Comprehensive Microstructural Characterization Research Articles

Comprehensive microstructural and optical characterization of the thermal stability of aluminum-titanium oxynitride thin films after high temperature annealing in air

The thermal stability of two AlyTi1-y(OxN1-x) layers prepared by cathodic vacuum arc deposition with different oxygen content was studied after high temperature annealing of the samples in air. These layers were designed to be part of solar-selective coating (SSC) stacks. Compositional and microstructural characterization of the thin films was performed before and after the thermal treatment by elastic recoil detection (ERD), transmission electron microscopy, and Raman spectroscopy. AlyTi1-yN sample was stable after 2 h of annealing at 450 °C. Initial stages of the formation of a surface oxide layer after annealing at 650 °C were observed both by ERD and Raman analysis. Contrarily, no changes were found after 2 h annealing treatment either at 450 and 650 °C in the composition and microstructure of AlyTi1-y(OxN1-x) sample. In both samples, the formation of a surface anatase TiO2 film was reported after 2 h annealing at 800 °C. These compositional and microstructural changes were correlated with the optical properties determined by spectroscopic ellipsometry. A transition from metallic to dielectric behavior with increasing annealing temperature was observed. These results complete the durability studies on the designed SSC based on AlyTi1-y(OxN1-x) materials, confirming that these stacks withstand breakdown at 600 °C in air.

Nanocomposite‐Seeded Epitaxial Growth of Single‐Domain Lithium Niobate Thin Films for Surface Acoustic Wave Devices

Epitaxial lithium niobate (LNO) thin films are an attractive material for devices across a broad range of fields, including optics, acoustics, and electronics. These applications demand high‐quality thin films without in‐plane growth domains to reduce the optical/acoustical losses and optimize efficiency. Twin‐free single‐domain‐like growth has been achieved previously, but it requires specific growth conditions that might be hard to replicate. In this work, a versatile nanocomposite‐seeded approach is demonstrated as an effective approach to grow single‐domain epitaxial lithium niobate thin films. Films are grown through a pulsed laser deposition method and growth conditions are optimized to achieve high‐quality epitaxial film. A comprehensive microstructure characterization is performed and optical properties are measured. A piezoelectric acoustic resonator device is developed to demonstrate the future potential of the nanocomposite‐seeded approach for high‐quality LNO growth for radio frequency (RF) applications.

Comprehensive microstructural characterization of saline–alkali soils in the Yellow River Delta, China

ABSTRACT Soil salinization in the Yellow River Delta, China, is a matter of concern from agricultural and environmental perspectives. This study characterizes the relationship between the permeability and microstructure of saline–alkali soil. Soil microstructure, including mineral composition, pore distribution, and particle morphology of soils at different depths, was analyzed using X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy. Results indicated that the soil mean saturated hydraulic conductivity and porosity, excluding the surface soil, were only 6.0 × 10−5 cm/s and 33.0%, respectively, and the mean proportion of clay minerals (illite, chlorite and kaolinite) was 6.1%. The pore distribution results showed that small pores (2 ≤ d< 10 μm) and micropores (0.1 ≤ d< 2 μm) constituted the majority of the soil pores. We attributed the deterioration of the soil permeability to low soil porosity, which, in turn, was associated with soil pore size distribution and a closely inlaid ‘brick wall’-type particle arrangement. Quantitative analysis of the scanning electron microscopy images showed that the fractal dimension of pores and roundness of particles increased with depth, whereas the fractal dimension of particles and roundness of pores decreased with depth. However, the azimuth, perimeter, Fred diameter, and orientation of saline–alkali soil did not significantly change with the increase in depth. This study revealed that the microstructure of saline–alkali soil exhibits a good correlation with its permeability.

Microstructural Evolution upon 10-wt% Mo Alloying and Laser Surface Melting of M2 High Speed Steels Hardfacings

The article aims to comprehend the microstructural changes, in Plasma Transfer Arc (PTA) deposited M2 high speed steel (HSS) hardfacings upon incorporation of 10 wt% Mo alloying during deposition followed by laser surface melting. PTA deposited hardfacings were produced over 4140 steel. Then Mo alloyed and unalloyed PTA deposits were subjected to laser surface melting (LSM) process. A comprehensive microstructural characterization for all the resultant structures was carried out. Optical metallography using appropriate etching reagents and SEM microscopy in conjunction with XRD techniques were employed to ascertain the matrix structure and carbides morphology. The PTA microstructure was close to equilibrium structure of M2 HSS containing mixture of ferrite/austenite/martensite along with MC, M2C and M6C type carbides. While the LSM of M2 HSS caused higher fraction of martensite and finer grains in the structure resulting in increment in hardness. 10-wt% Mo addition changes the carbides from MC and rod like M2C to fibrous M2C and fishbone like M6C carbides. The LSM of Mo alloyed M2 HSS PTA deposits led to an overall decrease in the fraction of M6C carbides and fibrous M2C carbides accompanied by a decrease in hardness.

Synergetic influence of microconstituents on the damage accumulation and consequent effect on the flow behaviour in cast Mg–Ca–Sn alloys

Combined role of microconstituents (particles, eutectic fraction and its morphology) on the damage accumulation of three as-cast Mg–Ca–Sn alloys having distinctive microstructural features is studied. Flow stress normalized with that of a solid solution alloy brought out the sole contribution of microconstituents to the deformation. For identifying the influence of microconstituents on strain hardening rate, Kocks-Mecking analysis is utilized together with intermittent post-deformation microstructures. Comprehensive microstructural characterization as a function of strain facilitated to unravel the distinction in internal damage accumulation among the alloys. Further, the internal damage accumulation variation due to distinct variation in the microconstituents is attributed to the differences in hardening rate trends among the three alloys. The reduction in hardening rate is analyzed to occur due to damage accumulation through eutectic fracture or interphase cracking. Thus, the damage mechanism and its influence on strain hardening in cast magnesium alloys is proposed.

A Comprehensive Microstructural and Compositional Characterization of Allogenic and Xenogenic Bone: Application to Bone Grafts and Nanostructured Biomimetic Coatings

Bone grafts and bone-based materials are widely used in orthopedic surgery. However, the selection of the bone type to be used is more focused on the biological properties of bone sources than physico-chemical ones. Moreover, although biogenic sources are increasingly used for deposition of biomimetic nanostructured coatings, the influence of specific precursors used on coating’s morphology and composition has not yet been explored. Therefore, in order to fill this gap, we provided a detailed characterization of the properties of the mineral phase of the most used bone sources for allografts, xenografts and coating deposition protocols, not currently available. To this aim, several bone apatite precursors are compared in terms of composition and morphology. Significant differences are assessed for the magnesium content between female and male human donors, and in terms of Ca/P ratio, magnesium content and carbonate substitution between human bone and different animal bone sources. Prospectively, based on these data, bone from different sources can be used to obtain bone grafts having slightly different properties, depending on the clinical need. Likewise, the suitability of coating-based biomimetic films for specific clinical musculoskeletal application may depend on the type of apatite precursor used, being differently able to tune surface morphology and nanostructuration, as shown in the proof of concepts of thin film manufacturing here presented.

Effect of deformation induced B2 precipitates on the microstructure and mechanical property of Al0.3CoCrFeNi high-entropy alloy

Precipitation-hardening high-entropy alloys (HEAs) based on a single face-centered cubic HEAs with a small amount of alloying elements have become a research of interest and technological importance for many structural applications. Here, we report different ordered B2 precipitates in an Al0.3CoCrFeNi HEA obtained by thermo-mechanical processing. Subsequent tensile deformation to 10% and annealing at 700 °C for 40min accomplished the dense nano-sized B2 particles along the grain boundaries and twinning boundaries by which explore ways to improve the strength while maintaining the plasticity at high level for the present HEA. Furthermore, nucleation of the B2 phase was discussed based on the comprehensive microstructure characterization.

Microstructure, fatigue, and impact toughness properties of additively manufactured nickel alloy 718

A complete understanding of processing-structure-property-performance relationship of additively manufactured (AM) components are critical from an application standpoint. Therefore, in the current investigation, a comprehensive microstructural characterization and mechanical properties (tensile, fatigue and impact toughness) evaluation of nickel alloy 718 AM by the laser powder bed fusion (L-PBF) technique have been performed. AM builds were made from powders manufactured via different atomization conditions. Although the standard post-heat treatment procedure led to the removal of severe interdendritic segregation both grain boundary and intra-grain precipitation of δ phase occurred. Regardless of δ phase presence, axial fatigue properties of both the AM builds were similar to design handbook wrought fatigue data. However, due to the δ phase, impact toughness properties were comparable to the wrought material conditions that exhibited δ phase. Fractured surfaces of Charpy impact samples exhibited crack propagation extensively along the boundaries decorated by δ precipitates.

Laser metal deposition of vanadium-rich high speed steel: Microstructural and high temperature wear characterization

A comparative high temperature wear study was conducted between two alloys: laser metal deposited vanadium-rich (V-rich) high speed steel (HSS) and spun cast carbide enhanced indefinite chilled double poured (CE-ICDP) iron. Laser Metal Deposition (LMD) of V-rich HSS alloy was performed by using a 4.0 kW Nd:YAG laser at three different laser scan speeds to investigate the effect thereof on the carbide size and morphology, phase constitution and mechanical properties (such as micro-hardness and wear resistance) of the laser metal deposits.A comprehensive microstructural characterization of these alloys revealed that the dendritic microstructure of the V-rich HSS alloy consisted of martensitic matrix and VC carbides. Increasing the laser processing speeds significantly changed the morphologies of VC carbides from square and round to angular and rod-like shapes. The micro-hardness of the V-rich HSS was improved from 760 HV to 835 HV by increasing the laser processing speed. During high temperature (500 °C) pin-on-disc wear tests, the V-rich HSS showed excellent wear resistance compared to CE-ICDP iron. It was found that V-rich HSS with square and round shape VC carbides (V-rich 10 mm/s) showed the most improved tribological performance with oxidative wear found to be the dominant wear mechanism at this temperature.

Exploring the relationship between the microstructure and strength of fresh and tempered martensite in a maraging stainless steel Fe–15Cr–5Ni

Hierarchical microstructure engineering is an efficient design path for ultra-high strength steels. An excellent example of this is maraging stainless steel, which achieves its high-performance by combining the hierarchic martensitic microstructure and nano-sized precipitates. Relating this complex microstructure with mechanical properties, e.g. strength, is not trivial. In the present work, we therefore explore the relationship between the hierarchic microstructure, evolving with the tempering of a Cu-containing maraging stainless steel 15–5 PH, and its strength. Comprehensive microstructure characterization, including the quantification of dislocation density, effective grain size, precipitates and retained austenite fraction is performed after quenching and tempering at 500 °C. The microstructure data is subsequently used as input for assessing the evolution of individual strength contributions and thus the increase in strength of tempered martensite contributed by Cu precipitation strengthening is evaluated. It is found that the Cu precipitation and dislocation annihilation are two major factors controlling the evolution of the yield strength of the tempered martensite. The Cu precipitation strengthening is also modelled using our previous Langer-Schwartz-Kampmann-Wagner model based predictions of the Cu precipitation, and modelled precipitation strengthening is compared with the evaluated Cu precipitation strengthening from the experiments. The work exemplifies the promising approach of combining physically based precipitation modelling and precipitation-strengthening modelling for alloy design and optimization. However, more work is needed to develop a generic predictive framework.

Effects of reference materials on texture coefficients determined for a CVD α-Al2O3 coating

α-Al2O3 coatings deposited onto cemented carbide tools by chemical vapor deposition (CVD), are widely employed in cutting applications due to their excellent wear resistance, thermal stability and chemical inertness. Technological advancements have allowed the control of crystallographic texture, enabling the exploration of anisotropic wear resistance of α-Al2O3. Within this study, a comprehensive microstructure characterization of an α-Al2O3 coating is presented, with the goal to provide useful guidelines for the determination of its crystallographic texture using texture coefficients (TCs). Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) pole figures were firstly used to determine the (001) fiber texture of the representative coating. TCs were then calculated, utilizing XRD intensities from three different reference materials (i.e. an α-Al2O3 coating powder, NIST SRM 676a and NIST SRM 1976b), for the same as-deposited coating. Based on the results obtained, a set of guidelines is advised for the determination of texture coefficients: (a) X-ray intensity acquisition only for symmetric θ/2θ scans, (b) removal of X-ray background intensity, (c) use of reflections that have a high peak to background X-ray intensity ratio, (d) application of peak fitting functions when overlaps occur, (e) exclusion of second order reflections at the presence of first order reflections, (f) texture coefficients provided should be accompanied by the respective coating thickness, and (g) use of an appropriate powder standard reference material.

An investigation of oxidation/hot corrosion-creep interaction at 800 °C in a Ni-base superalloy coated with salt mixture deposits of Na2SO4-NaCl-NaVO3

Creep-rupture behaviour of Nimonic263 alloy coated with 87.5 wt.%Na2SO4+5 wt.%NaCl +7.5 wt.%NaVO3(3SM) and 74 wt.%Na2SO4+21.7 wt.%NaCl+4.3 wt.%NaVO3(3SM-A) was investigated in the stress range of 180–350 MPa at 800 °C. The 3SMs are responsible for the reduction in creep-rupture life. Hot corrosion products found in the crack regions of the crept specimens deformed under the influence of 3SMs. Sulphidation followed by oxidation was dominant at 250 MPa, whereas oxidation was dominant at 215 and 180 MPa. Comprehensive microstructural characterization and analysis suggest that three-salt mixtures(3SMs) accelerate oxygen and sulphur penetration along the grain boundaries during creep, and enhance the stress assisted grain boundary oxidation resulting in a reduction in creep-rupture life.

Radiation tolerance of commercial and advanced alloys for core internals: a comprehensive microstructural characterization

Thirteen austenitic stainless steels, nickel-base alloys, and ferritic alloys were irradiated using 2 MeV protons at 360 °C to a damage level of 2.5 displacements per atom (dpa). Comprehensive microstructural characterization was performed for irradiation-induced features, including dislocation loops, voids, precipitates, and radiation induced segregation (RIS). Dislocation loops formed in all alloys except 14YWT, while voids were observed in alloys 316 L, 310, C22, and 14YWT. Irradiation-induced formation of γ′ precipitates was observed in alloys 316 L, 310, 800, and 690; the irradiation-enhanced, long-range ordered Ni2Cr phase (Pt2Mo-type) was observed in alloys 690, C22, 625, 625Plus, 625DA, and 725; and G-phase was observed in alloy T92. No irradiation-induced precipitates were observed in alloys X750, 718 or 14YWT. Precipitation of the γ′ phase can be understood through segregation and clustering of Si, Al, and Ti. Overall, austenitic stainless steels are generally susceptible to irradiation damage in the form of loops, voids, precipitates, and RIS. Ni-base alloys have this same type of dislocation loops and RIS behaviors but are more resistant to void swelling. Ferritic alloys showed better resistance to loop formation, void swelling and irradiation-induced precipitation. From the degree of irradiation-induced microstructural change, alloy T92 was identified as the most radiation resistant among these alloys.

Bioactive Zn-doped hydroxyapatite coatings and their antibacterial efficacy against Escherichia coli and Staphylococcus aureus

Zinc-doped hydroxyapatite (HA + Zn) has aroused increasing interest in bone contact applications, for its capacity to modulate osteoblast activity and for the antibacterial properties of zinc ions. In this study, HA + Zn coatings deposited by the Solution Precursor Plasma Spraying (SPPS) process were characterized. This method employs solution feedstocks providing simple and effective ion doping and resulting in the deposition of lamellae of smaller size than the conventional Atmospheric Plasma Spraying method. After a comprehensive microstructural characterization of the coatings, their bioactivity, cytotoxicity, and antibacterial efficacy were investigated. The coatings' bioactivity was confirmed by soaking them in Simulated Body Fluid (SBF), a commonly-used protocol to evaluate samples' bioactivity in vitro. Further, the coatings were non-cytotoxic against human osteoblast Saos-2-cells and, additionally, they showed antibacterial effects against Escherichia coli and Staphylococcus aureus. The HA + Zn coatings obtained showed higher efficacy against the gram-positive S. aureus than against the gram-negative Escherichia coli. These findings demonstrate the potential of the SPPS method for fabricating Zn-doped hydroxyapatite coatings, and show that Zn ions impart antibacterial properties against E. coli and S. aureus bacteria.

Asymmetric Cracking in Mar-M247 Alloy Builds During Electron Beam Powder Bed Fusion Additive Manufacturing

In the electron beam powder bed fusion (EB-PBF) process, a substantial number of high-gamma prime Ni-based superalloys are considered as non-printable due to a high propensity to form cracks. In this research, we focused on computational modeling framework to predict solidification-related cracking phenomena in EB-PBF processes. The cracking analysis was performed on cylindrical overhang structures where the cracks are observed only on one side of the part. Comprehensive microstructural characterization correlated the cracking tendency to low-melting point liquid-film formation along columnar grain boundaries with high misorientation angles due to partitioning of alloying elements. Uncoupled numerical thermal and mechanical models were used to rationalize the relationship between process parameters, build geometry, and cracking. The simulations showed asymmetric temperature distributions and associated asymmetric tensile thermal stresses over a cross section due to differences in section modulus and periodic changes in beam scanning directions. The results provide a potential pathway based on spatially varying beam scanning strategies to reduce the cracking tendency during additive manufacturing of complex geometries on the overhang structure in high-gamma prime nickel-based superalloys.

Visible light absorption of surface-modified Al2O3 powders: A comparative DFT and experimental study

Surface modification of Al2O3 powders, prepared using reproducible sol-gel synthetic route with small colorless organic molecules, induces charge transfer complex formation and the appearance of absorption in the visible spectral region. Comprehensive microstructural characterization involving transmission electron microscopy, X-ray diffraction analysis, and nitrogen adsorption–desorption isotherms, revealed that γ-crystalline alumina powders consist of mesoporous particles in the size range from 0.1 to 0.3 μm, with specific surface area of 54.8 m2/g, and pore radius between 3 and 4 nm. The attachment of catecholate-type of ligands (catechol, caffeic acid, gallic acid, dopamine and 2,3-dihydroxy naphthalene), salicylate-type of ligands (salicylic acid and 5-amino salicylic acid), and ascorbic acid, to the surface such γ-Al2O3 particles leads to the formation of colored powders and activates their absorption in visible-light spectral region. To the best of our knowledge, similar transformation of an insulator (Al2O3), with the band gap energy of 8.7 eV, into a semiconductor-like hybrid material with tunable optical properties has not been reported in the literature before. The density functional theory (DFT) calculations with periodic boundary conditions were performed in order to estimate the energy gaps of various inorganic/organic hybrids. The calculated values compare well with the experimental data. The good agreement between the calculated and experimentally determined band gaps was found, thus demonstrating predictive ability of the theory when proper model is used.

475°C Embrittlement of Duplex Stainless Steel—A Comprehensive Microstructure Characterization Study

Abstract The effect of 475°C embrittlement on microstructure development of grade 2205 duplex stainless steel was investigated. Spinodal decomposition products and associated precipitates in ferrite, austenite, and at interphase boundaries were characterized using analytical transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques. Micro-analyses confirmed the presence of Cr-enriched α'' and Cr-depleted α' spinodal structures in the ferrite after 5 h of aging at 475°C. Long-term aging for 255 h resulted in heavily-faulted R-phase precipitates with sizes of ∼50–400 nm, χ-phase, and ε-Cu in the ferrite, TiN and Cr2N precipitates in the austenite, and a continuous network of M23C6-carbides at interphase boundaries. A significant hardness increase was observed after 255 h of aging, which was accompanied by a reduction of ferrite fraction. X-ray diffraction (XRD) stress measurements showed a general reduction of residual stresses in both ferrite and austenite with aging. Electron backscatter diffraction (EBSD) showed increased local misorientations, primarily close to precipitate interfaces within the ferrite, indicating the development of strain heterogeneities in the microstructure. The data presented provided a better understanding of 475°C embrittlement in duplex stainless steel, suggesting that not only the ferrite alone is responsible for embrittlement. A comprehensive microstructure characterization study has been provided and the explanation for 475°C embrittlement of duplex stainless steel has been discussed.

Microstructural characteristics of the nickel-based alloy IN738LC and the cobalt-based alloy Mar-M509 produced by selective laser melting

This study investigates selective laser melting (SLM) of the nickel based superalloy IN738LC and the cobalt based alloy Mar-M509, and identifies the influence of process and material parameters on the resulting microstructure. Comprehensive microstructural characterization was performed using electron backscattered diffraction analysis. Significant differences between IN738LC and Mar-M509 were observed with respect to grain size, grain shape and texture sharpness. Alloy IN738LC exhibits coarse and elongated grains with a sharp texture and thus a pronounced mechanical anisotropy. Alloy Mar-M509 shows smaller grains with only moderate structural and mechanical anisotropy. The different microstructural and mechanical characteristics are attributed to the different recovery and recrystallization behavior of IN738LC and Mar-M509. The high stacking fault energy (SFE) of IN738LC results in pronounced recovery of lattice defects without affecting the basic grain structure, whereas the low SFE in Mar-M509 favors recrystallization with the effect of significant grain refinement and weakening of the solidification texture. The effect of microstructure and the structural anisotropy on the orientation-dependent values of the Young’s modulus and the mechanical properties are further discussed.

Solution‐Processed Ultrathin Aluminosilicate Nanotube–Poly(vinyl alcohol) Composite Membranes with Partial Alignment of Nanotubes

AbstractThe fabrication of ultrathin (300 nm) aluminosilicate nanotube–poly(vinyl alcohol) composite membranes with partial vertical alignment of the nanotubes (up to 33 %), by solution‐casting methods on porous polymeric substrates, is reported. A high loading (up to 60 vol %) of nanotubes is achieved. A comprehensive microstructural characterization of the membranes is performed by a combination of SEM, TEM, grazing‐incidence wide‐angle X‐ray scattering measurements, and simulations. This investigation shows that the nanotubes are individually dispersed and partially aligned in the polymeric matrix by the use of appropriate matrix and substrate materials. Permeation measurements of gas probe molecules (CO2 and CH4) on two different types of membranes, one containing bare nanotubes and the other containing amine‐functionalized nanotubes, also support the proposed microstructure of the thin nanocomposite membranes.

Switch of p–n electricity of reduced-graphene-oxide-flake stacked films enabling room-temperature gas sensing from ultrasensitive to insensitive

The p–n–p double transformation and ultrasensitive–insensitive gas sensing of reduced-graphene-oxide-flake stacked films (rGOFSFs) were controlled by changing the annealing temperature from 100 to 400°C on a sodium silicate substrate, as a result of the competition between the thermal reduction, decomposition, and diffusion of Na, as determined from comprehensive microstructure characterization. The slightly reduced p-type GOFSF showed ultrasensitive gas sensing at room temperature (RT), with a response of 58% to 1ppm ethanol and a high detection limit (sub-ppm). Interestingly, the rGOFSF can become n-type and insensitive to gas sensing, with a low response of −0.5% to 50ppm ethanol, by simply increasing the annealing temperature to 200–300°C. The capability of controlling carrier types and ultrasensitivity–insensitivity transition to gas is useful for developing nanodiodes and RT gas sensors.