Thomas J. Headley

INCONEL 718: A solidification diagram

As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to generate a constitution diagram for INCONEL 718* in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made using the Scheil solidification model, and the fraction of each phase in the eutectic is calculatedvia the lever rule. These are compared with experimentally determined values and found to be in good agreement.

Structural and compositional transformations of biomass chars during combustion

Abstract In an investigation of the physical and chemical transformations of biomass chars during combustion, we have subjected two chars, produced from the pyrolysis of pine and switchgrass, to combustion at 1600 K in a laminar flow reactor. In order to obtain time-resolved data on the structural and compositional transformations of the biomass chars. samples are extracted from the reactor at different residence times and subjected to a variety of analytical techniques: elemental analysis, scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction analysis, and high-resolution transmission electron microscopy. The results point to several changes in both the organic and inorganic constituents of the chars. The early stages of conversion are characterized by devolatilization, which leads to the removal of amorphous material and the release of oxygen- and hydrogen-rich gases. After devolatilization, combustion is accompanied by: vaporization of some metals (particularly Na and K), surface migration and coalescence of inorganic material, and the incorporation of metals (particularly Ca) into silicate structures. The latest stages of combustion reveal the transformation of inorganic constituents from amorphous phases to crystalline forms. Some short-range order appears in the carbon-rich portions of the chars as combustion proceeds, but the high levels of oxygen originally present in these chars foster cross-linking, which limits the extent of order ultimately attained. The transformations of the biomass chars are compared with those of coal chars, and the implications of these observations—with respect to reactivity and ash behavior—are discussed.

The welding metallurgy of HASTELLOY alloys C-4, C-22, and C-276

The welding metallurgy (solidification and solid state transformations) of HASTELLOY* Alloys C-4, C-22, and C-276 has been determined. Varestraint hot-cracking tests performed on commercial alloys revealed a weldability ranking as follows: C-4 > C-22 > C-276. All alloys would be expected to have good weldability, with Alloy C-4 having a very low hot-cracking tendency, comparable to 304L stainless steel. Microstructures of gas-tungsten-arc welds of these alloys have been characterized by scanning electron microscopy and analytical electron microscopy. Intermetallic secondary solidification constituents have been found associated with weld metal hot cracks in Alloys C-276 and C-22. In Alloy C-276, this constituent is a combination ofP and ώ phases, and in Alloy C-22, this constituent is composed of σ,P, and ώ phases. With phase composition data obtained by AEM techniques and available ternary (Ni-Cr-Mo) phase diagrams, an equivalent chemistry model is proposed to account for the microstructures observed in each alloys weld metal.

Evolution of char chemistry, crystallinity, and ultrafine structure during pulverized-coal combustion☆☆☆

Abstract The carbonaceous structure of partially reacted char samples, generated by direct injection of pulverized coal into a laboratory entrained flow reactor, was characterized by four techniques: elemental analysis, carbon dioxide vapor adsorption, x-ray diffraction, and fringe-imaging using high-resolution transmission electron microscopy. It is observed that the early stages of heterogeneous oxidation proceed in parallel with the latter stages of carbonization, leading to preferential loss of hydrogen, a reduction in surface area, and the development of crystalline order. Typical combustion times and peak temperatures are insufficient to bring about true (three dimensional) graphitization for most coals, but rather, lead to the growth of regions of turbostratic order. This ordering is seen to occur over a time scale comparable to the combustion process itself—here, on the order of 100 ms at particle temperatures of 1800 K and oxygen concentrations of 12 mol%. This work presents evidence that the reactivity of chars in the latter stages of burnout, which is critically important to the explanation and prediction of unburned carbon in flyash, is significantly impacted by the evolution of the carbonaceous matrix. Although significant evolution of internal surface area and hydrogen content (indicative of aromatic ring coalescence) occurs during early char combustion, these two phenomena do not play a major role in the char deactivation noted in previous investigations. Among the four indicators of carbon structure evaluated herein ( H C ratio, carbon dioxide surface area, crystallite dimensions by x-ray diffraction, and HRTEM images), the volume fraction of ordered material as determined by HRTEM fringe-imaging correlates best with the observed reactivity loss for Illinois #6 coal chars.

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M6C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M6C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(mL, mS) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.

Growth and morphology of cadmium chalcogenides: the synthesis of nanorods, tetrapods, and spheres from CdO and Cd(O2CCH3)2

In this work, we investigated the controlled growth of nanocrystalline CdE (E = S, Se, and Te) via the pyrolysis of CdO and Cd(O2CCH3)2 precursors, at the specific Cd to E mole ratio of 0.67 to 1. The experimental results reveal that while the growth of CdS produces only a spherical morphology, CdSe and CdTe exhibit rod-like and tetrapod-like morphologies of temporally controllable aspect ratios. Over a 7200 s time period, CdS spheres grew from 4 nm (15 s aliquot) to 5 nm, CdSe nanorods grew from dimensions of 10.8 × 3.6 nm (15 s aliquot) to 25.7 × 11.2 nm, and CdTe tetrapods with arms 15 × 3.5 nm (15 s aliquot) grew into a polydisperse mixture of spheres, rods, and tetrapods on the order of 20 to 80 nm. Interestingly, long tracks of self-assembled CdSe nanorods (3.5 × 24 nm) of over one micron in length were observed. The temporal growth for each nanocrystalline material was monitored by UV-VIS spectroscopy, transmission electron spectroscopy, and further characterized by powder X-ray diffraction. This study has elucidated the vastly different morphologies available for CdS, CdSe, and CdTe during the first 7200 s after injection of the desired chalcogenide.

Effect of B‐site cation stoichiometry on electrical fatigue of RuO2//Pb(ZrxTi1−x)O3//RuO2 capacitors

There have been numerous reports that Pb(ZrxTi1−x)O3 (PZT) thin‐film capacitors with RuO2 electrodes and compositions near the morphotropic phase boundary exhibit minimal decrease in switched polarization with electric‐field cycling. We show that the fatigue performance of RuO2//PZT//RuO2 capacitors strongly depends on PZT film composition. Specifically, we demonstrate that the rate of polarization fatigue increases with increasing Ti content for PZT thin films of tetragonal crystal symmetry deposited on RuO2 electrodes. As the Ti content of the PZT films increased, the film gain morphology changed from columnar to granular and the volume percent of a fluorite‐type second phase decreased. These microstructural trends and the possibility that the electrode material acts as a sink for oxygen vacancies are discussed to explain the fatigue dependence on B‐site cation ratio for PZT films with RuO2 electrodes.

Alpha-recoil damage in natural zirconolite (CaZrTi2O7)☆

Radiation effects in natural zirconolites which have received alpha doses from 4.4 × 1024 alphas/m3 to greater than 1026 alphas/m3 have been studied by transmission electron microscopy. The monoclinic zirconolite crystal structure is retained up to doses of 4.4 × 1024 alphas/m3. At intermediate doses (3 × 1025 alphas/m3), the material consists of a mixture of amorphous and crystalline regions, the latter having an average fcc fluorite structure. At doses greater than 1026 alphas/m3 the zirconolite is highly disordered and electron diffraction-amorphous. There is no evidence for remnant long range periodicity, and the broad electron diffraction halos are consistent with scattering from a nonperiodic atomic arrangement. Evidence is presented indicating that the crystallinity reported by others [2] in these high dose zirconolites may be due to alteration, the presence of impurities, or electron beam-heating. All samples with doses greater than 4.4 × 1024 alphas/m3 displayed porosity consisting of near-spherical micro-voids. This porosity may be caused by He-bubble formation during U and Th decay.

Residual carbon from pulverized-coal-fired boilers. 2. Morphology and physicochemical properties

The morphology and bulk physicochemical properties of residual carbon in eight fly ash samples from commercial power plants were investigated. Enriched carbon samples extracted from the bulk fly ash were characterized by high-depth-of-field optical microscopy, reflected-light microscopy, scanning electron microscopy, elemental analysis (C, H, O), and CO2 adsorption. The crystalline structure of the carbon was characterized by X-ray diffraction, optical reflectance, and high-resolution transmission electron microscopy fringe imaging. The results were compared with measurements on laboratory-generated chars in the early-to-intermediate stages of combustion. Compared with those chars, the residual carbon is of similar elemental composition, petrographic composition and surface area but higher crystallinity. The fuel-related mechanisms that can contribute to carbon carryover in boilers are discussed, including inertinite persistence, mineral matter encapsulation and char deactivation by pregraphitization, as well as the implications for utilization of residual carbon.

A comparison of the solidification behavior of INCOLOY 909 and INCONEL 718

The solidification behavior of two commercial aerospace superalloys, INCOLOY 909 and INCONEL 718, has been examined. Specifically, differential thermal analysis (DTA) revealed that INCOLOY 909 terminates solidification with the formation of a single minor constituent at ≈1198 °C. INCONEL 718 terminates solidification with the formation of two minor constituents, at ≈1257 °C and ~1185 °C, respectively. Metallography performed on the DTA samples confirmed that a single minor constituent was present in INCOLOY 909 while two minor constituents were present in INCONEL 718. Differential thermal analysis samples were also examined by electron probe microanalysis to reveal the patterns of elemental segregation. Arc welds of these alloys were examined by transmission and analytical electron microscopy (TEM and AEM). It was observed that the arc welds of INCOLOY 909 contained only a y/Laves eutectic-like constituent, while the arc welds of INCONEL 718 contained both y/Laves and γ/MC eutectic-like constituents. Compositional analyses of these minor phases revealed that all were enriched in Nb relative to the bulk alloy. The Laves phases were also enriched in Si relative to the bulk alloy concentration. Comparisons of the observed solidification sequences in these alloys with other Nb-bearing austenitic matrix alloys are made.