Comprehensive Microstructural Characterization Research Articles

Effect of microstructural heterogeneities on scatter of toughness in multi-pass weld metal of C–Mn steels

Often, scatter in mechanical properties of multi-pass steel weldments is qualitatively attributed to the underlying heterogeneous microstructure brought about by spatial variations of multiple thermal cycles. In this research, a method for quantitative heterogeneity calculation based on measured variations of microstructure and hardness throughout the multi-pass weld metals including various reheated regions was explored. Published data from multi-pass welds with controlled titanium additions (7–32 wt ppm) were correlated to comprehensive microstructure characterization. The scatter in 7 wt ppm Ti steel welds was larger than that of 32 wt ppm Ti steel welds. This change in scatter is correlated to spatial distributions of microstructural heterogeneity parameter, rather than the average value of heterogeneity parameter for the whole weldment.

Iridium oxides based gate interface of AlGaN/GaN high electron mobility transistors formed by high temperature oxidation

We report on a high temperature forming of iridium oxides (IrO2) gates of circular AlGaN/GaN high electron mobility transistors (C-HEMTs) to be predetermined for high temperature applications. IrO2 gate interfacial layer is formed by high temperature oxidation (T=500–800°C, for 1min) of 15nm thick Ir gate contact layer. A comprehensive microstructural and electrical characterization of the IrO2 gates is carried out to explain the improved transport properties and thermal stability of the gate interfaces. It is found that the transformation of Ir gate layer into its crystalline IrO2 phase at oxidation temperatures of 700°C and 800°C provides the high barrier gate interface with prevailing thermionic emission transport mechanism. Accelerated reliability tests are used to confirm C-HEMT thermally stable performance deduced from both the gate interface and 2DEG channel stability. The introduced AlGaN/GaN C-HEMTs with high temperature grown IrO2 gates seem to be very attractive for both the high temperature operated electronic and sensor devices.

High Temperature Thermal Gradient Interaction Chromatography (HT‐TGIC) for Microstructure Analysis of Polyolefins

AbstractSummary: High temperature thermal gradient interaction chromatography (HT‐TGIC) is a newly developed technique to analyze comonomer distributions in polyolefins. This paper documents the key differences between crystallization elution fractionation (CEF) and HT‐TGIC, and the advantages of using multiple detectors in HT‐TGIC to provide comprehensive microstructure characterization. A demonstration of the technique using a specifically designed blend, provides comprehensive data in less than 1.5 hours of analysis time by HT‐TGIC. This paper also reports that HT‐TGIC has excellent short term repeatability.

Microstructure and mechanical properties of the superalloy ATI Allvac ® 718Plus™

ATI Allvac ® 718Plus™ is a novel nickel-based superalloy, which was designed for heavy-duty applications in aerospace turbines. In the present study the high-resolution investigation techniques, atom probe tomography, electron microscopy and in situ high-temperature small-angle neutron scattering were used for a comprehensive microstructural characterization. The alloy contains nanometer-sized spherical γ′ phase precipitates (Ni 3(Al,Ti)) and plate-shaped δ phase precipitates (Ni 3Nb) of micrometer size. The precipitation kinetics of the γ′ phase can be described by a classical model for coarsening. The precipitation strongly influences the mechanical properties and is of high scientific and technological interest.

Fabrication and characterization of amorphous silica nanostructures

Large-scale amorphous silica nanostructures, including nanowires, nanotubes and flowerlike nanowire bunches depending on the position, have been fabricated on silicon wafer through a cheap route under the assistance of gold and germanium. Accompanying the observation of blue–green light emission, comprehensive micro-structural characterization reveals that the growth of nanostructures is catalyzed only by gold whereas the final morphology of nanostructures depends on the location to germanium ball. Au 2Si, a compound of gold and silicon, is also disclosed as an intermediate state during the catalysis. Correspondingly, a growth scheme is proposed based on the experimental results and the vapor-liquid-solid mechanism.

Inertia Friction Welding Dissimilar Nickel-Based Superalloys Alloy 720Li to IN718

This article describes a comprehensive microstructural characterization of an inertia friction welded joint between nickel-based superalloys 720Li and IN718. The investigation has been carried out on both as-welded and postweld heat-treated conditions. The detailed metallographic analysis has enabled the relation of hardness profiles across inertia-welded alloy 720Li to IN718 and morphological changes of the precipitates present. The work demonstrates that inertia friction welding (IFW) 720Li to IN718 results in a weld free of micropores and microcracks and no significant chemical migration across the weld line. However, substantial differences in terms of grain structure and precipitation phase distribution variations are observed on each side of the dissimilar weld. The high γ′ volume fraction alloy 720Li exhibits a wider heat-affected zone than the mainly γ′′ strengthened IN718. Alloy 720Li displays only a small hardness trough near the weld line in the as-welded condition due to the depletion of γ′, while γ″-strengthened IN718 shows a soft precipitation-free weld region. Postweld heat treatment (PWHT) of the dissimilar weld at 760 °C, a typical annealing temperature for alloy 720Li, results in an overmatch of the heat-affected zone in both sides of the weld. The comparison of the as-welded and postweld heat-treated condition also reveals that IN718 is in an overaged condition after the stress relief treatment.

Depth‐Resolved Porosity Investigation of EB‐PVD Thermal Barrier Coatings Using High‐Energy X‐rays

Demands for designing prime reliant, energy‐efficient, and high‐performance thermal barrier coatings (TBCs) in gas turbines have led to a growing interest toward comprehensive microstructural characterization. Here we investigate the novel use of high‐energy X‐rays for small‐angle X‐ray scattering (SAXS), together with wide‐angle scattering and radiography, for the depth‐resolved characterization of TBCs grown by electron beam physical vapor deposition (EB‐PVD). The coating microstructure is found to consist of columns perpendicular to the substrate, extending through the thickness, with a [001] growth texture and significant intercolumnar porosity. In addition, overshadowing effects during deposition together with gas entrapment give rise to nanoscale intracolumnar porosity consisting of featherlike and globular pores. Radiography showed an increase in the total porosity, from 15% near the substrate to 25% near the coating surface, which is ascribed to an increase in the intercolumnar spacing at the top of the coating. By contrast, the small‐angle scattering studies, which are sensitive to fine features, showed the pore internal surface area to be greatest near the substrate.

Comprehensive microstructural characterization and predictive property modeling of plasma-sprayed zirconia coatings

Quantitative microstructure characterization to better understand processing-microstructure-property correlations is of considerable interest in plasma sprayed coating research. This paper quantifies, by means of small-angle neutron scattering (SANS) data, microstructure (porosity, opening dimensions, orientation and morphologies) in plasma sprayed partially-stabilized zirconia (PSZ) coatings, primarily used as thermal barrier coatings. We report on the investigation of the influence of feedstock characteristics on microstructure and establish its influence on the resultant thermal and mechanical properties. The microstructural parameters determined by SANS studies are then assembled into a preliminary model to develop a predictive capability for estimating the properties of these coatings. Thermal conductivity and elastic modulus were predicted using finite element analysis and ultimately compared to experimental values.

Comprehensive microstructural characterization in modified 9Cr-1Mo ferritic steel by ultrasonic measurements

Modified 9Cr-1Mo ferritic steel (T91/P91) has been subjected to a series of heat treatments consisting of soaking for 5 minutes at the selected temperatures, starting from the α-phase region (1073 K) to the γ + δ-phase region (1623 K), followed by oil quenching. Hardness measurements, microstructural features, and grain-size measurements by the linear-intercept method have been used for correlating them with the ultrasonic parameters. Ultrasonic velocity and attenuation measurements, and spectral analysis of the first backwall echo have been used for characterization of the microstructures obtained by various heat treatments. As the soaking temperature increased above Ac1, the ultrasonic velocity decreased because of the increase in the volume fraction of martensite in the structure. There were sharp changes in the ultrasonic velocities corresponding to the two critical temperatures, Ac1 and Ac3. Ultrasonic longitudinal- and shear-wave velocities were found to be useful in identifying the Ac1 and Ac3 temperatures and for the determination of hardness in the intercritical region. However, ultrasonic attenuation and spectral analysis of the first backwall echo were found to be useful to characterize the variation in the prior-austenitic grain size and formation of δ ferrite above the Ac4 temperature. The scattering coefficients have been experimentally determined for various microstructures and compared with the theoretically calculated value of the scattering coefficients for iron reported in literature.

Integrated Characterization at a Canadian National Facility

Abstract The pressure of shrinking budgets and downsizing has made it increasingly difficult to meet requests for comprehensive microstructural characterization. In many laboratories, especially those in industry, the expense of maintaining and staffing electron, X-ray and ion beam facilities has resulted in fewer analytical capabilities. At the same time, modern materials and products continue to diversify and increase in complexity, requiring increasingly sophisticated techniques. This growing paradox has increased the need for multiclient, shared-access ‘clusters’ of microanalytical instruments. The above federal laboratory is mandated to serve both the Canadian public (in a broad sense) and Canadian industry (in the specific sense of collaborative and contract research projects). As a result, the Characterization Group has evolved into an integrated lab competency which compliments other competencies in materials processing (full-scale foundry, industrial rolling facility, Gleeble processing simulator) and mechanical behavior (fracture mechanics, stress and deformation modeling). Building on established expertise in metallography, microprobe, X-ray and TEM, the group has doubled in size (to 20 professionals and 10 major capabilities) and now includes SIMS and surface analysis (Table 1).