Investigation Of Microstructure Evolution Research Articles

Investigation of microstructure evolution and redox behavior of Ni-GDC cermet by cyclic re-oxidation in CO2

The microstructure and reactivity of Ni-GDC cermet under cyclic oxidation in CO2 conditions were investigated using temperature-programmed reduction (TPR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The XRD results revealed that NiO was formed during the re-oxidation process in CO2. SEM and TPR results demonstrated that nanostructured NiO aggregates were formed on the surface of the sample after being re-oxidized at 400 °C for 1200 min, resulting in an α peak with high reduction activity and a conversion of only 16% from Ni to NiO. In contrast, the samples re-oxidized at 600 °C for 600 min and 800 °C for 240 min were completely oxidized, resulting in reconstructed and sintered NiO with larger particle size, and a β peak with lower reactivity and a conversion of 100%. Redox cycling was further conducted on completely oxidized samples, which showed a decrease in β peak temperature from the first to seventh cycle. The migration and redistribution of sintered NiO towards the exterior were confirmed by comparing SEM images after the first and fourth cycles, which favored an increase in the reactive area. Therefore, this study reveals that lower re-oxidation temperatures (≤400 °C) and an appropriate increase in redox cycles are beneficial for forming surficial nanostructures while hindering the formation of large sintered NiO particles.

Manipulating scanning strategies towards controlled microstructure of laser remelted Mg–3Al–1Zn alloy

Various laser scanning strategies of multitrack products, consisting of a default Raster pattern (R1), a Raster pattern with a changed line order (R2), and a default Zigzag pattern (Z1), with different overlap rates ranged from 0.7 to 0.5, have been conducted to investigate microstructure evolution for Mg–3Al–1Zn alloy. As the overlap rate changes from 0.7 to 0.5, the average grain size decreases and the texture is slightly concentrated, except for R2 pattern. For effect of different scanning pattern, a relatively small average grain size can be obtained using the R1, R2 patterns compared to using the Z1 pattern; a relatively weak texture can be obtained using the R1, Z1 patterns compared to using the R2 pattern. The utilization of Z1 pattern results in a further texture weakening compared to that of the R1 pattern. Microstructure evolution of scanning strategy is mainly determined by low-undercooling-required epitaxial growth along temperature gradient directions at the solid/liquid interface. Combined with computational thermal-fluid dynamics simulation, a strong fluid flow within the molten pool promotes side expansion of the molten pool, resulting in the larger overlap area, the corresponding three-times remelting domain and continuous deflections of grain major axes.

Investigation of microstructural evolution and mechanical properties for in-service nickel-based superalloy

In this study, the microstructural evolutions are systematically investigated in the IN718 alloy that has been in-service for eight years. Correspondingly, the effect of in-service process on mechanical properties are explored in terms of tensile properties, fatigue performance and crack growth behavior. Results show that the grain sizes at different sampling locations vary from 9.8 to 15.5 μm, exhibiting the inhomogeneous microstructures for the in-service IN718 alloy. The volume fraction of δ phase and kernel average misorientation (KAM) at outermost layer are the maximum ones, followed by the middle and innermost layers. In addition, the high magnitude of δ phase is caused by the transformation from strengthening phase γ′′. From the perspective of mechanical properties, the in-service IN718 alloy can maintain excellent yield strength and elongation at the high temperature of 650 °C, while the tensile performance is degenerated at room temperature (RT) compared with those of new IN718 alloy. The fatigue life at 650 °C is similar to that at RT under the stress levels lower than 600 MPa. As the stress level increases to 800 MPa, an obvious decrease in fatigue life at 650 °C compared with that at RT. This special fatigue performance is revealed based on the fractography examinations under different loading conditions. Moreover, the investigation of fatigue crack growth for the in-service IN718 alloy is carried out under different stress ratio and temperatures.

Investigation of microstructure evolution and electromagnetic shielding mechanism of absorbed MWCNTs-FeCo composites by one-step in situ deposition using high-speed laser cladding

Electromagnetic shielding materials can inhibit or reduce the harm of electromagnetic radiation and have become an important means to protect against electromagnetic pollution. In order to overcome the inconveniences of in situ or conformal manufacturing on the specific equipment, and the resulting assembly errors lead to the reduction of electromagnetic interference shielding effectiveness (EMI SE). As a new technology, high-speed laser cladding can directly design multi-walled carbon nanotubes (MWCNTs)-FeCo alloy composites as absorbents, and it is the first time to realize the rapid manufacturing of electromagnetic shielding materials by one-step in-situ deposition without adding auxiliary materials. According to the temperature field and stress field, it is found that the through-crack defects of electromagnetic shielding coating mainly resulted from the residual stress concentration of microcracks near the bottom fusion line. When the cladding strategy is 1600 W power deposition after preheating at 400 °C, a high-quality cladding layer can be obtained. When the addition of MWCNTs is 1.5 wt%, the material exhibits good electromagnetic shielding performance under the synergistic effect of dielectric loss, conductivity loss, and magnetic loss. And the average EMI SE is 23 dB in the range of 2-18GH. The research shows that high-speed laser cladding can simultaneously meet the requirements of flexible material design and efficient preparation, which realizes the integration of electromagnetic shielding material design and manufacturing.

A spatially resolved analysis of dislocation loop and nanohardness evolution in proton irradiated Zircaloys

Proton irradiation is increasingly used as a surrogate for neutron irradiation, providing similar damage structures at significantly lower costs and less time compared to neutron irradiation. However, in contrast to neutrons, protons produce a depth dependent damage profile that incorporates a plateau region and a Bragg peak in the tens of microns region depending on the material and proton energy. Here we demonstrate that this depth dependent damage can be utilised to obtain new understanding about irradiation induced damage at different dose levels from a single sample by combining spatially resolved micro-beam synchrotron X-ray diffraction-based dislocation analysis and nanoindentation. For this purpose, and to study the damage evolution starting from very early stages, we have investigated microstructure evolution and hardening behaviour of Zircaloy-2 and Zircaloy-4 proton irradiated samples between 0.01 dpa and 17 dpa at 350 °C. The results highlight good agreements of SRIM-based damage depth predictions with irradiation-induced dislocation appearance and nanohardness. However, at even relatively low damage levels within the plateau region, dislocation line density and nanohardness profiles appear saturated across the entire irradiated region, even when the dpa levels differed significantly. When relating dislocation loop line densities to nanohardness, it was found that from a certain point hardness increased only very slightly with increasing line densities. This apparent discrepancy can be explained by a decreasing loop size for the highest line densities identified by from the diffraction line profile analysis and the application of a Dispersed Barrier Hardening model that relates obstacle strength to dislocation loop size.

Solution slot-die coating perovskite film crystalline growth observed by <i>in situ</i> GIWAXS/GISAXS

Solution method is an important means of fabricating optoelectronic devices. During the thin film sample preparation, organic or inorganic perovskite semiconductor material usually needs to be finished in a glove box. However, most of the traditional experimental characterizations under the air environment, it is hard to reflect the reality of the structure and performance between film and device, therefore it is urgently needed to solve the microstructure evolutions of these semiconductor films based on <i>in situ</i> real-time representation technique. In this work, we report a synchrotron-based grazing incidence wide and small-angle scattering (GIWAXS and GISAXS) <i>in situ</i> real-time observation technique combined with a mini glove box, thereby realizing the standard glove box environment (H<sub>2</sub>O, O<sub>2</sub> content all reached below 1×10<sup>–6</sup>) under remote control film spin coating or slot-die preparation and various sample post-processing. Meanwhile, this technique can real-time monitor the microstructure and morphology evolution of semiconductor film during fabrication. Based on the <i>in situ</i> device and GIWAXS, SnO<sub>2</sub> ETL interface induced perovskite growth crystallization process shows that CQDs additive can result in three-dimensional perovskite, with the random orientation growth changing into highly ordered vertical orientation, meanwhile can effectively restrain the low-dimensional perovskite domain formation, helping to reveal the film microstructure transformation of inner driving force and providing the perovskite device preparation process optimized with experimental and theoretical basis. The conversion efficiency of large-area fully flexible three-dimensional perovskite thin film solar cells prepared by the roll-to-roll total solution slit coating method is increased to 5.23% (the area of a single device is ~15 cm<sup>2</sup>). Therefore, using the <i>in situ</i> synchrotron-based glove box device, the microstructure evolution and the associated device preparation conditions of perovskite and organic semiconductor thin films can be controlled, and the thin film growth interface characteristics and film quality can be further controlled, which is the key technology to control the optimization process conditions of semiconductor thin films and devices.

Investigation of microstructural evolution and mechanical properties Al13Fe4 produced by casting and spark-plasma sintering

In this paper, the preparation and evolution of the polycrystalline Al13Fe4 microstructure were investigated during the casting and spark plasma sintering process (SPS). Experimental results showed that an increase in the cooling rate (90–100 °C/min) for both methods of synthesis led to the formation of cracks and chips in the bulk of the material. Cracks during casting are formed by the passage of a peritectic reaction. The high cooling rate does not allow contact melting products to fill the pores that coalesce into cracks. With SPS, the main driving force for the formation of cracks is the effect of the thermal expansion coefficient. The analysis of XRD and results of mechanical testing revealed the maximum tensile micro stresses (425–430 MPa) in samples obtained at a high cooling rate. A decrease in cooling rate promotes the formation of high-density polycrystalline Al13Fe4. It is noted that the material prepared by SPS has a fine-grained structure by using Al13Fe4 powder as the initial material. The highest indicators of flexural strength are 60 ± 10 MPa, Vickers hardness HV = 8.2 ± 0.8 GPa, fracture toughness K1c = 1.5 ± 0.1MPa × m−0.5, modulus of elasticity 180 ± 10 GPa. The fractography of the surfaces after the flexural test has common features of a quasi-brittle trans crystalline fracture with a “river” structure.

Study of microstructure and mechanical properties of quasi-continuous network structured (Ti3AlC2-Al3Ti)/2024Al composites based on hot rolling

To investigate microstructure evolution and improve mechanical properties of quasi-continuous network structured (Ti3AlC2-Al3Ti)/2024Al composites, Hot rolling with entire 40 % deformation under 500 °C was conducted. The results show that rolling deformation refined precipitations in matrix and elongated network structure, causing increment of matrix connectivity and decrement of local reinforcements volume of network structure, as well as activated DRX behavior and caused grain refinement and sub-grain appearance. After rolling deformation, the composites showed an increasing trend in both strength and ductility, the superior properties for as-rolled 5 vol% (Ti3AlC2-Al3Ti)/2024Al composite increased by 38 % in ultimate tensile strength (390 MPa), 29.1 % in yield tensile strength (205 MPa) and almost 1 time in elongation (9.8 %), while for 10 vol% composite was 430 MPa, 232 MPa and 4.3 %, respectively. The increment in ductility of the as-rolled composites results from both DRX behavior and decrement of local reinforcements volume in network structure, while the superior strengthening mechanisms can be mainly attributed to grain refining of matrix, load transfer of reinforcements and thermal mismatch.

Investigation of microstructural evolution in a hybrid additively manufactured steel bead

Abstract Motivated by the beneficial effects of friction stir processing (FSP) for microstructural grain refinement, equiaxed grain production, and minimizing metallurgical defects, additive bead (AB) produced by the gas metal arc welding-wire arc additive manufacturing (GMAW-WAAM) technique was subjected to FSP. This was because deposited additive bead often develops defects such as shrinkage, voids, solidification cracking, during liquid to solid transformation. In this study, a low carbon steel double pass additive bead with 32 % lateral overlap was fabricated by the GMAW-WAAM technique followed by hybridization through FSP in the overlapped region (OR). The peak temperature estimation during bead deposition and FSP on bead was done through modeling by using ABAQUS. The microstructural analysis was carried out by using optical microscopy, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. The microstructure of OR of deposited additive bead is dominated by a combination of ferrite and bainite while that of hybrid additive bead (HAB) is dominated by a combination of ferrite and martensite. Further, the analysis revealed the effects of FSP on the OR in the form of grain refinement from 5.56 µm to 3.50 µm and a decrease in the low angle grain boundaries from 35.4 % to 10.6 %. The continuous dynamic recrystallization is observed since the bainitic fraction in the overlapped region decreased along with an increase in the fraction of martensite in the friction stir processed zone. The kernel average misorientation is observed to decrease after FSP from 1.001 of AB to 0.608. The microhardness test reveals the decrease in the hardness after FSP.

Investigation of microstructure evolution on different planes in laser welding of aluminum alloy

The solidification behavior of a molten pool is a critical factor affecting the mechanical properties of welded joints. This paper develops a multi-scale model combining the macroscale heat transfer and fluid flow model with the microscale phase field model for calculating the microstructure evolution on two different planes that are perpendicular to the thickness direction in the laser welding of the aluminum alloy. To obtain the time-varying temperature gradient (G) and solidification velocity (R) used in the simulation, a transient solidification conditions model is proposed. These models are validated by comparing the simulation results with the experimental results. The results indicate that G decreases, while R increases during solidification process. G/R decreases on both two planes, which results in the transformation of the microstructure from planar to cellular and then to the columnar grain. Additionally, it is found that the primary dendrite arm spacing of columnar grains on the lower plane is smaller, which is related to lower G−1/2R−1/4.

An Integrated Simulation of Multiple-Pass U-10Mo Alloy Hot Rolling and Static Recrystallization

AbstractTo achieve a desired microstructure and minimize the thickness variation in rolled foils, researchers must understand the effects of foil fabrication process variables on microstructure evolution. We developed an integrated simulation of deformation and recrystallization that employs the finite element method (FEM) and the kinetic Monte Carlo (KMC) Potts model, respectively, to investigate microstructure evolution during multiple-pass hot rolling and heat treatment in polycrystalline U-10Mo fuel. Scanning electron microscopy and electron backscatter diffraction images of microstructures were directly used as input in FEM calculation of deformation, and the calculated strains were used to determine the driving force of nucleation and growth of recrystallized grains in the Potts model. Grain structures predicted by the Potts model were used to update the grain structure and material properties for FEM. Simulation alternated between FEM and the Potts model to simulate grain structure evolution during multiple rolling and heat treatments. The initial model parameters were determined by benchmarking the recrystallization kinetics against experimental data. Then, the model was applied to predict the grain structure evolution. Results showed that our model can capture the coupling between deformation and recrystallization and can quantitatively reproduce the observed U-10Mo recrystallization and grain growth kinetics. The simulation results demonstrated that the developed model can predict U-10Mo grain structures as a function of initial microstructure and foil fabrication parameters.

Investigation of microstructure evolution on the fracture toughness behaviour of brass/low carbon steel/brass clad sheets fabricated by cold roll bonding process

The property of fracture toughness is one of the essential parameters in choosing a material that shows resistance to crack growth. In this research, the effect of microstructure evolution on the fracture toughness of Brass (CuZn10)/Low Carbon Steel (St14)/Brass (CuZn10) composite fabricated by the Cold Roll Bonding (CRB) process was investigated. For this purpose, the Brass/Low Carbon Steel/Brass three layered samples were annealed at 450 °C, 600 °C, 750 °C and 850 °C for 2 h. Uniaxial tensile test, microhardness, peeling test, EDS line scan analysis, optical microscope (OM) and scanning electron microscope (SEM) were used to characterize the properties of the composite. The compact tension (CT) test was also used to obtain the fracture toughness of samples. The EDS line scan analysis results showed that no intermetallic compound is formed at the interface of Steel and Brass layers under any temperature. However, increasing the annealing temperature causes the diffusion layer to expand from 5.13 μm to 9.02 μm for samples annealed at 450 °C and 850 °C, respectively, which aids in increasing the bond strength of the layers. Recrystallization changes the layers' microstructure, so the samples' fracture toughness was measured in the roll direction (RD) and transverse direction (TD). It was found that the value of fracture toughness is very different for the as-rolled sample in RD and TD. The most significant decrease in fracture toughness at different temperatures occurs after annealing at 600 °C, wherein the direction of RD compared to the temperature of 450 °C, more than 35% decline was recorded. In fact, after rolling, the microstructure of the layers is elongated in the direction of rolling, which makes the force-crack mouth opening in different directions not the same. Furthermore, as the temperature increases and full recrystallization occurs, the grains become equiaxed, which in addition to reducing the fracture toughness, also reduces the sensitivity to the rolling direction. Based on the obtained results, it was found that the temperature of 600 °C can be chosen as the optimal annealing temperature for three layers of Brass/Steel/Brass composite, in which the appropriate mechanical properties and fracture toughness can be obtained.

A phase field methodology for simulating the microstructure evolution during laser powder bed fusion in-situ alloying process

A recently-developed [1] multi-component phase field model has been utilized to investigate microstructure evolution during in-situ alloying of a blended elemental Ti-1Al-8V-5Fe alloy powder via the Laser Powder Bed Fusion process. The process of in-situ alloying, where elemental powder is used instead of pre-alloyed powder, was studied by performing two simulations having: (1) a uniform initial composition, and (2) a spatially varying initial composition to represent different powder particles. Specifically, the grain morphology, solute distribution, competitive growth and nucleation under the two different scenarios were simulated and compared. To assist the microstructure simulations, a macro-scale finite element model was developed to simulate the heat transfer during LPBF process. The thermal history data calculated by the finite element model was provided to the phase field model in order to simulate transient dendritic growth behaviour. The results show that a set of evenly-spaced columnar dendrites form in the uniform initial composition case, whereas when the initial composition is spatially varying, non-uniform dendrites having elongated shape can develop. It is also shown that competitive growth between dendrites is influenced by nucleation. For the spatially varying initial composition case, the results indicate that full alloying is difficult to achieve during the LPBF printing process; this incomplete alloying greatly influences the dendrite morphology and solute distribution.

Investigation of microstructure evolution and mechanical properties of 2A10 aluminum alloy during ultrasonic vibration plastic forming

Ultrasonic vibration-assisted plastic forming technology is promising since it can reduce the forming force and improve the quality of surface. However, the microstructure and deformation uniformity in the whole sample were rarely taken into account. In this paper, 2A10 Al alloy, which is commonly used in aviation rivets, is selected as the research object for ultrasonic assisted compression test. The test results show that ultrasonic vibration can significantly reduce the flow stress during Al deformation and increase the material hardness. Ultrasonic vibration provides energy for dislocation movement, which makes easier the dislocation to overcome an obstacle. In addition, it also refine the grain, increases the deformation degree of the small deformation region, and reduces the difference of deformation degree between different areas, resulting in improved deformation homogeneity.

Investigation of microstructure evolution and mechanical properties of near-eutectic Fe2-yCoNiCryCx high entropy cast iron

Near-eutectic Fe2-yCoNiCryCx high entropy (HE) cast iron was designed and characterized. The FeCoNiCrC0.45 eutectic HE cast iron was determined being composed of FCC phase and M7C3 carbide. Their crystallographic orientation relationship at the outer region is [101]//[01 1‾ 0] and {110}//{112‾ 0}. The orientation of eutectic M7C3 carbide changes from [01 1‾ 0] to [123‾ 0] as the grain grows from the outer to inner of sample. With the increase of carbon addition, the alloy composition transits from hypoeutectic to hypereutectic where the primary phase changes from FCC phase to M7C3 carbide. With the increase of chromium addition, the phase constituent transits from a mixture of FCC, M7C3 and graphite to that only with FCC and M7C3. The yield strength, compressive strength and hardness for Fe2-yCoNiCryCx HE cast iron are more sensitive to the chromium than carbon. Fe1.2CoNCr0.8C0.45 HE cast iron exhibits the superior properties with the ultimate strength of 1934 MPa and elongation rate of 21.3%, which is mainly attributed to the strengthening mechanism induced by the precipitation of M7C3 carbide with higher fraction. It provides a clue for the design of new cast iron with high-strength, high-elongation and high-hardness.

Kinematical barriers enhanced dislocation strengthening mechanisms in cold-worked austenitic steels

We experimentally investigated microstructure evolution and strengthening behaviors of three cold-worked austenitic steels that deform via dislocations, twinning and stacking faults. The development of stacking faults and twins acting as kinematical barriers for dislocation motion was found to be influenced by stacking fault energy and accumulated dislocations simultaneously, which in turn accelerate dislocation storage rate, strengthen the alloys, and develop high strain hardening rate. We thus proposed a nonadditive strengthening equation to rationalize experimental observation by combining the dislocation-based two-internal-variable model coupled with kinematical barriers.

Investigation of microstructure evolution and properties in silica‐based ceramic cores with alumina as a mineralizer

AbstractIn this work, silica‐based ceramic cores with alumina as a mineralizer were prepared via an injection molding method, and the effects of alumina on the microstructural evolution and properties at 1450°C (simulating the process of equiaxed castings) and 1550°C (simulating the process of columnar/single crystal castings) were investigated. It was found that alumina promoted the cristobalite crystallization of fused silica refractory during sintering but inhibited the devitrification rate in the subsequent heating. The flexural strength of silica‐based ceramic cores at an ambient temperature and 1450°C improved with an increasing alumina content, whereas the opposite trend appeared at 1550°C. The creep resistances of silica‐based cores were improved significantly and then slightly deteriorated with an increasing alumina content from 5% to 20%, depending on the competition effects of alumina hindering the viscous flow of liquid silica (favorable), but suppressing the devitrification rate (unfavorable). The results of this work show that silica‐based cores need to follow different compositional design principles for equiaxed and columnar/single‐crystal turbine blade castings.

Investigation of microstructure evolution, mechanical and corrosion properties of SAF 2507 super duplex stainless steel joints by keyhole plasma arc welding

Keyhole plasma arc welding (PAW) of the super duplex stainless steel (SDSS) is expected to achieve full-penetration joints of medium-thickness plates in a single pass with higher efficiency. However, the unique key-holing processes due to the special constricted arc behaviors cause much different thermal cycles of the liquid metals as well as changed microstructures. SDSS 2507 with a thickness of 10 mm was joined by PAW to investigate the joints' microstructure and properties. Microstructure characterization reveals substantial differences in the content of austenite, ranging from 68.9% in weld metal (WM) to 32.1% in the heat-affected zone (HAZ). The oxide inclusions in the WM enhance the hardness of the welded joints, but provide the path for crack propagation to decrease its impact toughness. While complicated microstructure is found in HAZ forming ferrite and diverse austenite, such as grain boundary austenite (GBA), intragrain austenite (IGA), and Widmanstätten austenites (WA). A small amount of secondary austenite (γ2) and chromium nitride are also found in HAZ, and they significantly deteriorate its corrosion resistance. The grain boundary misorientation angle distribution of the austenite in WM is found to be dominated by the low angle boundaries (LABs). However, its content decreases from the WM to base metal (BM). Selective corrosion occurs in all samples, where ferrite is preferentially dissolved when the extrinsic voltage is low (<0.5 V) and austenite is preferentially dissolved when the extrinsic voltage is high (>0.5 V), indicating the different electrochemical activity of the two phases.

Investigation of microstructure evolution and mechanical properties of gas tungsten arc welded dissimilar titanium (CP-Ti/Ti–6Al–4V) alloys

Economical welding of dissimilar titanium alloys has always been a challenge for the aerospace and nuclear industries, where these alloys are extensively used because of their high strength-to-weight ratio. The present investigation deals with the development of a novel shielding setup for the successful welding of commercially pure titanium (α-phase) to Ti–6Al–4V ((α + β)-phase) without any atmospheric contamination using gas tungsten arc welding. Commercially pure titanium (CP-Ti) and Ti–6Al–4V sheets of 3 mm thickness were butt-welded autogenously and using CP-Ti filler metal. Metallographic studies, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, microhardness measurements, tensile testing, and fractography were carried out to understand the evolution of the microstructure and characterize the welds. Grain coarsening in heat-affected zone and weld, and variation in the distribution of phases were observed from the optical micrographs and energy-dispersive X-ray spectroscopy spectrum of different zones. An unsymmetrical variation of hardness was observed in the weld region, and a 15% reduction in hardness value (280 HV0.2) was obtained at the weld center of using filler metal weld than autogenous weld. Comparable strengths with little drop from as-received CP-Ti (307 MPa) were observed for welded specimens. Welds produced using CP-Ti filler metal (272 MPa) had higher strength than autogenous welds (214 MPa). As-received tensile specimens fractured nearly from the center of gauge length, but welded tensile specimens fractured near the base/heat-affected zone boundary of the CP-Ti side.

Investigation of Microstructural Evolution and Mechanical Properties of IN617 Superalloy During Long-Term Operating at High Temperature

Investigation of Microstructural Evolution and Mechanical Properties of IN617 Superalloy During Long-Term Operating at High Temperature