A model for dislocation creep in polycrystalline Ni-base superalloys at intermediate temperatures

A model for creep at intermediate temperatures in polycrystalline Ni-based superalloys is presented. The model is based on describing stacking fault nucleation, propagation and subsequent shear within the matrix and precipitates. The critical energy for stacking fault nucleation is obtained by minimising the energy to form a stacking fault from dislocation partials, which is promoted by local stress concentrations. The extent of stacking fault shear at a precipitate is estimated using a force balance at the interface to determine the critical shear distance The model results are validated against creep experimental data in several polycrystalline superalloys showing good agreement. Individual contributions to creep from key microstructural features, including grain size and distribution, are studied to identify which ones are more significant. Similarly, it is shown that one of the main factors controlling the creep rate is the stacking fault energy in the as it dictates the stacking fault nucleation and shear rates. Parameter analysis on alloying additions typically used in commercial superalloys demonstrates which elements have the strongest effect on creep, highlighting how the present model can be used as tool for alloy and microstructure design against dislocation creep

A multiscale study on the morphology and evolution of slip bands in a nickel-based superalloy during low cycle fatigue

Plastic deformation during low cycle fatigue in fcc materials with low stacking fault energy is accumulated in slip bands, which become preferential sites for crack initiation. Whilst these dislocation structures have been studied before, little has been done to assess the effect and evolution of the individual slip lines within them. In this study, samples of a γ′precipitate strengthened nickel-based superalloy are fatigued at room temperature and 700˚C for 1, 40 and 500 cycles.The resulting dislocation structures are characterised via Electron Channeling Contrast Imaging and Transmission Electron Microscopy. We introduce a new methodology to measure slip band parameters such as the slip line spacing and shear step length by analysing the holes left by sheared precipitates in γ′-etched secondary electron micrographs. Statistics of these parameters are obtained and compared for different conditions. Advantages of this technique include resolution at the scale of individual planes, acquisition of true three-dimensional data and applicability in the bulk of the material. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.Consejo Nacional de Ciencia y Tecnología Cambridge Commonwealth European and International Trust Roberto Rocca Education Program Royal Academy of Engineering Rolls-Royce plc Engineering and Physical Sciences Research Counci

Ruthenium(II)-bis(4'-(4-ethynylphenyl)-2,2':6', 2''-terpyridine) - A versatile synthon in supramolecular chemistry. Synthesis and characterization

A homoleptic ethynyl-substituted ruthenium(II)-bisterpyridine complex representing a versatile synthon in supramolecular chemistry was synthesized and analyzed by NMR spectroscopy, mass spectrometry and X-ray diffractometry. Furthermore, its photophysical properties were detailed by UV/Vis absorption, emission and resonance Raman spectroscopy. In order to place the results obtained in the context of the vast family of ruthenium coordination compounds, two structurally related complexes were investigated accordingly. These reference compounds bear either no or an increased chromophore in the 4̀-position. The spectroscopic investigations reveal a systematic bathochromic shift of the absorption and emission maximum upon increasing chromophore size. This bathochromic shift of the steady state spectra occurs hand in hand with increasing resonance Raman intensities upon excitation of the metal-to-ligand charge-transfer transition. The latter feature is accompanied by an increased excitation delocalization over the chromophore in the 4̀-position of the terpyridine. Thus, the results presented allow for a detailed investigation of the electronic effects of the ethynyl substituent on the metal-to-ligand charge-transfer states in the synthon for click reactions leading to coordination polymers

Sinking properties of some phytoplankton shapes and the relation of form resistance to morphological diversity of plankton – an experimental study

Form resistance (Phi) is a dimensionless number expressing how much slower or faster a particle of any form sinks in a fluid medium than the sphere of equivalent volume. Form resistance factors of PVC models of phytoplankton sinking in glycerin were measured in a large aquarium (0.6 x 0.6 x 0.95 m). For cylindrical forms, a positive relationship was found between Phi and length/ width ratio. Coiling decreased Phi in filamentous forms. Form resistance of Asterionella colonies increased from single cells up to 6-celled colonies than remained nearly constant. For Fragilaria crotonensis chains, no such upper limit to Phi was observed in chains of up to 20 cells ( longer ones were not measured). The effect of symmetry on Phi was tested in 1 - 6-celled Asterionella colonies, having variable angles between the cells, and in Tetrastrum staurogeniaeforme coenobia, having different spine arrangements. In all cases, symmetric forms had considerably higher form resistance than asymmetric ones. However, for Pediastrum coenobia with symmetric/asymmetric fenestration, no difference was observed with respect to symmetry. Increasing number and length of spines on Tetrastrum coenobia substantially increased Phi. For a series of Staurastrum forms, a significant positive correlation was found between arm-length/cell-width ratio and Phi: protuberances increased form resistance. Flagellates (Rhodomonas, Gymnodinium) had a Phi 1. The highest value ( Phi = 8.1) was established for a 20-celled Fragilaria crotonensis chain. Possible origin of the so-called 'vital component' ( a factor that shows how much slower viable populations sink than morphologically similar senescent or dead ones) is discussed, as is the role of form resistance in evolution of high diversity of plankton morphologies

A heterotriangulene polymer for air stable organic field effect transistors

We report on a novel air stable p type heterotriangulene polymer PTA for large area organic field effect transistor OFET applications. The newly synthesized amorphous organic semiconductor was characterized concerning morphological, optical, electrical and interface related properties and revealed a saturation mobility of similar to 4.2 x 10 3 cm 2 V 1 s 1 and an on off current ratio of similar to 10 5 in bottom gate bottom contact BG BC OFETs. The influence of several interface modifications was investigated in order to optimize the device performance. PTA FETs exhibited excellent air stability over several months and a superior performance compared to the widely used poly 3 hexylthiophene based OFETs. Moreover, we show that hydrochloric acid in chloroform leads to protonation of the nitrogen atoms on the PTA polymer, resulting in a significant change of the electrical characteristics of OFET

Raman band widths of anhydrite II reveal the burning history of high‐fired medieval gypsum mortars

When used as a mineral binder, gypsum is thermally dehydrated and mixed with water, resulting in a paste hardening in the backreaction to calcium sulphate dihydrate (CaSO4 · 2 H2O). Although nowadays mainly hemihydrate-based (CaSO4 · ½ H2O) binders are employed, higher firing temperatures in medieval kilns yielded anhydrite II (CaSO4). Except for the discrimination of the metastable phases anhydrite III and I due to different crystal structures, variations within the production temperature range of anhydrite II (approximately 300 to 1180°C) were not analytically accessible until recently. This study describes the development of an analytical technique, which is based on steady changes of band widths in room-temperature Raman spectra of anhydrite II as a function of burning temperature. Raman microspectroscopic mapping experiments enable to pinpoint individual unreacted grains of thermal anhydrite in mortars and to discriminate them from natural anhydrites originating from the raw gypsum. The determination of band full widths at half maximum of down to 3 cm−1 and differences between them of a few tenths of wavenumbers is not a trivial task. Thus, a focus of this work is on peak fitting and strategies for correction of instrument-dependent band broadening, which is often neglected also beyond the field of mortar analysis. Including other potential influences on band widths, burning temperatures of 400 to 900°C can be retraced in high-fired medieval gypsum mortars with an uncertainty of approximately ± 50 K, as demonstrated with sample material of a stucco sculpture dated around 1400