Interior Of Grains Research Articles

Microstructures, tensile properties and serrated flow of Al CrMnFeCoNi high entropy alloys

Abstract Microstructures and mechanical properties of dual-phase AlxCrMnFeCoNi (x=0.4, 0.5, 0.6, at.%) alloys were investigated. Thermomechanical processing leads to a microstructural evolution from cast dendritic structures to equiaxed ones, consisting of face-centered cubic (fcc) and body-centered cubic (bcc) phases in the two states. The volume fraction of bcc phase increases and the size of fcc grain decreases with increasing Al content, resulting in remarkably improved tensile strength. Specifically, the serrated flow occurring at the medium temperatures varies from type A+B to B+C or C as the testing temperature increases. The average serration amplitude of these Al-containing alloys is larger than that of CoCrFeNiMn alloy due to the enhanced pinning effect. The early small strain produces low-density of dislocation arrays and bowed dislocations in fcc grains while the dislocation climb and shearing mechanism dominate inside bcc grains. The cross-slip and kinks of dislocations are frequently observed and high-density-tangled dislocations lead to dislocation cells after plastic deformation with a high strain.

Comprehensive Elucidation of Grain Boundary Behavior in All‐Inorganic Halide Perovskites by Scanning Probe Microscopy

AbstractAll‐inorganic halide perovskite (AIHP) is becoming one of the most promising generation materials of perovskite photovoltaics for commercialization due to its thermodynamic stability and soared efficiency. Depending on material properties, grain boundary (GB) has detrimental or beneficial effect on device photovoltaic performance. However, less attention is paid to GB behavior in AIHPs. Herein, it is concluded that the microscopic GBs are the major sites for photocarrier generation and transport, as well as the ionic pathway dominating the current hysteresis behavior in AIHP solar cells. Kelvin probe force microscopy (KPFM) measurements reveal a lower surface potential at GBs as compared to grain interiors (GIs), suggesting a significant upward band bending around GBs. Conductive atomic force microscopy (c‐AFM) measurements show a higher current flow in the vicinity of GBs, indicating enhanced carrier separation and collection taking place at GBs. Furthermore, the existence of ion motion is evidenced both by the single‐point c‐AFM measurement and voltage‐controlled KPFM measurement, which accounts for the common current hysteresis behavior in AIHP solar cells. These investigations provide an advanced understanding of the role of GBs in AIHP and further inspiration on how to optimize device performance up to the levels of organic–inorganic hybrid perovskite.

Effect of Cr/Mo carbides on corrosion behaviour of Fe_Mn_C twinning induced plasticity steel

Fe-Mn-C twinning-induced-plastic (TWIP) steel was prepared by adding Cr and Mo, and the effect of Cr/ Mo carbides on the corrosion behaviour of TWIP steel was studied. Different contents of carbide were obtained by aging treatment. The parameters for heat treatment method were 1150 ℃ and 2 h solid solution treatment, aging at temperature 700 °C, heat preservation for 6 and 24 h. Results show that the carbide precipitation phase occurred preferentially in the grain boundary and partly at the interior of grain when the aging time increased. The carbide precipitation phases were primarily MoC and Cr3C2. Cr/ Mo carbide and austenite matrices exhibited phase potential difference and formed a local galvanic couple. More carbides were precipitated, and more galvanic couples were generated as the aging time increased, which resulted in serious corrosion.

Microstructure, texture, mean free path of dislocations and mechanical properties of Ti–6Al–4V alloy during uniaxial compression at elevated temperatures

In the present work, microstructure, texture, mean free path of dislocations and mechanical properties of Ti–6Al–4V (Ti64) alloy during uniaxial compression testing has been studied in the temperature range of 298 K–873 K. Significant deformation twins were observed in the samples beyond 20% reductions at temperatures of 298 K and 673 K only. Such twins were further found to be of non-Schmid type and were confirmed through VPSC (Visco-plastic self-consistent) simulation. The deformation bands were observed at all studied temperatures in the grains of specific orientations. These deformation bands were found to nucleate at the grain boundaries and gradually progressed towards the interior of the parent grains. The deformation bands facilitated the origination of deformation twins inside the grains, i.e. at the boundary between deformation band and a parent grain. Bulk texture measurements showed that the texture strength of (0001) pole increased up to 30% reduction at all temperatures of deformation. However, the dominant texture strength remained along (12‾10) pole, i.e. the initial texture of the sample before subjected to uniaxial compression. An initial decrease and subsequent increase in hardening rate was observed in the samples deformed at room temperature. The maximum decrease in the rate of hardening was observed at a temperature of 673 K and up to 18% strain followed by a relative increase thereafter. Further, the observed decrease in the rate of hardening was found to be relatively lower at a temperature of 873 K compared to that at 673 K. The Ti64 alloy showed better strength compared to that of the commercially pure (CP) titanium samples tested in the similar temperature range of 298 K–873 K which can be attributed to the dominant dislocation-solute interaction in the former case.

Effects of stress and temperature on creep behavior of a new third-generation powder metallurgy superalloy FGH100L

In this paper, a new third generation powder metallurgy nickel-based superalloy, designated as FGH100L, was prepared by Spray Forming, followed by Hot Isostatic Pressing, Isothermal Forging and Heat Treatment. The effects of stress and temperature on the creep properties of the alloy were studied by analyzing the creep mechanism and fracture characteristics. The results show that the creep life of the alloy decreases significantly with the increase of stress and temperature. The creep fracture indicates mixed intergranular and transgranular fracture characteristics for all the tested specimens. However, at low stress or low temperature conditions, the intergranular fracture is more prominent in the fracture source region and almost no shear lip region can be observed in the final rupture region. With the increase of stress and temperature, the transgranular fracture becomes more prominent, and the shear lip region is enlarged. The misorientation within the grain decreases with the increase of stress and temperature, indicating that the higher the stress and the temperature, the lower the local strain the alloy can bear. Under the creep condition of high temperature and high stress, the misorientation is much larger around the original grain boundary than in the interior of the original grain, indicating that the internal strain of the alloy is mainly concentrated near the original grain boundary under this condition.

Microstructure and Mechanical Properties of Mg–8Y–2Ho–2Zn Alloy with Long Period Stacking Ordered Phase

In this study, microstructures, phase compositions, phase morphologies of the as-cast and as-extruded Mg–8Y–2Ho–2Zn (wt%) alloys were investigated by optical microscope, scanning electron microscope, energy-dispersive spectrometer, transmission electron microscope and X-ray diffraction. The mechanical properties at room temperature and elevated temperatures were evaluated by tensile test. The primary secondary phases in the as-cast alloy are 18R-type long period stacking ordered (LPSO) phase. After extrusion, the microstructure mainly consists of considerable fine dynamically recrystallized (DRX) grains, the long strip 18R-type LPSO-phase at grain boundary along the extrusion direction and the fine lamellar 14H-type LPSO-phase in the interior of DRX grains. When the temperature is increased, the strength of the alloy is reduced, but the elongation progressively rises. The as-extruded alloy shows higher strength and larger elongation than the as-cast alloy at each test temperature. The superior mechanical properties of the as-extruded alloy are ascribed to various strengthening mechanisms induced by co-addition of Y and Ho element, e.g., solid solution strengthening, fine-grain strengthening and strengthening of fine LPSO-phase.

Fabrication and characterization of an Al-based nanocomposite with high specific strength and good elongation using large amount CMA nanoparticles

In the present research, preparation and characterization of both powder and bulk of a new Al-CMA nanocomposite were investigated. For this, the composites were prepared by mechanical alloying/milling (MA/MM) of 10 wt % Al3Mg2 and pure Al mix powders and then cold pressing, sintering and finally hot extrusion. According to the microscopic observations, a uniform distribution of Al3Mg2 in the Al and a clean interface between them were obtained after ball milling. In addition, analysis of particle size distribution revealed that the presence of Al3Mg2 particles and amount of process control agent had a significant influence on the composite powder refinement. In addition, the measured Carr index of 13.5% indicated that the ball milled powder offered good flowability. High-resolution microscopic studies showed that formation of twins might be one of the crystallite size decreasing mechanisms during MA/MM process. The initial powder boundary in the powder samples could not be completely deleted by cold pressing and sintering, but could be reduced by following hot extrusion. The microstructure of extruded nanocomposites comprised nanometer Al grains and Al3Mg2 nanoparticles dispersed interior grains which produced by dynamic recrystallizations (DRX). The Al3Mg2 particles experienced vigorous size reduction during MA/MM, which play an important role in Al grain refining during DRX via Zener pinning. These conditions along with clean interface for the 15 h milled sample could be resulted in notable enhancement of 515% in yield strength (YS), 603% in ultimate compressive strength (UCS), 181% in young’s modulus and 406% in microhardness compared with pure Al. A more interesting point was the elongation up to 24% owing to the uniform distribution of nanoparticles in Al grains. These results from Al–Al3Mg2 nanocomposites discover newer feasibilities for impressive manufacturing of high strength and low-density composites with improved ductility, which could make them possible candidates for a wide range of industries especially for weight critical applications.

Feasibility of grain refinement method for AZ91 alloy using commercial Al-SiC composite

SiC is well-known as an effective carbon inoculation material for grain refinement of Mg-Al-based alloys. In this study, grain refinement ability of stir-casted Al-SiC composite for Mg-Al-based alloy was investigated to confirm the feasibility of industrial application. Commercialized Al-20 vol.-% SiC composite ingot was used in AZ91 alloy as a grain refiner, and grain size of AZ91 alloy dramatically reduce from 650 to 74 µm and become uniform with 0.7 wt-% Al-SiC composite addition. And acicular type Al-Mn phase existed at the grain boundaries are changed to polygonal type and positioned inside grains after grain refinement. These morphological changes of small grain size and uniform second phases contributed to the excellent mechanical properties of AZ91 alloy.

In Situ Micro-Pillar Compression to Examine Radiation-Induced Hardening Mechanisms of FeCrAl Alloys

The effects of 5 MeV Fe2+ ion irradiation at 300 °C on the microstructure evolution and deformation behavior of a FeCrAl C26M alloy are presented. It has been found that dislocation loop density increases an order of magnitude from 1 dpa to 16 dpa irradiations, whereas, the dislocation loop size saturates with increasing damage. Micropillars, 600 nm in diameter and 1.3 um in height, were fabricated and compressed inside grains with , and crystallographic orientations, respectively. {112} has been identified as the primary slip system. The irradiation-induced hardening generally follows the prediction of Orowan dispersed barrier hardening equation, even though the exact increasement of yield stress after irradiation is presented heterogeneously across three orientations. Detailed transmission electron microscopy (TEM) studies were performed to quantify the Burgers vector and the distribution of irradiation induced dislocations. It is revealed that localized shear instability is caused by avalanche slip events of ½ dislocations gliding out of tested pillars. Simultaneously, a large number of sessile/immobile dislocations formed in the vicinity of slip band, leading to the hardening at elevated strains.

Effects of Rotary Swaging Parameters and Artificial Ageing on Mechanical Properties and Microstructure of 2024 Precipitation-Hardenable Aluminium Alloy.

This work concerns mechanical properties in relation to microstructural changes in hardenable EN AW-2024 aluminium alloy in wrought and heat treated condition. The treated material benefits from synergistic effects of hardening mechanisms. Grain boundary strengthening and work hardening were activated in this material by rotary swaging. Rotary swaging is a method which shows great promise for industrial use. Precipitation hardening was achieved thanks to the material’s age hardening ability. First, the material was artificially-aged in a furnace at 140–180 °C. Second, natural ageing was used. Mechanical properties of the as-treated material were tested and microstructural processes were explored using electron microscopy and differential scanning calorimetry. The treatment route which delivered the best results was as follows: solution annealing 500 °C/1 h + water cooling + rotary swaging + artificial ageing 160 °C/21 h. This led to a yield strength close to 640 MPa, and ultimate strength above 660 MPa, and elongation of 8%. Electron backscatter diffraction observation revealed that in this condition, the ratio of Low-angle to High-angle grain boundaries is 80:20. The microstructure contains both T-phase in the interior of grains, whose particles are normally oval-shaped, and S-phase, which is present in two shapes: small oval particles or coherent needles aligned to <100> direction.

Microstructure Evolution of a Simulated Coarse-Grained Heat-Affected Zone of T23 Steel During Aging

In this work, simulated CGHAZ of T23 steel was produced via a thermomechanical simulator, and then the CGHAZ specimens were aged at 650 °C for 0 to 240 hours to simulate the microstructure evolution of as-welded CGHAZ during service. Microstructure change and carbide precipitation were observed by OM, SEM, EBSD and TEM + EDS. Carbide precipitation kinetics in T23 steel at 650 °C was calculated for comparison with the experiment results. The hardness change of CGHAZ during aging was detected, and the effect of microstructure evolution on hardness was analyzed. The results showed that the CGHAZ of T23 steel exhibited a mixed microstructure of martensite and bainite with high hardness in as-welded condition. After aging at 650 °C, the microstructure recovered, recrystallization occurred, the dislocation density decreased, and the lath width increased. Consequently, the hardness dropped, the drop depending on the aging time. In the early stage of aging (before 24 hours), the precipitations inside the grain were mainly M3C, M7C3 and a small number of M23C6 carbides, while the precipitation at the grain boundaries was M23C6. The precipitation of M23C6 caused the hardness to drop rapidly. When aged for 24 to 48 hours, MX precipitated inside grains extensively. The precipitation hardening produced by MX could slow down the decline of hardness. As the aging proceeded, carbide precipitated and transformed as follows: M3C → M3C + M7C3 + M23C6 → M3C + M7C3 + M23C6 + MX → M23C6 + MX + M6C. W-rich carbides precipitated in some grain boundaries of CGHAZ during aging, which may be related to the W segregation at those grain boundaries.

Void Formation and Plastic Deformation Mechanism of a Cold-Rolled Dual-Phase Steel During Tension

The void formation and plastic deformation micromechanisms of a cold-rolled DP600 steel during tensile loading were studied by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The SEM observations revealed that the main void nucleation mechanism in the DP600 steel is decohesion at the ferrite–martensite interfaces. The voids were mostly observed between the closely spaced martensite islands situated at the boundaries of relatively finer ferrite grains. The EBSD results indicated a strain gradient developed from the ferrite–martensite and ferrite–ferrite interfaces into the interior of ferrite grains during the tensile deformation, which led to a stress concentration at these interfaces. Moreover, it was demonstrated that local misorientation inside the finer ferrite grains surrounded by martensite islands was higher than that for the coarser ferrite grains, which made the former more prone to void initiation.

Rheological Solidification Behavior and Mechanical Properties of AZ91-Sn Alloys

The solidification and tensile deformation behaviors of rheo-cast AZ91-Sn alloys were revealed to study the effects of Sn alloying on improvement of AZ91 alloy’s mechanical properties. Two kinds of Mg17Al12 phases precipitated from the supersaturated magnesium matrix during rheo-solidification were observed: coarse discontinuous precipitates (DP) at grain boundaries and small-sized continuous precipitates (CP) inside grains. With increasing Sn content, the amount of Mg17Al12 phases was increased whilst the amount of Al atoms in the matrix was decreased. Due to the higher melting point of Mg2Sn than Mg17Al12, Mg2Sn precipitated earlier from the melt, and therefore provided heterogeneous nuclei for Mg17Al12 during the eutectic reaction. Due to grain refinement and solid solution strengthening, AZ91-2.4Sn (mass%) gained 52% increase in tensile strength and 93% increase in elongation compared with pure AZ91 alloy. The higher-density twins and microcracks induced by Sn alloying relaxed stress concentration during plastic deformation, so the fracture mode was transformed from cleavage fracture of pure AZ91 alloy to ductile fracture of AZ91-Sn alloys.

Effect of DyHx addition on the elemental distribution, magnetization reversal and coercivity in sintered Nd–Fe–B magnets

Nd12.2Fe81.8B6 powders were mixed with DyHx powders to prepare sintered magnets for improving the coercivity. The addition of 1 wt% DyHx leads to a significant increase of coercivity from 7.2 kOe to 12.4 kOe, and for 3 wt% DyHx addition the coercivity increases to 17.1 kOe. However, with increasing the addition amount of DyHx the increasing trend of coercivity becomes slower, and the remanence decreases in the sintered magnets. The activation size is a little larger than the theoretical value of domain wall size in Nd–Fe–B, implying that the reversed domain wall involves the defect region at grain outer-layer that promotes the nucleation of reversed domain wall in the interior of grain. Dy atoms prefer to diffuse into Nd–Fe–B and substitute for Nd due to the theomodynamics origin. For 1 wt% DyHx powders addition some of Dy atoms could diffuse into the outer-layer of main phase, which may modify the grain boundary structure and form (Nd, Dy)–Fe–B shell regions. So the nucleation field of reversed domain is improved at the grain outer-layer, and the coercivity increases largely in the sintered magnets. For more amount addition of DyHx powders, most of Dy atoms enter into the interior of Nd–Fe–B phase. The effect of enhancing the magnetocrystalline anisotropy at the grain outer-layer would be weakened, which should be responsible for the decrease of increasing amplitude of coercivity. Optimizing the addition amount of Dy alloys and regulating the elemental distribution are necessary to balance the coercivity, the remanence and energy product in the sintered magnets.

Unveiling Defect-Mediated Charge-Carrier Recombination at the Nanometer Scale in Polycrystalline Solar Cells.

Solar cells made of polycrystalline thin-films can outperform their single-crystalline counterparts despite the presence of grain boundaries (GBs). To unveil the influence of GBs, high spatial resolution characterization techniques are needed to measure local properties in their vicinity. However, results obtained using single technique may provide limited aspects about the GB effect. Here, we employ two techniques, near-field scanning photocurrent microscopy (NSPM) and scanning transmission electron microscope based cathodoluminescence spectroscopy (STEM-CL), to characterize CdTe solar cells at the nanoscale. The signal contrast from the grain interiors (GIs) to the GBs, for high-efficiency cells where CdTe is deposited at a high substrate temperature (500 °C) and treated by CdCl2, is found reverse from one technique to another. NSPM reveals increased photocurrents at the GBs, while STEM-CL shows reduced CL intensity and energy redshifts of the spectral peak at the GBs. The results are attributed to the increased nonradiative recombination and the band bending mediated by the surface defects and the shallow-level defects at GBs, respectively. We discuss the advantages of sample geometry for room-temperature STEM-CL and present numerical simulations as well as analytical models to extract the ratio of GB recombination velocity to minority carrier diffusivity that can be used for evaluating the GB effect in other polycrystalline solar cells.

Comparison of Hot Deformation Behavior Characteristics Between As-Cast and Extruded Al-Zn-Mg-Cu (7075) Aluminum Alloys with a Similar Grain Size.

The hot compressive behavior and processing maps of as-cast and extruded 7075 aluminum alloys with a similar grain size (320–350 μm) were studied and compared, which allows us to directly observe the effect of segregated phases in the as-cast microstructure on the deformation behavior and hot workability of 7075 alloys. In the as-cast alloy, the compound phases segregated along the interdendritic interfaces within the interiors of original grains provided the additional sites for continuous dynamic recrystallization via the particle stimulation nucleation mechanism. As a result, the as-cast alloy exhibited higher fractions of recrystallized grains and smaller grain sizes than the extruded alloy after compression. The stress exponent values of the as-cast alloy were smaller than those of the extruded alloy. In the processing maps, the domain associated with high power dissipation efficiencies (≥35%) occurred in a wider temperature range in the as-cast alloy compared to the extruded alloy. The segregated phases that remained undissolved in the as-cast alloy after compressive deformation could be effectively eliminated during the solid solution treatment (753 K for 2 h) for T6 aging applied after hot compression. The current results suggest the possibility and advantage of omitting the extrusion step when preparing 7xxx aluminum forging or extrusion feedstocks for hot working. The proposed method can be applied to other precipitation hardenable aluminum alloys.

Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics

AbstractColloidal assemblies have applications as photonic crystals and templates for functional porous materials. While there has been significant progress in controlling colloidal assemblies into defined structures, their 3D order remains difficult to characterize. Simple, low‐cost techniques are sought that characterize colloidal structures and assist optimization of process parameters. Here, structural color is presented to image the structure and dynamics of colloidal clusters prepared by a confined self‐assembly process in emulsion droplets. It is shown that characteristic anisotropic structural color motifs such as circles, stripes, triangles, or bowties arise from the defined interior grain geometry of such colloidal clusters. The optical detection of these motifs reliably distinguishes icosahedral, decahedral, and face‐centered cubic colloidal clusters and thus enables a simple yet precise characterization of their internal structure. In addition, the rotational motion and dynamics of such micrometer‐scale clusters suspended in a liquid can be followed in real time via their anisotropic coloration. Finally, monitoring the evolution of structural color provides real‐time information about the crystallization pathway within the confining emulsion droplet. Together, this work demonstrates that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters.

Microstructure and fatigue behavior of a laser additive manufactured 12CrNi2 low alloy steel

In this research, 12CrNi2 low alloy steel was successfully prepared by laser melting deposition (LMD). The mechanisms driving high cycle fatigue fracture of the as-built LMD 12CrNi2 low alloy steel were investigated and a concurrent process-microstructure-property relationship was established through microstructural analysis. The results showed that the crystal structures of the as-built LMD 12CrNi2 steel mainly consisted of the ferrite and a small amount of Cr23C6 carbides. No preferred texture was observed as a result of the complicated heat flux direction during fabrication. Based on the examination of the fracture surface, fatigue cracks of the as-built LMD 12CrNi2 steel initiated from subsurface defects for all the cases. Crack propagation zones showed a mixed mode of transgranular and intergranular fracture in a brittle manner, whereas the final fracture zones displayed dimples typical of ductile fracture. The kernel average misorientation (KAM) map indicated that the strain localization predominantly occurred at the grain boundaries and slightly appeared at the interior of the ferrite grains. Schmid factor distribution results implied that fatigue cracks originated from grains with {123}<111> slip system due to the prior activation of {123}<111> slip system.

Orientation gradient on surface of non-oriented electrical steel annealed by γ → α transformation

The microstructures and textures of the non-oriented electrical steel sheets under various annealing conditions were investigated. The orientation gradient in the interior of grains appeared on the surface of steel sheets after α → γ → α transformation in various atmospheres. The orientation gradient that appeared on the surface was inherited into the center of the steel sheet in H2 atmosphere, while that appeared only in the grains of the surface layer during γ → α transformation with N2 atmosphere. The thin oxide scale was generated in various atmospheres containing traces of O2 and H2O. The formation mechanism of orientation gradient is related to the thermal stress caused by the difference of thermal expansion coefficients between the oxide scale and the thin ferrite on the surface. The combined effect of phase transformation stress and thermal stress caused the plastic deformation of the thin ferrite grains on the surface in the initial stage of γ → α transformation, and then the orientations of the thin ferrite grains gradually rotated and caused the orientation gradient in the interior of grains.

Nanoscale analysis of ion irradiated ODS 14YWT ferritic alloy

In this work, the nanoscale microstructure of an advanced oxide dispersion strengthened (ODS) 14YWT ferritic alloy (SM13 heat) with nominal composition Fe–14Cr–3W-0.4Ti-0.3Y2O3 (wt. %) has been characterized by atom probe tomography (APT) before and after ion irradiation with 70 MeV Fe9+ ions at 450 °C to a total dose of 21 dpa. A detailed solute cluster analysis of APT data reveals that, in the manufacturing process, larger nanoparticles form in or close to the grain boundaries respective to those inside grains. The evolution of the nanoparticles after irradiation seems to be related to their location, as a higher increase in the number density and in the Y:Ti ratio is observed for the nanoparticles in or close to grain boundaries. APT analysis also shows Cr, W and C segregation to grain boundaries enhanced by the irradiation. A previous study of this same alloy before and after irradiation reports that the mechanical properties do not seem to be affected, but the microstructure was not investigated to confirm. The present work confirms little microstructural evolution after irradiation in this 14YWT alloy, indicating tolerance at the given irradiation conditions.