The carbon distribution behavior and crystal structure of as-quenched martensite in Fe-1Mn-C (C: 0.07 to 0.8 mass pct) steels were quantitatively investigated by atom probe tomography (APT) and X-ray ...diffraction with Rietveld analysis. APT revealed that the martensite steels contained quantities of carbon in solid solution far beyond its solubility in body-centered cubic (bcc)-Fe in all the alloys investigated; the carbon atoms were non-homogeneously distributed as carbides or aggregates on dislocations due to autotempering. Tetragonality was observed in the steels with interstitial solute carbon concentrations in the range of 0.1 to 0.7 mass pct, but was not evident below 0.1 mass pct. The appearance and disappearance of tetragonality in the low-carbon steels may be explained by the disordered bcc ↔ ordered body-centered tetragonal (bct) mechanism, considering the partial tetragonality due to the heterogeneity of the interstitial solute carbon distribution. The existence of tetragonality in the autotempered low-carbon steels can alternatively be understood by a mechanism based on the kinetic decrease of tetragonality during cooling, where the microscopic strain release is the rate-controlling process. The excess carbon solubility in the autotempered low- and medium-carbon martensite is due to the existence of tetragonal distortions, owing to the slow kinetics of the tetragonality decrease during cooling.
High-strength low-to-medium carbon martensitic steel is increasingly used in the automobile industry. This study investigated the room temperature aging behavior of as-quenched autotempered Fe–C lath ...martensitic steels (C: 0.07–0.77 mass%) using kinetic analysis of hardness change and interrupted atom probe (AP) analysis to clarify the dominating factor of hardening. Age-hardening at 23°C was confirmed in the autotempered lath martensitic steels, including low-carbon steel with a carbon content of less than 0.25 mass%. The AP and kinetic analyses of hardness evolution indicated that the growth of carbon clusters at dislocations dominates the hardening of martensite. The maximum hardness increment in lath martensite increased with initial excess solute carbon Csol in the matrix, but the increment in unit Csol was smaller than that in carbon-supersaturated ferrite. The smaller hardness increase in martensite may indicate the concurrent softening due to the relaxation of the residual lattice strain in martensite by carbon clustering. Interrupted AP analysis of the prolonged aging over two years indicated that the transformation from carbon clusters to iron carbides occurs via an in-situ transformation of the clusters. The microscopic heterogeneity in carbon distribution in the order of martensite blocks and the gradual decrease in excess Csol during room temperature aging were also confirmed by AP analysis. The persistence of the heterogeneity and excess solute carbon in the martensite matrix after aging and tempering is also discussed.
The tetragonality and carbon distribution in tempered Fe-0.6C-1Mn martensite were investigated by X-ray diffraction and atom probe tomography to elucidate strain relaxation in the tetragonal lattice ...during tempering and its relationship with the solubility of excess carbon in martensite. Even though tetragonality (
c
/
a
) decreased with an increase in the tempering temperature, it persisted at low levels up to 400 °C. Si addition suppressed the decrease in tetragonality at 400 °C by inhibiting recovery in the dislocated matrix. Such persistence implies that dislocation migration is crucial for the complete release of tetragonal lattice strain at such a temperature, in addition to the decrease in the amount of solute carbon in martensite. A low level of tetragonality was observed for martensite containing carbon in the solid solution below the critical value of ~ 0.2 mass pct, at which a bcc structure was predicted. The amount of solute carbon after tempering was linearly correlated with tetragonality in the solute carbon content range of 0.07 to 0.6 mass pct, and the correlation coefficient was similar to those for as-quenched auto-tempered martensite and bainitic ferrite; these results indicate that the amount of excess carbon is simply determined by the amount of tetragonal lattice distortions remaining after carbide precipitation and recovery.
The dynamic optical properties of perovskite CH3NH3PbI3 single crystals were studied by means of time-resolved photoluminescence (PL) spectroscopy at room temperature. The PL peak under one-photon ...excitation exhibits a red-shift with elapsing time, while two-photon PL is time-independent and appears at lower energy levels. The low-energy two-photon PL can be attributed to emissions from the localized states because of strong band-to-band absorption and photon re-absorption of the emitted light in the interior region. We revealed that the PL behaviors can be explained by the diffusion of photocarriers generated in the near-surface region to the interior region. The excitation fluence dependence of the one-photon PL dynamics is also discussed in terms of the electron–hole radiative recombination and carrier diffusion effects.
To elucidate the mechanisms of deformation and a state of plastic stability at the front of Lüders bands during a tensile test, metastable austenitic transformation-induced plasticity (TRIP) steels ...with different dislocation densities and ferritic steels were characterized via macroscopic-DIC-based stress–strain investigations and scanning electron microscopy (SEM). A direct correlation between stress–strain curves and measured strain distributions in the tensile specimen indicated that the Lüders front represents a transition region from a state of plastic instability to one of stability, whereby a general rule relating the Lüders strain ( ) and increments in the true stress in the Lüders band ( ) to a lower yield stress ( ) can be described as irrespective of the amount of deformation-induced martensite in the band or crystal structure of the steel. The inclination angle of the Lüders front with respect to the tensile direction changed from 55° to 90° with a reduction in the measured strain ratio (−εyy/εxx) in the Lüders band, and the change agreed with the tendency calculated by the plasticity model, assuming the pure shear occurs under the minimum shear strain criterion. SEM observations of the sheet surface and the front cross-section in the TRIP steel showed the formation of multiple inclined ~20 µm-wide shear deformation zones that accompanied a reduction in thickness. All the observed geometrical characteristics of the Lüders front were qualitatively described by a mechanism involving minimizing the misalignment from the fixed tensile axis caused by ‘shear’ deformation.
We have investigated the dynamic optical properties of CH3NH3PbI3 (MAPbI3) perovskite thin films at low temperatures using time-resolved photoluminescence, optical transient absorption (TA), and THz ...TA spectroscopy. Optical spectroscopic results indicate that the high-temperature tetragonal phase still remains in the MAPbI3 thin films at low temperatures in addition to the major orthorhombic phase. The fast charge transfer from the orthorhombic phase to the tetragonal phase is likely to suppress the formation of excitons in the orthorhombic phase. Consequently, the near-band-edge optical responses of the photocarriers in both the tetragonal and orthorhombic phases of the MAPbI3 thin films are more accurately described by a free-carrier model, rather than an excitonic model even at low temperatures.
The precipitation of iron carbides is a crucial factor that determines the properties of tempered martensite. However, the effect of alloying elements on the carbon concentration of ε carbide has not ...yet been clarified. In this work, we studied the effect of alloying elements on the carbon concentration of ε carbide using first-principles calculations and a three-dimensional atom probe. The first-principles calculations showed that ε carbide with a lower carbon concentration tends to form by the inclusion of Si. The carbon concentration in ε carbide measured by the three-dimensional atom probe was consistent with the first-principles calculations.
A high‐purity methylammonium lead iodide complex with intercalated dimethylformamide (DMF) molecules, CH3NH3PbI3⋅DMF, is introduced as an effective precursor material for fabricating high‐quality ...solution‐processed perovskite layers. Spin‐coated films of the solvent‐intercalated complex dissolved in pure dimethyl sulfoxide (DMSO) yielded thick, dense perovskite layers after thermal annealing. The low volatility of the pure DMSO solvent extended the allowable time for low‐speed spin programs and considerably relaxed the precision needed for the antisolvent addition step. An optimized, reliable fabrication method was devised to take advantage of this extended process window and resulted in highly consistent performance of perovskite solar cell devices, with up to 19.8 % power‐conversion efficiency (PCE). The optimized method was also used to fabricate a 22.0 cm2, eight‐cell module with 14.2 % PCE (active area) and 8.64 V output (1.08 V/cell).
No rush: High‐efficiency CH3NH3PbI3 perovskite solar cells (19.8 % power‐conversion efficiency, PCE) and large‐area eight‐cell modules (14.2 % PCE, 8.64 V) were readily fabricated from a purified perovskite precursor material, CH3NH3PbI3⋅DMF (see picture). The low volatility of the pure DMSO solvent (as compared to DMF/DMSO) extended the allowable time for low‐speed spin programs and relaxed the precision needed for the antisolvent addition step.
The article examines the influence of delta-wing V-type baffles (DW-PVBs) on the average Nusselt number, local Nusselt number distribution, pressure losses, and thermal performance behaviors in a ...channel. Delta-wing perforated V-type baffles (DW-PVBs) mounted in a regular manner on the bottom of a channel produced two pairs of longitudinal counter-rotating vortices to enhance chaotic fluid mixing and destabilize the boundary layer, hence boosting the heat transfer. The geometric characteristics of the delta-wing V-type baffle (DW-PVBs) located on the bottom of the channel were examined at relative baffle blockage and pitch ratios (BR =
h
/
H
= 0.3 and
p/H
= 1.5), and five delta-wing attack angles,
θ
= 0
o
(solid V-shaped baffle), 22.5°, 45°, 67.5°, and 90°. The present DW-PVBs mounted on the channel were designed to mitigate pressure loss due to flow blockage. The experiment was done by permitting air to flow through a channel at Reynolds numbers (Re) ranging from 6000 to 24,000. The present results show that the friction factor using the DW-PVBs decreased considerably with increasing
θ
values. The experimental results revealed that small
θ
values yielded greater heat transfer and resistance than large
θ
values. The DW-PVBs with
θ
= 22.5° performed better than inserts with other
θ
values in terms of heat transfer rate. It was also observed that the DW-PVBs with
θ
= 45° gave the maximum thermal performance factor (TPF), while presenting a 13.64–17.26% lower friction factor than the solid V-shaped baffle. Furthermore, it was also found that the DW-PVBs with
θ
= 0°, 22.5°, 45°, 67.5°, and 90° gave peak TPF values of up to 1.87, 1.89, 1.91, 1.87, and 1.84 at the lowest Reynolds number, 6000.
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