Similar Microstructure Research Articles

Enhanced creep performance in a polycrystalline superalloy driven by atomic-scale phase transformation along planar faults

Predicting the mechanical failure of parts in service requires understanding their deformation behavior, and associated dynamic microstructural evolution up to the near-atomic scale. Solutes are known to interact with defects generated by plastic deformation, thereby affecting their displacement throughout the microstructure and hence the material's mechanical response to solicitation. This effect is studied here in a polycrystalline Ni-based superalloy with two different Nb contents that lead to a significant change in their creep lifetime. Creep testing at 750°C and 600 MPa shows that the high-Nb alloy performs better in terms of creep strain rate. Considering the similar initial microstructures, the difference in mechanical behavior is attributed to a phase transformation that occurs along planar faults, controlled by the different types of stacking faults and alloy composition. Electron channeling contrast imaging reveals the presence of stacking faults in both alloys. Microtwinning is observed only in the low-Nb alloy, rationalizing in part the higher creep strain rate. In the high-Nb alloy, atom probe tomography evidences two different types of stacking faults based on their partitioning behavior. Superlattice intrinsic stacking faults were found enriched in Nb, Co, Cr and Mo while only Nb and Co was segregated at superlattice extrinsic stacking faults. Based on their composition, a local phase transformation occurring along the faults is suggested, resulting in slower creep strain rate in the high-Nb alloy. In comparison, mainly superlattice intrinsic stacking faults enriched in Co, Cr, Nb and Mo were found in the low-Nb alloy. Following the results presented here, and those available in the literature, an atomic-scale driven alloy design approach that controls and promotes local phase transformation along planar faults at 750°C is proposed, aiming to design superalloys with enhanced creep resistance.

Comparative analysis for pyrolysis of sewage sludge in tube reactor heated by electromagnetic induction and electrical resistance furnace

A comparative study was conducted on the pyrolysis of sewage sludge in tube reactors heated by electromagnetic induction (EMI) and conventional electrical resistance furnace (ERF). A minimal effect of pyrolysis temperature and initial moisture content on the distribution of pyrolytic products was obtained. Compared with the counterpart from ERF pyrolysis, the bio-char from EMI pyrolysis exhibited less ash content (46.38 wt%) and higher organic matter content (53.62 wt%). SEM and FTIR test showed similar microstructure characterizations in the two bio-chars. The specific area of bio-char from EMI pyrolysis was 8.6 m2/g. EMI pyrolysis increased the total content of aliphatic/aromatics in the bio-oil from 10.8 wt% to 15.6 wt% and the hydrogen/carbon monoxide in the bio-gas from 33.8 vol% to 41.1 vol% because of possible cracking and reforming reactions. Increased sulfur content in the bio-oil and decreased hazard gas content (such as hydrogen sulfide and sulfur dioxide) in the bio-gas were obtained during EMI pyrolysis. The actual energy consumption for EMI and ERF pyrolysis were 4.62 MJ/kg and 6.65 MJ/kg. Increasing the feedstock content would reduce the energy consumption unit energy consumption. Less system energy loss during EMI pyrolysis might explain the higher energy recovery from EMI pyrolysis than that from ERF. Despite some disadvantages, EMI pyrolysis shows potential in real-plant applications.

Influence of Early-Age Curing Regimes on Late-Age Hydration of UHPC

Abstract This paper investigates the influence of metakaolin (MK) as well as various early-age curing regimes on late-age hydration of ultrahigh-performance concrete (UHPC) immersed in hot water. To reduce the consumption of cement, we partially substitute cement by limestone (LS) in the UHPC mixture. After 24 h of hardening, UHPC specimens are demolded and exposed to different curing regimes, including standard water curing at 20°C for up to 28 days as well as steam curing at 55°C or 90°C for 24 or 48 h followed by standard water curing for up to 28 days. Afterward, the UHPC mortar and paste specimens are transferred into the water at 80°C and left for predesigned testing ages (7, 14, 21, and 28 d). We evaluate the compressive strength of UHPC mortar as well as hydration and microstructural properties of UHPC paste with the analysis of chemically bound water, differential thermal analysis, and mercury intrusion porosimetry. The results indicate that the compressive strength of the UHPC has an increasing tendency from 28 to 42 days and then a decreasing tendency. The addition of MK alleviates the late-age strength development tendency of the steam-cured UHPC specimens compared with the control mixtures without MK. The hydration results verify this strength development tendency of UHPC from 28–56 days. The ternary MK- and LS-blended UHPC exhibits similar strength development tendency and microstructure with binary paste incorporated with MK at later ages.

Compact representations of microstructure images using triplet networks

The microstructure of a material, typically characterized through a set of microscopy images of two-dimensional cross-sections, is a valuable source of information about the material and its properties. Every pixel of the image is a degree of freedom causing the dimensionality of the information space to be extremely high. This makes it difficult to recognize and extract all relevant information from the images. Human experts circumvent this by manually creating a lower-dimensional representation of the microstructure. However, the question of how a microstructure image can be best represented remains open. From the field of deep learning, we present triplet networks as a method to build highly compact representations of the microstructure, condensing the relevant information into a much smaller number of dimensions. We demonstrate that these representations can be created even with a limited amount of example images, and that they are able to distinguish between visually very similar microstructures. We discuss the interpretability and generalization of the representations. Having compact microstructure representations, it becomes easier to establish processing–structure–property links that are key to rational materials design.

Machine-learning assisted laser powder bed fusion process optimization for AlSi10Mg: New microstructure description indices and fracture mechanisms

In this study, a machine-learning approach based on Gaussian process regression was developed to identify the optimized processing window for laser powder bed fusion (LPBF). Using this method, we found a new and much larger optimized LPBF processing window than was known before for manufacturing fully dense AlSi10Mg samples (i.e., relative density ≥ 99%). The newly determined optimized processing parameters (e.g., laser power and scan speed) made it possible to achieve previously unattainable combinations of high strength and ductility. The results showed that although the AlSi10Mg specimens exhibited similar Al-Si eutectic microstructures (e.g., cell structures in fine and coarse grains), they displayed large difference in their mechanical properties including hardness (118 - 137 HV 10), ultimate tensile strength (297 - 389 MPa), elongation to failure (6.3 - 10.3%), and fracture toughness (9.9 - 12.7 kJ/m2). The underlying reason was attributed to the subtle microstructural differences that were further revealed using two newly defined morphology indices (i.e., dimensional-scale index Id and shape index Is) based on several key microstructural features obtained from scanning electron microscopy images. It was found that in addition to grain structure, the sub-grain cell size and cell boundary morphology of the LPBF fabricated AlSi10Mg also strongly affected the mechanical properties of the material. The method established in this study can be readily applied to the LPBF process optimization and mechanical properties manipulation of other widely used metals and alloys or newly designed materials.

Assessing the scale contributing factors of three carbide-free bainitic steels: A complementary theoretical and experimental approach

The bainitic ferrite plate thickness is the main parameter that controls the strength of this type of microstructures. Such thickness has been proved to mainly depend on the austenite yield strength, the driving force for the transformation and the transformation temperature. However, no research has focused on how these parameters evolve throughout the transformation and how this evolution can affect the outcome. In this study, thermal and thermomechanical treatments have been performed in three selected steels. The treatments have been designed in such a way that all the mentioned parameters are comparable, aiming to obtain similar microstructures in terms of bainitic ferrite plate thickness. However, significant differences have been found among the microstructures, with variations in plate thickness larger than 100 nm. These results indicate that there might be other factors that take part in the scale of bainitic microstructures. To explain these differences and based on the kinetics of the transformation and on the carbon content of austenite at the end of the transformation, a possible explanation has been proposed.

Microstructure and mechanical properties of new die-cast quaternary Al-Cu-Si-Mg alloys

Three new ultrafine hypoeutectic die-cast alloys based on quaternary Al-Cu-Si-Mg system were developed. The CALPHAD thermodynamic modelling of Al-Cu-Si-Mg system based on Scheil description was conducted to predict the volume fractions of the eutectic mixtures. The designed volume fraction of eutectic mixtures of Al-xCu-2.2Si-1.1Mg (x = 5, 6.6 and 10.6 wt%) alloys were determined to be 0.2, 0.25 and 0.3, respectively. With the increase in eutectic fraction, the yield strength and ultimate tensile strength increased from 219 MPa to 267 MPa, and 344.7 MPa–395 MPa respectively, while the elongation to fracture decreases from 7.72% to 3.4%. The microstructure of the alloys mainly consists of Si, Al2Cu, α-Al and Al4Cu2Mg8Si7 (Q) phases. The Al5Cu2.2Si1.1Mg and Al6.6Cu2..2Si1.1Mg alloys had similar microstructures that consisted of coarse binary/ternary and fine quaternary eutectic regions, surrounding α-Al matrix. The Al10.6Cu2.2Si1.1Mg alloy contained α-Al grains and almost only one type of eutectic structure. The orientation relationships among Al2Cu, α-Al and Q phases were found in the eutectic region. Al2Cu and α-Al has a coherent interface, while the interfaces between Q and Al2Cu as well as Q and α-Al, were found to be semi-coherent. Our results broaden a new approach to the design of die-cast alloys with multi-phase and multi-component microstructure for high-performance applications.

Effect of Ti addition on the sliding wear behavior of AlCrFeCoNi high-entropy alloy

Originally defined high-entropy alloys (HEAs) are generally in a solid-solution state, which are solution-strengthened by multiple-elements with desired toughness. However, the mixture of multiple elements likely result in certain microstructure features, which could also benefit the HEAs. In this study, AlCrFeCoNi and AlCrFeCoNiTi0.5 high entropy alloys (HEAs) were fabricated, and their microstructures and sliding wear behaviors (in comparison with stainless steel) were investigated using XRD, SEM, EDS and a pin-on-disc tribometer, respectively. It is shown that the two HEAs consist of ordered Al–Ni/Al–Ni–Ti rich BCC and disordered Fe–Cr rich BCC domains. The AlCrFeCoNi alloy has an intertwined structure with plate-like Fe–Cr rich domains embedded in the Al–Ni rich matrix. The AlCrFeCoNiTi0.5 alloy shows a similar microstructure but has honeycomb-like Fe–Cr rich interdendrites. The Ti addition markedly increases the hardness of the alloy, leading to considerably enhanced resistance to sliding wear. Through wear tests with and without air-flow to minimize frictional heating along with worn surface analysis, we demonstrate that the Ti addition also enhances the oxide scale, making additional contribution to the wear resistance.

Machine-Learning Assisted Laser Powder Bed Fusion Process Optimization for AlSi10mg: New Microstructure Description Indices and Fracture Mechanisms

In this study, a machine-learning approach based on Gaussian process regression was developed to identify the optimized processing window for laser powder bed fusion (LPBF). Using this method, we found a new and much larger optimized LPBF processing window than was known before for manufacturing fully dense AlSi10Mg samples (i.e., relative density ≥ 99%). The newly determined optimized processing parameters (e.g., laser power and scan speed) made it possible to achieve previously unattainable combinations of high strength and ductility. The results showed that although the AlSi10Mg specimens exhibited similar Al-Si eutectic microstructures (e.g., cell structures in fine and coarse grains), they displayed large difference in their mechanical properties including hardness (118 - 137 HV 10), ultimate tensile strength (297 - 389 MPa), elongation to failure (6.3 - 10.3%), and fracture toughness (9.9 - 12.7 kJ/m 2 ). The underlying reason was attributed to the subtle microstructural differences that were further revealed using two newly defined morphology indices (i.e., dimensional-scale index I d and shape index I s ) based on several key microstructural features obtained from scanning electron microscopy images. It was found that in addition to grain structure, the sub-grain cell size and cell boundary morphology of the LPBF fabricated AlSi10Mg also strongly affected the mechanical properties of the material. The method established in this study can be readily applied to the LPBF process optimization and mechanical properties manipulation of other widely used metals and alloys or newly designed materials.

Experimental study of rate-dependent uniaxial compressive behaviors of two artificial frozen sandy clay soils

The rate-dependent uniaxial compressive behavior of frozen clay soil is strongly affected by its minerals, void ratio, stress history, and pore fluid salinity. Previous studies show that an extremely low deformation rate applied on a clay soil tends to result in a lower bound compressive strength. However, the lower bound stress-strain behavior of frozen clay soil under the uniaxial compressive condition was not studied. A thorough understanding of the rate-dependent behavior of frozen clay and its relationship with temperature requires a large number of specimens with similar compositions and micro-structures. In this study, a series of laboratory tests were carried out on artificial frozen sandy clay soils to measure the rate-dependent uniaxial compressive behavior at temperatures ranging from −15 °C to 0 °C. We used two types of artificial frozen clay soils with pre-determined clay fraction, clay mineralogy, stress history, and moisture content. Our results conclude that a low temperature and a high deformation rate tend to generate brittle failure with post-peak softening behavior. A temperature close to the freezing temperature and a low deformation rate result in a diffuse failure associated with strain hardening. Temperature-dependent uniaxial compressive mechanical properties were measured and modeled using empirical relations, which are highly dependent on the applied deformation rate. A series of step-loaded relaxation tests were carried out on those artificial clay soils and creep parameters were estimated using relaxation test results and a power law creep model. A new approach of deriving the lower bound of stress-strain curve of frozen soil is proposed accordingly. The idea of determining the lower bound of stress-strain curve is based on the isotache concept, which was previously used in the soil consolidation theory.

Laser Directed Energy Deposition of Bulk 316L Stainless Steel

Laser directed energy deposition (DED) of high-density parallelepiped 316L stainless steel specimens has been performed while varying laser power, scanning velocity, powder mass flow rate and layer thickness. An extensive set of analyses has been performed into the resulting dimensions, surface roughness, microstructure, hardness, tensile strength, build substrate temperature, deposition efficiency, production time and specific energy input with the aim of differentiating variations in component performance and process efficiency for components that have already been optimized in terms of density. Average layer height is found to be an approximately linear function of the powder line mass flow rate, while samples with the same powder feed per unit length have similar microstructures despite differences in scanning velocity and total powder feed rate. The hardness of specimens is generally higher than bulk 316L stainless steel, while yield and ultimate tensile strength are similar or higher than the bulk material but with dependence on the load direction. Deposition efficiency is strongly influenced by the scanning strategy and laser off time, with values as low as 42% with a “raster” scanning strategy and up to 84% with a “snake” scanning strategy. The large number of analyses performed on high-density components provides important new insight into factors affecting build quality and process efficiency that go beyond simply maximizing density, suggesting that significant further improvements can be obtained. Such knowledge is exploitable for application-specific process optimization and implementation of open and closed-loop height control strategies for production of bulk 316L stainless steel components.

Simultaneous Recruitment of Stem Cells and Chondrocytes Induced by a Functionalized Self-Assembling Peptide Hydrogel Improves Endogenous Cartilage Regeneration.

The goal of treating articular cartilage (AC) injury is to regenerate cartilage tissue and to integrate the neo-cartilage with surrounding host cartilage. However, most current studies tend to focus on engineering cartilage; interface integration has been somewhat neglected. An endogenous regenerative strategy that simultaneously increases the recruitment of bone marrow mesenchymal stem cells (BMSCs) and chondrocytes may improve interface integration and cartilage regeneration. In this study, a novel functionalized self-assembling peptide hydrogel (KLD-12/KLD-12-LPP, KLPP) containing link protein N-peptide (LPP) was designed to optimize cartilage repair. KLPP hydrogel was characterized using transmission electron microscopy (TEM) and rheometry. KLPP hydrogel shared a similar microstructure to KLD-12 hydrogel which possesses a nanostructure with a fiber diameter of 25–35 nm. In vitro experiments showed that KLPP hydrogel had little cytotoxicity, and significantly induced chondrocyte migration and increased BMSC migration compared to KLD-12 hydrogel. In vivo results showed that defects treated with KLPP hydrogel had higher overall International Cartilage Repair Society (ICRS) scores, Safranin-O staining scores and cumulative histology scores than untreated defects or defects treated with KLD-12 hydrogel, although defects treated with KLD-12 and KLPP hydrogels received similar type II collagen immunostaining scores. All these findings indicated that the simple injectable functionalized self-assembling peptide hydrogel KLPP facilitated simultaneous recruitment of endogenous chondrocytes and BMSCs to promote interface integration and improve cartilage regeneration, holding great potential as a one-step surgery strategy for endogenous cartilage repair.

Effect of high temperature service on the complex through-wall microstructure of centrifugally cast HP40 reformer tube

Centrifugally cast reformer tubes are used in petrochemical plants for hydrogen production. Due to the conditions of hydrogen production, reformer tubes are exposed to high temperature which causes creep damage inside the microstructure. In this study, two different ex-service HP40 alloy reformer tubes which come from the same steam reformer unit have been compared by microstructural characterisation performed at a range of length scales from mm to um. Analyses performed by EBSD (Electron Backscatter Diffraction), EDS (Energy Dispersive X-ray Spectroscopy) and PCA (Principal Component Analysis) show that both tubes have similar microstructural constituents, with the presence of an austenitic matrix and M23C6, G phase and M6C carbides at the grain boundaries. Even if both tubes have a similar microstructure, one tube due to it localisation inside the steam reformer unit presents a region with more micro cracks which may indicate that this tube have accumulated more creep damage than the other one.

Microstructure and Texture Evolution of AZ31 Alloy Prepared by Cyclic Expansion Extrusion with Asymmetrical Extrusion Cavity at Different Temperatures.

This work is to study the microstructure and texture evolution of AZ31 alloy prepared by cyclic expansion extrusion with an asymmetrical extrusion cavity (CEE-AEC) at different deformation temperatures. The result shows AZ31 alloy undergoes continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) during CEE-AEC processing. At the initial stage of deformation, AZ31 alloys exhibit similar bimodal microstructure of coarse deformed grains surrounded by fine DRXed grains. As the passes increase, the cumulative strain increases, and the coarse grains of all samples are almost replaced by fine equiaxed grains. The average grain sizes and the basal texture intensities of the deformed samples increase as the deformation temperature increases. In addition, due to the existence of an asymmetrical cavity, as the passes increase, the basal textures of all samples are deflected with maximum intensities increase, and even an unusual bimodal texture is formed, resulting in a soft orientation that is easy to basal slip.

Microstructural banding of directed energy deposition-additively manufactured 316L stainless steel

The microstructures of 316 L stainless steel created by rapid solidification are investigated by comparing the similar microstructures of individual hatches of directed energy deposition additive manufacturing (DED-AM) and those of single, laser surface-melted tracks formed on a solid plate. High recoil pressure, which is exponentially dependent on the laser beam power density, induces convection of the melt pool, which causes formation of microstructural bands in the as-solidified microstructure. The microstructural bands are associated with changes in the chromium concentration and are a significant component of the inhomogeneous microstructure of DED-AM.

Mix design optimization and early strength prediction of unary and binary geopolymer from multiple waste streams

Geopolymer has received increasing amounts of attention recently due to its potential utilization of industrial and urban wastes. However, the variability of source materials and the complexity of mixture design hinder geopolymer applications derived from various waste streams. There is a need for a practical and quick scanning tool for material evaluation and mixture design optimization. Six types of industrial and urban wastes, two types of reagents, and two curing temperatures were employed in this study to systematically evaluate the feasibility of using isothermal calorimetry to optimize the geopolymer mixture design and predict the three-day strength. Test results show that isothermal calorimetry has the potential to quantify the compositional differences between source materials, identify the different kinetics of geopolymers, and determine the mechanical properties of final products. For the source materials with similar microstructure and fineness, fairly strong correlations between heat and strength could be found with R2 = 0.91 for the NaOH solution and R2 = 0.90 for the composite solution.

Viability of button melting for accelerated development of chromium carbide overlay

To accelerate overlay design and development, button melting method is compared with wire fabrication and welding. For the same overlay alloy, similar microstructure and hardness are produced by both methods. It is viable to use button melting in replace of wire fabrication. However, there is no dilution from base metal in button melting, and the microstructure consists of more directionally solidified grains due to the predominant directional cooling. Button melting also produces faster cooling rates that enhance formation of martensite in the steel matrix.

Role of β(FeAl) nanoparticles in abnormal grain growth in the annealing of cast Cu-Al-Mn-Fe shape memory alloys

In this study, the low-cost production of Cu-Al-Mn-Fe shape memory alloy single crystals exceeding 46 mm by abnormal grain growth was realized only through annealing their cast alloys. The results show that the misorientation formed during annealing may be responsible for such abnormal grain growth process. It was confirmed that this misorientation resulted from the dissolution of bcc β(FeAl) nanoparticles during heat treatment at a sufficiently temperature of approximately 1173 K. The rate of migration of the abnormal grain boundary was experimentally measured to be approximately 9.3 × 10-5 m s-1 within 2 min of the commencement of abnormal grain growth. Additionally, the range of composition of the alloys that can lead to abnormal grain growth was determined. When the Cu-13.0Al-6.5Mn-3.2Fe single crystal close to the [100] direction was deformed to a pre-strain of 12%, full shape recovery happened without any residual strain. At that time, the superelastic strain was approximately 9%. Such a superelastic characteristic remained nearly constant over 50 cycles, showing excellent fatigue resistance. The superelastic properties of the present Cu-13.0Al-6.5Mn-3.2Fe single crystal are compared to those of commercial Ni-Ti-based shape memory alloys. Therefore, it can be considered as a new kind of superelastic material having practical applications. The obtained results should be of great significance in the development of Cu-based shape memory alloys. Furthermore, it is expected that a similar microstructure can be designed for the production of more metallic single crystals.

Interfacial transformation of preoxidized Cu microparticles in a formic-acid atmosphere for pressureless Cu–Cu bonding

Wide bandgap (WBG) power devices have been attracting increasing attention for next-generation electronic applications. Cu–Cu bonding has been developed to accommodate the high temperature and current characteristics encountered in WBG semiconductors service conditions. In this study, we demonstrate and optimize a pressureless Cu–Cu bonding method by reductively sintering preoxidized Cu microparticles in a formic-acid atmosphere, and provide information on the bonding mechanism and relationship between behaviors of Cu microparticles and bonding quality. The transformation of Cu microparticles was examined at different preoxidation intervals, which were followed by a reduction reaction. The Cu oxide produced by preoxidation is critical for reliable Cu–Cu bonding because it forms linkages between Cu microparticles during the process; additionally, it fills the gaps between the individual particles to increase bonding density. In both thermal aging and cycling test, it was found that the Cu–Cu bonding exhibited similar microstructures and oxidation behavior, but followed different degradation and failure modes.

Liquid-State Interfacial Reactions of Sn and Sn-Ag-Cu Solders with p-Type (Bi,Sb)2Te3 Thermoelectric Material

Liquid-state interfacial reactions of p-type (Bi,Sb)2Te3 thermoelectric (TE) material with Sn and Sn-3.0 wt.%Ag-0.5 wt.%Cu (SAC305) solders, respectively, were examined at 250°C on a commercial highly orientated (Bi,Sb)2Te3 substrate with a (110) soldering plane. For the reactions with Sn, the initial reaction phase was the porous SnTe intermetallic compound (IMC). Then, a dense SnTe layer with many tiny cracks formed between the porous phase and TE substrate. With longer aging time, the dense SnTe gradually changed to the porous phase due to Sb dissolution. In the subsequent stage, the reaction phase zone included porous SnTe, an alternating layer microstructure of SnTe and liquid solder, and SnTe/Sn3Sb2 alternating layers from the solder to TE. The IMC growth was extremely fast, being approximately 15 μm/min, and nearly linear with aging time, suggesting reaction control. For the reactions with SAC305, a similar microstructure of porous SnTe and the dense layer was observed. Compared with the reactions with Sn, however, the IMC growth rate was greatly suppressed by ~ 80%. This can be attributed to the Ag-rich phase layer between the dense SnTe and (Bi,Sb)2Te3. Ag was verified to be the key element for dramatically changing the interfacial reaction behavior.