Optimizing the Implementation of High-Temperature Fixed Points through the Use of Thermal Modeling

High-temperature fixed points (HTFP) have the potential to make a step-change improvement in high-temperature metrology, significantly reducing the uncertainty of scale realization of the current ITS-90 and improving dissemination of high-temperature scales to industry. However, in a practical implementation, the performance of HTFP could be limited, by, for example, injudicious use of insulation in the vicinity of the fixed point, furnace gradients, or incomplete filling. This article investigates some of these aspects for a selection of HTFP. Steady-state modeling of the influence of insulation on the radiance temperature was performed for Co–C (1,324°C), Pd–C (1,492°C), Pt–C (1,738°C), Ru–C (1,953°C), and Re–C (2,474°C) fixed points. This included studying mitigation scenarios through the insertion of different types and designs of insulation. The optimum design was identified to minimize the temperature drop in a particular furnace. It was found that, for the furnace and fixed-point combination modeled, the actual effect of the insulation was almost insignificant. Transient modeling was performed for a Re–C fixed point, to track the evolution of the radiance temperature through the melting transition. The starting point of the model was the beginning of the melt. The evolution of radiance temperature with time in “perfectly” filled cells was modeled with a range of linear temperature gradients across the eutectic cell. The gradient had a significant effect on the duration of the transition and on the structure of the melt itself. Despite the model’s simplicity, it qualitatively demonstrated that the melt transition temperature, as identified by the point of inflection, could be significantly affected by the presence of furnace gradients.     

Microstructural analysis of fusion and heat affected zones in electron beam welded ALLVAC 718PLUS™ superalloy

The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed.     

Creep behavior and related structural defects in Al2O3-Ln2O3 (ZrO2) directionally solidified eutectics (Ln = Gd, Er, Y)

The eutectic architecture of “in situ” composites prepared by solidification from the melt in the Al₂O₃-Ln₂O₃ (ZrO₂) systems gives rise to materials with a high creep resistance. With the objective to elucidate the high temperature deformation micro-mechanisms, microstructural features are investigated on crept specimens. Compressive creep experiments have been carried out between 1400 and 1550 °C for various eutectic compositions. Different deformation regimes depending on considered systems and conditions of stress and temperature are revealed. Transmission electron microscopy studies emphasize the activation of different slip systems in the alumina phase and the deformation by dislocation climb processes controlled by bulk diffusion.     

Investigation of the Behavior of the Co–C Eutectic Fixed Point

The behavior of the Co–C eutectic fixed point was investigated at INRIM. Several cells of different design and volume, and filled with cobalt of different purity were constructed and investigated with both Pt/Pd thermocouples and radiation thermometers. The melting behavior was investigated with respect to the melting rate, the pre-freezing rate, and the annealing time. The melting temperatures, as defined, were not significantly affected by the different testing conditions, even if the shape and duration of the plateaux were influenced. Several tens of melt and freeze cycles were performed with the different cells. The spread in the results for all of the different conditions was very limited in extent, giving rise to a standard deviation of less than 0.04 °C; a repeatability of better than 0.02 °C was found with both Pt/Pd thermocouples and radiation thermometers. The results of our measurements are encouraging and confirm the suitability of Co–C as a reference point for the high-temperature range in a possible future temperature scale. Investigations of long-term stability remain ongoing.     

电弧熔炼态NiAl-39V共晶合金的组织及力学性能

采用真空非自耗电弧熔炼制备了Ni-30.5A1-39V (at%)合金,利用光学显微镜(OM),X射线衍射(XRD),扫描电镜(SEM)分析了合金不同凝固位置处的相组成和组织形态,结果表明:Ni-30.5Al-39V合金的凝固组织由NiAl+V片层共晶组成。通过断裂韧性和高温压缩的测试表明:NiAl-39V合金的断裂韧性是NiAl合金的3倍,裂纹偏转和裂纹键合是其主要韧化机理。高温强度提升约2倍左右,复相强化和固溶强化是高温强度提升的主要原因。最后,基于Arrhenius模型构建NiAl-39V合金本构关系为ε=5.398[sinh(0.037128σ)]~(2.3)exp(-109.95×10~3/RT),其激活能为109.95 kJ·mol~(-1)。     

Pattern formation in solidification

Regular patterns form in many solidification processes. Examples occur during lamella and rod-like eutectic growth and when single-phase cells or dendrites are formed. The scale and regularity of the microstructure can determine the properties of the cast materials and is thus important practically. The purpose of the present paper is to show that common features occur in all processes.

Steady-state analysis indicates that a wide range of possible spacings could occur during eutectic, cellular or dendritic growth. The degree of freedom is removed by considering the mechanism determining the minimum and maximum spacing on a specimen. It is found that the minimum spacing occurs when the array first becomes stable for a lamella or rod-like eutectic, for cell growth and for some dendrites. For low temperature gradient, high-velocity dendrites, the minimum spacing is determined by thespacing when the dendrites irst become near enough to interact. The maximum spacing for eutectics andfor cells is determined by tip splitting. The maximum spacing for dendrites occurs when a tertiary arm becomes a new primary. Good agreement is obtained between theory and experiment using this approachto predict spacing limits. The average spacing on a specimen can approach either limit depending o past history. The two extreme spacings are found to span the spacing of the minimum undercooling foreutectic and cellular growth and this allows an average spacing to be estimated using a single condition.

It is concluded that three conditions are necessary to form regular structures. A mechanism must exist to eliminate members of the array when the spacing is too small. A mechanism must exist to form new members of the array when the spacing is too wide. The structure must be stable to fluctuations in the range of spacing between the two limits.     

New polaritonic materials in the THz range made of directionally solidified halide eutectics

Directionally solidified alkali halide binary eutectics have been recently proposed as THz polaritonic metamaterials based on their ordered microstructure and the suitable phonon–polariton resonances in the THz range of the spectrum. In the present work we focus on the search of new available eutectic systems both binary and ternary eutectics with well-ordered fibrous or lamellar microstructures and interparticle distances from 1 to several tens of microns. Simple effective homogenization models have been used to calculate effective permittivity and transmittance in the THz range of eutectic slices. This lets us identify the electromagnetic spectral ranges where hyperbolic dispersion is expected together with a significant transmittance value. The hyperbolic dispersion range shifts with microstructure size, that is, with growth parameters, showing that the materials response can be finely tuned by the manufacture conditions. Applications with these materials cover the electromagnetic range from 5 to 20 THz.     

A Nickel–Carbon Eutectic Cell for Contact and Non-contact Thermometry

The highest-temperature, defining fixed point of the International Temperature Scale of 1990 (ITS-90) is the copper freezing point (1,084.62°C). Many international metrology institutes are investigating the use of transition temperatures of metal–carbon alloys as references for the calibration of temperature measuring instruments above the copper point, making it possible to reduce the calibration uncertainty of pyrometers in radiation thermometry and thermocouples in contact thermometry. This research is being performed mainly by radiation thermometry laboratories that have developed specific cells with blackbody cavities containing relatively small quantities of metal–carbon alloys. Parallel to this, some laboratories have also developed cells with these same alloys, but of a different design, suitable for the calibration of thermocouples. This report concerns the development of a nickel–carbon eutectic cell (≅1,329°C) at Inmetro, with which either a radiation thermometer or thermocouple can be calibrated. The measurements of the temperature of this cell were performed using the reference radiation thermometer of the Pyrometry Laboratory and Pt/Pd thermocouples that were constructed, stabilized, and calibrated at the Thermometry Laboratory. Details of the cell fabrication, as well as the instrumentation used for the measurements are given. The results of a comparison between the two different types of measurement are reported, including the uncertainty budgets of both methods.     

Orientation relationships of unidirectionally aligned GdAlO3/Al2O3 eutectic fibers

Unidirectionally solidified rare-earth activated GdAlO₃(GAP)/Al₂O₃ eutectic crystal with well-aligned fibrous structure exhibits excellent light guiding property and can be used as a scintillator plate for high-resolution X-ray imaging. In this paper, the microstructures and orientation relationships of the GAP/Al₂O₃ eutectic fibers were investigated. The regular GAP single crystal fibers with a hexagonally close-packed arrangement grew straight in the same direction along the solidification direction, and were embedded in a c-axis oriented Al₂O₃ single crystal matrix. The majority of GAP fibers had the orientation relationships of [0 1 0]GAP//[0 0 0 1]Al₂O₃ to the growth direction and $(100)\text{GAP}//(11\overline{2}0){\text{Al}}_2{\text{O}}_3$ to the interface plane, while slight misorientation angle of both [0 1 0]GAP axis and (1 0 0)GAP plane were observed. In the GAP/Al₂O₃ interface boundary, the lattice misfit between the two phases was relieved by insertion of extra half-planes on the Al₂O₃ side.     

Testing molten metal oxide catalysts over structured ceramic substrates for diesel soot oxidation

Onboard tests of an oxide (CoO_x) and a molten mixed oxide catalyst (CoPbO_x) supported on cordierite wall flow filters (Corning Durotrap CO EX80) were performed on rollers with a 1.9-l light duty vehicle for three different driving conditions including two constant speeds with different loadings and a standard European cycle (NEDC). The balance point temperatures obtained in these tests were used to compare the catalytic activity under onboard conditions with laboratory measurements. Onboard tests showed similar catalytic activities with the previous micro-reactor experiments performed in “loose contact” mode [D. Uner, M.K. Demirkol, B. Dernaika, Appl. Catal. B: Environ., in press]. The concentration profiles of the active layer determined by SEM-EDX analysis after use and after aging revealed that the mobile component, PbO_x, migrated in the flow direction and accumulated at the closed end of the channel. There is no direct evidence for the evaporative loss of PbO_x. Regions of high activity indicated by low carbon amounts with unique Pb–Co ratios were determined by SEM analysis.