrostatic pressure. It increases with pressure and decreasing temperature. Depending on temperature, a certain pressure is necessary to induce the phase transformation. In addition, the onset pressure depends on hydrostaticity; it is lowered by shear stresses. The reverse transformation evolves over a long period of time at ambient conditions due to the destabilization of the hcp phase. The effect of the phase transformation on the microstructure and texture development and corresponding microhardness of the HEA at room temperature is demonstrated. The phase transformation leads to an inhomogeneous microstructure, weakening of the shear texture, and a surprising hardness anomaly. Reasons for the hardness anomaly are discussed in detail
Phase transformation induced by high pressure torsion in the high-entropy alloy CrMnFeCoNi
The forward and reverse phase transformation from face-centered cubic (fcc) to hexagonalclose-packed (hcp) in the equiatomic high-entropy alloy (HEA) CrMnFeCoNi has been investigatedwith diffraction of high-energy synchrotron radiation. The forward transformation has been inducedby high pressure torsion at room and liquid nitrogen temperature by applying different hydrostaticpressures and large shear strains. The volume fraction of hcp phase has been determined by Rietveldanalysis after pressure release and heating-up to room temperature as a function of hydrostaticpressure. It increases with pressure and decreasing temperature. Depending on temperature, acertain pressure is necessary to induce the phase transformation. In addition, the onset pressuredepends on hydrostaticity; it is lowered by shear stresses. The reverse transformation evolves overa long period of time at ambient conditions due to the destabilization of the hcp phase. The effectof the phase transformation on the microstructure and texture development and correspondingmicrohardness of the HEA at room temperature is demonstrated. The phase transformation leadsto an inhomogeneous microstructure, weakening of the shear texture, and a surprising hardnessanomaly. Reasons for the hardness anomaly are discussed in detail
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Microstructure, Texture, and Strength Development during High-Pressure Torsion of CrMnFeCoNi High-Entropy Alloy
The equiatomic face-centered cubic high-entropy alloy CrMnFeCoNi was severely deformed at room and liquid nitrogen temperature by high-pressure torsion up to shear strains of about 170. Itsmicrostructurewas analyzed by X-ray line profile analysis and transmission electronmicroscopy and its texture by X-ray microdiffraction. Microhardness measurements, after severe plastic deformation, were done at room temperature. It is shown that at a shear strain of about 20, a steady state grain size of 24 nm, and a dislocation density of the order of 1016 m-2 is reached. The dislocations are mainly screw-type with low dipole character. Mechanical twinning at room temperature is replaced by a martensitic phase transformation at 77 K. The texture developed at room temperature is typical for sheared face-centered cubic nanocrystalline metals, but it is extremely weak and becomes almost random after high-pressure torsion at 77 K. The strength of the nanocrystalline material produced by high-pressure torsion at 77 K is lower than that produced at room temperature. The results are discussed in terms of different mechanisms of deformation, including dislocation generation and propagation, twinning, grain boundary sliding, and phase transformation. © 2020 by the authors. Licensee MDPI, Basel, Switzerland
Microstructure, Texture, and Strength Development during High-Pressure Torsion of CrMnFeCoNi High-Entropy Alloy
The equiatomic face-centered cubic high-entropy alloy CrMnFeCoNi was severely deformed at room and liquid nitrogen temperature by high-pressure torsion up to shear strains of about 170. Its microstructure was analyzed by X-ray line profile analysis and transmission electron microscopy and its texture by X-ray microdiffraction. Microhardness measurements, after severe plastic deformation, were done at room temperature. It is shown that at a shear strain of about 20, a steady state grain size of 24 nm, and a dislocation density of the order of 1016 m−2 is reached. The dislocations are mainly screw-type with low dipole character. Mechanical twinning at room temperature is replaced by a martensitic phase transformation at 77 K. The texture developed at room temperature is typical for sheared face-centered cubic nanocrystalline metals, but it is extremely weak and becomes almost random after high-pressure torsion at 77 K. The strength of the nanocrystalline material produced by high-pressure torsion at 77 K is lower than that produced at room temperature. The results are discussed in terms of different mechanisms of deformation, including dislocation generation and propagation, twinning, grain boundary sliding, and phase transformation