Homogeneous Grain Research Articles

Dynamic Recrystallization in Magnesium Alloy AZ31 Under Large Plain Strain Warm Deformation Conditions

In the present study, plane strain compression was used to investigate the large plastic deformation behavior and microstructural evolution in magnesium alloy AZ31 with a view to understand the microstructural restoration mechanisms under the applied test conditions. While the stress–strain curves at all test conditions were indicative of dynamic recrystallization (DRX), the amounts of flow softening under different temperature–strain rate conditions was found to be different. The extent of recrystallization increased with increasing deformation temperature or decreasing strain rate with microstructures having higher extent of recrystallization revealing more homogeneous grain structure. DRX was found to be discontinuous in nature and a slightly different mechanism of DRX is proposed based on electron backscattered diffraction (EBSD) analysis. This consists of an inward bulging of the boundary due to the dislocation density being higher inside the larger deformed grains. Consequently, the deformation-induced dislocations outside such inward bulges help in anchoring them leading to the formation of recrystallized nuclei.

Ionic-liquid induced enhanced performance of perovskite light-emitting diodes

Organic–inorganic hybrid perovskites (OIHP) have attracted a great deal of attention as promising light-emitting materials, because of their low-cost production via simple solution processing, high color purity, and easily tunable bandgaps. Nevertheless, the fabrication of OIHP films for light-emitting diodes generally requires complicated post-treatments, such as an anti-solvent dripping process, owing to the poor film forming property of the perovskite precursor solution. In this work, we employed an ionic liquid, methylammonium acetate (MAAc), as an additive to optimize the growth of FAPbBr3 perovskite films via an anti-solvent-free approach. The influence of MAAc on the crystallization kinetics of FAPbBr3 phase and optoelectronic properties of the resulting perovskite films have been systematically investigated. Our results demonstrate that MAAc is an effective modifier for creating perovskite films with reduced grain size, homogeneous grain distribution, and smooth surface, leading to an enhanced average lifetime of the excitons. Perovskite light-emitting diodes (PeLEDs) containing FAPbBr3 films obtained via incorporating 10 vol% MAAc as an additive exhibited considerably suppressed leakage current, and a significant enhancement in current efficiency (CE) from 0.027 to 4.85 cd A−1. These findings reveal that the incorporation of a suitable additive could play a beneficial role in the formation of well-optimized perovskite films, which would ultimately lead to the realization of PeLEDs with improved performance.

Cyclic strain amplitude-dependent fatigue mechanism of gradient nanograined Cu

Different grain coarsening behaviors (i.e. abnormal and homogeneous) are prevalently observed in gradient nanograined (GNG) Cu under stress controlled high-cycle and strain controlled low-cycle fatigue tests, respectively. In this paper, to comprehensively understand the intrinsic fatigue mechanism of gradient nanograined structures, both high and low cycle fatigue behaviors of GNG Cu are investigated under strain-controlled fatigue tests with a wide strain amplitude ranges. Cyclic behavior transition from abnormal grain coarsening at small strain amplitude to homogeneous grain coarsening at large strain amplitude is observd in GNG Cu. Microstrucural analysis reveals that the grain coarsening behavior in either abnormal or normal (homogeneous) mode is closely related to the spatial distribution of the cyclic plastic strain in the GNG layer (localized or delocalized) under cyclic loading. Such unique cyclic strain amplitude-dependent fatigue behavior is inherent to the gradient nanostructure, which fundamentally differs from the conventional strain localizing mechanism in metals with homogeneous structures under cyclic loading.

Influence of a UV-ozone treatment on amorphous SnO2 electron selective layers for highly efficient planar MAPbI3 perovskite solar cells

The effect of ultraviolet-ozone (UVO) irradiation on amorphous (am) SnO2 and its impact on the photoconversion efficiency of MAPbI3-based perovskite solar cells were investigated in detail. UVO treatment was found to increase the amount of chemisorbed oxygen on the am-SnO2 surface, reducing the surface energy and contact angle. Physicochemical changes in the am-SnO2 surface lowered the Gibbs free energy for the densification of perovskite films and facilitated the formation of homogeneous perovskite grains. In addition, the Fermi energy of the UVO-treated am-SnO2 shifted upwards to achieve an ideal band offset for MAPbI3, which was verified by theoretical calculations based on the density functional theory. We achieved a champion efficiency of 19.01 % with a statistical reproducibility of 17.01 ± 1.34 % owing to improved perovskite film densification and enhanced charge transport/extraction, which is considerably higher than the 13.78 ± 2.15 % of the counterpart. Furthermore, UVO-treated, am-SnO2-based devices showed improved stability and less hysteresis, which is encouraging for the future application of up-scaled perovskite solar cells.

Dual Interfacial Engineering Enables Efficient and Reproducible CsPbI2Br All-Inorganic Perovskite Solar Cells.

CsPbI2Br all-inorganic perovskite has shown superior photovoltaic properties particularly excellent phase and thermal stability, while the complicated film growth process requires additional research. Herein, the nucleation and crystallization process of the CsPbI2Br perovskite film is assisted by methyl acetate anti-solvent treatment. Additionally, a tailored SnO2 nanoparticle/TiO2 nanocrystal structured double electron transport layers (ETLs) is designed to remove the interfacial energy barrier, thus enhancing charge transfer and decrease charge recombination at the CsPbI2Br/ETL interfaces. Through synergistically dual interfacial engineering, we have demonstrated the preparation of a compact CsPbI2Br polycrystalline film with ordered and homogeneous grain size as well as ideal interfacial energy level alignment. In consequence, stable CsPbI2Br all-inorganic perovskite solar cells with the best power conversion efficiency of 15.86% has been successfully achieved together with a high open-circuit voltage of 1.23 V and a fill factor of 82.29%. We believe that the results here demonstrate efficient approaches to achieve high-quality inorganic perovskites for promoting their optoelectronic applications.

A novel fabrication technique of toughened TiC-based solid solution cermets using mechanochemical synthesis

Ceramic–metal composite powders (Ti,M)C-Ni (M = Mo, W and Ta) were prepared from elemental powders using a one-step mechanochemical synthesis method through high-energy ball milling. The microstructure and mechanical properties of the resulting cermets were also investigated. The analytical results revealed that ball milling of Ti-M-C-Ni elemental powders triggered a combustion reaction between Ti and C. The reaction resembled a fast sintering process which facilitated alloy elements atoms dissolve into the newly formed TiC, synthesizing the (Ti,M)C-Ni composite powders. The microstructure of the sintered (Ti,M)C-Ni cermets consisted of the core-rim structure grains and homogenous grains. High-resolution transmission electron microscope analysis results revealed that there was a misorientation between the ceramic phase and the binder phase in the cermets, but the interface between the core and the rim was coherent. The cermets exhibited mechanical properties that were comparable to conventionally produced cermets, particularly in fracture toughness which was as high as 12.45 MPa m1/2. The excellent cohesion between the Ni binder and the (Ti,M)C solid solution and the reduction in the interfacial stress in ceramic grains were believed to be responsible for the enhanced toughening of the cermets.

Fabrication of highly transparent Er3+, Yb3+:Y2O3 ceramics with La2O3/ZrO2 as sintering additives and the near-infrared and upconversion luminescence properties

Abstract Highly transparent Er3+, Yb3+:Y2O3 ceramics were fabricated by pressureless vacuum sintering method using La2O3 and ZrO2 as compound sintering additives. The microstructure, optical transmittance, upconversion and near-infrared spectral properties of the ceramics were investigated by X-ray diffraction, scanning electron microscopy, energy-dispersive spectrometry and spectroscopic analysis methods. The sintered ceramics displayed a pure cubic Y2O3 phase structure and a nearly pore-free microstructure with homogeneous grain morphology and grain-size distribution. The highest transmittance of Er3+, Yb3+:Y2O3 ceramics sintered at 1800 °C for 14 h with 9 at% of La2O3 and 3 at% of ZrO2 as compound additives was more than 82% in the range of 380–3000 nm. Under 980 nm wavelength excitation, the ceramics exhibited intense blue, green and red upconversion emission, and broad-bandwidth near-infrared emission centered at 1536 nm, which gives it the potential to act as a high-performance solid-state 1.54 μm near-infrared and upconversion laser gain media.

Achieving a unique combination of strength and ductility in CrCoNi medium-entropy alloy via heterogeneous gradient structure

Despite having otherwise outstanding mechanical properties, the practical application of CrCoNi medium-entropy alloy (MEA) is limited by its insufficient room temperature yield strength. Unfortunately, most traditional approaches for increasing strength usually lead to the degradation of ductility, a dilemma known as strength–ductility trade-off. Here, we report a heterogeneous gradient structure (HGS) in this MEA produced by ultrasonic surface rolling processing that can achieve a unique property combination: 3.7 times higher engineering yield strength compared to that of the fully recrystallized microstructure with homogeneous grain size, and at the same time preserving a very high elongation-to-failure of 54.1%. The HGS along depth from the treated surface is characterized by surface nanocrystalline, heterogeneous nanolaminate structure layers and deformed dislocation structure layer. The excellent combination of high strength and good ductility comes from extraordinary work-hardening ability arising from the heterogeneous gradient structure and the abundant extremely thin nanoscale deformation twins.

Effect of sintering treatment on the microstructure of NiFe2O4 synthesized by the sonochemical method and the conventional method

Nickel ferrite ultrafine nanoparticles were synthesized by the sonochemical method. The sample was sintered at temperatures ranging from 1273 K to 1673 K for 3 h. XRD peaks of as-synthesized NiFe2O4, their Transmission Electron Microscopy (TEM), high resolution TEM, and selective area (electron) diffraction pattern demonstrate that the particles are in the completely crystalline state. The initial particle size of as-synthesized NiFe2O4 was found to be approximately <5 nm. Besides, NiFe2O4 samples were also synthesized by a conventional double sintering technique. The purpose was to compare the effect of sintering treatment on the microstructure quality of the NiFe2O4 prepared by these two techniques. Homogeneous coaxial grains did not form until 1573 K for conventionally prepared samples, while for the sonochemical method, homogeneous grains started to form even as low as 1373 K. Furthermore, other measurements were done only for the sintered samples prepared by the sonochemical method to evaluate the magnetic properties. An abrupt change in B–H loops was found with Ts for a maximum applied field of 1500 A/m. M–H loops with the maximum applied field of 1.6 × 103 KA/m and Mössbauer spectroscopy demonstrate that the samples are all at the ferrimagnetic stage. Curie temperatures Tc, determined from the temperature dependence of the initial permeability μ′, for the same samples almost remained unchanged, which confirms that the cation distribution is almost unchanged with the variation of Ts. A slight variation of cation distribution manifested in the variation of Tc with Ts conforms with the site occupancy of Fe3+ analyzed by Mössbauer spectroscopy.

Effect of Multidirectional Forging and Subsequent Annealing to the Microstructure of Al-Mg-Mn Type Alloy

The grain refinement is important to improve both service properties at room temperature and superplasticity at elevated temperatures. This study focuses on the effect of multidirectional forging in isothermal conditions on the microstructure of Al-Mg-Mn-type alloy. The evolution of dislocation and grain structure, and precipitates of Mn-rich phase during multidirectional forging in a temperature range of 200 to 500 °C was studied. Multidirectional forging at temperatures of 200 and 300 °C leads to the formation of shear bands in the deformed grains. The multidirectional forging at 400 and 500 °C leads to the formation of a bimodal grain structure with fine- and coarse-grained areas. Subsequent recrystallization annealing at 500 °C increases the grain size and decreases the fine grains fraction in the samples pre-deformed at 400-500°C, and, on the contrary, annealing leads to formation homogeneous and fine grain structure with size up to 6.5 μm in samples pre-deformed at 200 and 300 °C.

High-performance (K,Na)NbO3-based binary lead-free piezoelectric ceramics modified with acceptor metal oxide

Environmental awareness has led to legislation that restricts lead-containing piezoelectric material use; consequently, lead-free alternatives are urgently sought. Chemical modification of the multiphase piezoelectric system can be used to develop electrical properties of lead-free ceramics with thermal stability. This paper reports lead-free potassium sodium niobate (K, Na) NbO3 (KNN)-based piezoelectric ceramics modified with Fe2O3 and prepared via the solid-state route. The results demonstrate that via the partial substitution of Fe3+ for Nb5+ and Ti4+ ions in the B-sites of the perovskite structure, the polymorphic phase transition (PPT) region at the orthorhombic–tetragonal (O–T) phase boundary can be moved to approximately room temperature and the mass transportation during sintering can be enhanced, thereby promoting homogenous grain growth and increased densification. The substantially diminished polarization anisotropy between the O-T phase boundaries promotes domain wall mobility, which can substantially enhance the piezoelectric properties. An optimal piezoelectric coefficient value of d33 = 246 pC/N, an electromechanical coupling coefficient value of kp = 49%, a dielectric permittivity of εr = 1420, a remnant polarization of Pr = 23 μC/cm2 and a high Curie temperature of TC = 343 °C were realized at x = 0.08 mol% of Fe2O3. Due to their enhanced electrical properties and high thermal stability, KNN-based lead-free ceramics doped with low-cost Fe2O3 can replace commercial high-lead-content piezoelectric ceramics.

An Alternative Casting Technique to Improve the Creep Resistance of Cast INCONEL Alloy 740H

The increasing performance requirements of power plant designs, such as advanced-ultra supercritical (A-USC), require the use of Ni-based superalloys to replace high-strength, ferritic-martensitic steels for components subjected to temperatures above 898 K (625 °C) and for austenitic stainless steels components at temperatures above 973 K (700 °C). To date, commercial Ni-based superalloy INCONEL 740H has been shown to be appropriate for use in A-USC power plants as boiler components in a wrought product. However, large complex components in boilers as well as other casings in the turbine and valve chest require castings of a thick-wall nature. Using the alloy in its cast form would be significantly valuable in terms of range of component size, geometry and complexity. Previous investigations revealed short creep lives from cast INCONEL alloy 740H. In this investigation, an alternative casting route that utilized a melt procedure resulting in a fine-grain casting, and in conjunction with a computationally optimized homogenization heat treatment, not only controlled the grain size and grain boundary structure but minimized chemistry variability and segregation. A primarily equiaxed and homogenous grain size distribution was obtained from this approach with better repartition of M23C6 carbides along the grain boundaries. Furthermore, better than 38 pct increase was obtained for this material in comparison to the creep life obtained from the best performing conventionally cast material. More importantly, the fine-grain homogenized (FGH) casting route resulted in the Larson–Miller plot for this material that coincided with that of wrought alloy 740. At low creep stresses (with a test still in progress), the FGH casting is resulting in higher values of the LMP than the wrought alloy.

Effect of process parameters on track geometry, microstructural evolution on 316L stainless steel multi-layer clads

In the present work, the effect of process parameters such as laser power, scan speed and powder flow rate on the geometry and surface waviness of the bead during Laser Metal Deposition (LMD) was determined. A gas atomized powder of 316L stainless steel were employed, which were deposited on a stainless steel substrate at a rate of 2 kg/h. The experimental results indicated that the bead height, bead width and its wetting angle with respect to the substrate decreases with increasing scan speed and decreasing laser power. Here the scan speed and laser power exhibited contrasting effect on the shape and morphology of the bead. After analyzing the effects of process parameters, various deposition strategies were explored to fabricate a solid cylindrical sample followed by microstructural analysis and hardness measurements. The analysis of microstructures revealed the presence of solidification tracks, and columnar and homogeneous cellular dendritic grains, which are common features of additively manufactured alloys. Moreover, the optimized parameters of LMD were able to manufacture a 316L stainless steel with microhardness similar to that of as-cast and wrought samples of the same alloy.

The role of mesoscopic deformation heterogeneities in plastic flow and recrystallization of a magnesium sheet alloy

Unconventional rolling schemes characterized by high reductions and a low number of passes were employed to deform a new magnesium sheet alloy to large strains in order to obtain a heterogeneous deformation microstructure with large local variations in defect density. Microstructure heterogeneity was present in the form of large elongated grains, twinning, and shear banding. The rolled material was further deformed in tension at room temperature along different sheet directions to study the effect of such microstructure on plastic flow and planar anisotropy. Other rolled samples were subjected to annealing treatments before tensile testing to investigate the mechanisms of recrystallization in conjunction with deformation heterogeneity. Recrystallization chains were seen to form by stages of progressive subgrain rotation and subgrain growth giving rise to new grain boundaries capable of long-range migration during annealing. Unlike the as-rolled condition where planar anisotropy of the yield strength and maximum elongation was evident due to several reasons discussed in the study, the annealed condition with its homogeneous equiaxed grain structure and weak textures demonstrated an almost isotropic plastic flow behavior.

Investigation on the Durability of Ti-6Al-4V Alloy Designed in a Harmonic Structure via Powder Metallurgy: Fatigue Behavior and Specimen Size Parameter Issue

In the present work, the four-point bending loading fatigue properties of a heterogeneously distributed grain size microstructure consolidated from Ti-6Al-4V alloy powder are studied. The microstructure involved here, a so-called “harmonic structure”, possesses quasi-spherical large grain regions (“cores”) embedded in a continuous fine grain region (“shell”). Unlike the previous reports dealing with this issue, the effect of the specimen size on the fatigue characteristics is also probed, since two distinct specimen configurations are considered. Furthermore, the obtained experimental data are compared with the corresponding fatigue results derived from homogeneous coarse grain counterparts. Contrary to homogeneous structure material, discrepancies on both the fatigue strength and the fatigue crack initiation aspects are found for the harmonic structure material. Consequently, the present work aims to clarify the underlining phenomena involved in the specimen size effect detected for Ti-6Al-4V designed in the harmonic structure. A less active interface surface between the core and the shell combined with a wider critical volume in the large size specimen should be the main reasons of the fatigue strength discrepancy.

Effects of semi-solid treatment by electro-magnetic induction on micro-structure evolution and mechanical properties of the Mg-2.4Y-4Nd-0.5Zr-1Ni alloys

The semi-solid billets of Mg-2.4Y-4Nd-0.5Zr-1Ni (WE34-1Ni) alloys are fabricated by electro-magnetic induction heating semi-solid treatment at 2.05 kW and 4.10 kW from 580 °C to 625 °C. In this work, the microstructure evolution and mechanical properties of WE34-1Ni alloys are investigated, and the results reveal that with increasing semi-solid temperatures, the average grain size of the solid globules and liquid fraction at the grain boundary gradually increase while the shape factor fluctuates slightly. Compared with 2.05 kW power, the semi-solid billet with 4.10 kW power at 625 °C has more fine homogeneous grains, the lower average size of the solid globules, more liquid fraction. The semi-solid billet with 4.10 kW at 625 °C obtains ideal semi-solid spheroid structure with the solid grains surrounded by a small amount of liquid pools and the best mechanical properties of the semi-solid process parameters. Besides, the elongation as-extruded of WE34-1Ni alloys increased from 21.4 ± 0.7% to 33.2 ± 0.3% at 4.10 kW power and 625 °C via electro-magnetic induction heating semi-solid treatment.

Crack Behavior and Crystallographic Feature for TMCP690 Steel under Fatigue Loading

For a new type of thermomechanical controlled process steel, when stress intensity factor was about 32 MPa m1/2, several slip bands with different orientations occurred in cyclic plastic zone. Some slip bands had strong interaction with dominant crack, which led to crack bifurcation and aggregation. After that, electron back-scattered diffraction technique was employed to evaluate the cyclic plasticity effect on crystallographic features of kernel average misorientation (KAM), strain distribution and fraction of different categories of grains for fatigue loading sample in crack tip zone. Before fatigue loading, KAM distribution map revealed relatively homogeneous plastic deformation and few grains displayed distortion based on strain distribution map. After fatigue loading, KAM showed higher cyclic plasticity in the middle, and lower value on both sides. Strain network structure formed. Meanwhile, when fatigue loading was executed, proportion of recrystallized grains had a little variation from 3.15 to 5.58%. Ratio of substructured grains increased to 19.63%, and deformed grains reduced to 74.79%.

Hall-petch relationship and heterogeneous strength of CrCoNi medium-entropy alloy

In this work, we mainly focused on single phase face-centered cubic (FCC) type CrCoNi medium-entropy alloy (MEA), fabricated the homogeneous and heterogeneous grain structure through heavily deformation and subsequent annealing treatment, then investigated the Hall-petch relationship and heterogeneous strength of CrCoNi MEA. The experimental results show that, the large friction stress of fully-recrystallized CrCoNi MEA resulted from the severe lattice distortion, short-time partial-recrystallized annealing and abundant thermal twinning facilitated the heterogeneous structure with multiple gain size. The heterogeneous structure (i.e. partial-recrystallized structure) exhibited a superb strength-ductility synergy in comparison with nano-grained structure and/or ultrafine-grained structure in CrCoNi MEA. The results would provide the comprehensive understanding of CrCoNi MEA, as well as the essentials for further development and applications of FCC type multicomponent alloys.

Systematic investigation of structural, optical and dielectric properties of 0.5 mol% Eu:BaTiO3 ceramics

Lead-free ceramic with the stoichiometric composition of 0.5 mol% Eu:BaTiO3 (EBT) is prepared by following the conventional solid-state reaction method. Prepared EBT is sintered at 1250 °C for 4 h. X-ray diffraction (XRD) study confirms the formation of a single-phase cubic structure that belongs to Pm3m space group. Field emission scanning electron microscopy (FE-SEM) shows the homogenous grain growth throughout the sample and the average grain size is estimated to be ∼0.8 μm. The value of the band gap is estimated from diffuse reflectance spectra and found to be ∼3.24 eV for EBT ceramics. Wide range of frequency (100 Hz–5 MHz) and temperatures (room temperature-500 °C) dependent dielectric response suggest that prepared sample exhibits maximum dielectric constant (ε′) at low frequency and this increases with increasing temperature. The value ε′ varies from 1865 to 8581 for the temperature range 300–500 °C at 100 Hz. The sample shows strong dispersion at low frequency in its dielectric characteristics which is a temperature dependent phenomenon. The enhanced dielectric properties of EBT suggests the applicability of these materials in electronic components designed for harsh temperatures.

Scale‐up of pore‐level relative permeability from micro‐ to macro‐scale

AbstractScaling up relative permeability curves of wetting and nonwetting phase of drainage and imbibition processes from pore scale to macro scale is a challenge. A new method for scaling up relative permeability from micro‐ to macro‐scale is proposed based on electrical analogy of multiphase fluid flow at pore scale. The method is validated against four synthetic porous media generated using homogeneous and heterogeneous grain size distributions, each of which were cut into eight sub‐segments. Single‐phase and two‐phase flow properties were calculated for the main blocks and the subsequent sub‐segments using random network modelling technique. Then, the subsegments were randomly distributed in space to reconstruct the main blocks and the proposed scale‐up method was employed to calculate the relative permeability curves of the reconstructed blocks. Results were compared to the ones obtained directly from the network model of the original blocks and show good agreement between the calculated and scaled‐up relative permeability curves of primary drainage and secondary imbibition. Furthermore, the model was tested on real media. Eight network models were extracted from pore size distribution of core samples obtained from the Green River basin located in the Mesaverde Formation. Flow properties obtained from the network models were validated against experimental data and good agreement was observed. These network models show a higher level of heterogeneity at micro‐scale. Then, the scale‐up methods were employed in order to reconstruct the macro‐scale sample and predict its properties. Scale‐up methods successfully predict the single‐phase and two‐phase flow properties of the sample.