Christoph Somsen

Fabrication and characterization of nanoporous gold composites through chemical dealloying of two phase Al–Au alloys

We present a facile route to fabricate nanoporous gold composites (NPGCs) through chemical dealloying of two phase Al–Au alloys comprising Al2Au and AlAu intermetallic compounds under free corrosion conditions. The microstructures of the NPGCs were characterized using X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and transmission electron microscopy. The dealloying of Al2Au and AlAu separately proceeds, and results in the formation of the NPGCs. The microstructures of the NPGCs are composed of intracellular and intercellular areas which exhibit two kinds of nanoporous structures with different length scales of ligaments/channels. The nanoporous structure of the intracellular areas forms due to the dealloying of Al2Au and that of the intercellular area forms owing to the dealloying of AlAu. Moreover, the proportion of the intercellular areas in the NPGCs increases with increasing Au content in the starting Al–Au alloys. In addition, the length scale of ligaments/channels in the NPGCs can be adjusted by simply changing the dealloying solution. The NPGCs show abnormal coarsening behaviors when subjected to annealing at different temperatures. In comparison to nanoporous gold (NPG) with a homogeneous structure, the NPGCs exhibit higher Youngs modulus and yield strength. We can tailor the microstructures, properties and applications of these NPGCs through changing the composition of the starting Al–Au alloys, changing the dealloying solution, and adopting proper post treatment including annealing at high temperatures and acid treatment at room temperature.

Electrochemical Synthesis of Core–Shell Catalysts for Electrocatalytic Applications

A novel electrochemical method to prepare platinum shells around carbon-supported metal nanoparticles (Ru and Au) by pulsed electrodeposition from solutions containing Pt ions is presented. Shell formation is confirmed by characteristic changes in the cyclic voltammograms, and is further evidenced by monitoring particle growth by transmission electron microscopy as well as by energy-dispersive analysis of X rays (EDX). Scanning electrochemical microscopy and EDX measurements indicate a selective Pt deposition on the metal/carbon catalyst, but not on the glassy carbon substrate. The thus prepared carbon-supported core-shell nanoparticles are investigated with regard to their activity in electrocatalytic oxygen reduction, which demonstrates the applicability of these materials in electrocatalysis or sensors.

Intrinsic room temperature ferromagnetism in Co-implanted ZnO

We report on the structural and magnetic properties of a cobalt-implanted ZnO film grown on a sapphire substrate. X-ray diffraction and transmission electron microscopy reveal the presence of a( 1 0 ¯ 1 0)-oriented hexagonal Co phase in the Al2O3 sapphire substrate, but not in the ZnO film. Co clusters, with a diameter of about 5–6 nm, form a Co rich layer in the substrate close to the ZnO/Al2O3 interface. Magnetization measurements indicate that there exist two different magnetic phases in the implanted region. One originates from the Co clusters in Al2O3, the other one belongs to a homogeneous ferromagnetic phase with a ferromagnetic Curie temperature far above room temperature. In the latter case, the ferromagnetism can be attributed to Co substitution on Zn sites in the ZnO layer. We have observed magnetic dichroism at the Co L2,3 and O K edges at room temperature as well as the multiplet structure in x-ray absorption spectra around the Co L3 edge, supporting the intrinsic nature of the observed ferromagnetism in a Co-implanted ZnO film. The magnetic moment per substituted cobalt is found to be about 2.81 µB, which is very close to the theoretical expected value of 3 µB/Co for Co 2+ in its high spin state.

High-temperature ferromagnetism in Co-implanted TiO2 rutile

We report on structural, magnetic and electronic properties of Co-implanted TiO2(1 0 0) rutile single crystals for different implantation doses. Strong ferromagnetism at room temperature and above is observed in TiO2 rutile plates after cobalt ion implantation, with magnetic parameters depending on the cobalt implantation dose. While the structural data indicate the presence of metallic cobalt clusters, the multiplet structure of the Co L3 edge in the XAS spectra provides evidence that a sizeable portion of the dopants occupy substitutional Co 2+ sites. The detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted TiO2 plates. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the rutile phase, and the second one is a superparamagnetic phase which originates from the formation of metallic cobalt clusters in the implanted region. Using x-ray resonant magnetic scattering, the element specific magnetizations of cobalt, oxygen and titanium in Co-implanted TiO2 single crystals are investigated. Magnetic dichroism was observed at the Co L2,3 edges as well as at the O K edge. Anomalous Hall effect measurement indicates n-type carriers in Co-implanted TiO2 rutile. The interaction mechanism, which leads to ferromagnetic ordering of substituted cobalt ions in the host matrix, is also discussed.

Ingot metallurgy and microstructural characterization of Ti–Ta alloys

Abstract In the present work we perform a detailed investigation of ingot metallurgy processing routes of Ti-30Ta, a shape memory alloy with a good potential for applications at higher temperatures. There is currently considerable interest in high temperature shape memory alloys, both in industry (automotive and aerospace applications) and in academia. By means of scanning electron microscopy, we provide recommendations on the number of remelting cycles in the vacuum arc melting process, and on annealing temperatures/times in order to obtain chemical homogeneity. It is also shown that this is required to obtain well defined differential scanning calorimeter charts, which facilitates characterization and investigations of martensitic transformation in this alloy. Areas in need of further work are identified.

Nanoindentation of pseudoelastic NiTi shape memory alloys: Thermomechanical and microstructural aspects

Abstract Nanoindentation allows local characterization of the microstructural processes associated with pseudoelastic recovery in NiTi shape memory alloys. We discuss nanoindentation and tensile testing results, both carried out at different temperatures on a NiTi alloy with three different microstructures (containing no, small and large Ni-rich precipitates). Maximum shape recovery in indentation experiments and during tensile testing occurs in microstructures with fine precipitates. Thermomechanical constraints affect pseudoelasticity differently in micro- and macro-scale testing: Indentation at temperatures as close as possible to the transformation start temperatures is associated with better shape recovery than testing at higher temperatures, whereas maximum shape recovery is only observed at slightly higher temperatures in macroscopic testing. These results are rationalized by considering the effect of temperature on stress-induced martensitic transformation and dislocation plasticity.

Martensitic Transformations and Functional Stability in Ultra-Fine Grained NiTi Shape Memory Alloys

Martensitic transformations in NiTi shape memory alloys (SMAs) strongly depend on the microstructure. In the present work, we investigate how martensitic transformations are affected by various types of ultra-fine grained (UFG) microstructures resulting from various processing routes. NiTi SMAs with UFG microstructures were obtained by equal channel angular pressing, high pressure torsion, wire drawing and subsequent annealing treatments. The resulting material states were characterized by transmission electron microscopy and differential scanning calorimetry (DSC). The three thermomechanical processing routes yield microstructures which significantly differ in terms of grain size and related DSC chart features. While the initial coarse grained material shows a well defined one-step martensitic transformation on cooling, two-step transformations were found for all UFG material states. The functional stability of the various UFG microstructures was evaluated by thermal cycling. It was found that UFG NiTi alloys show a significantly higher stability. In the present work, we also provide preliminary results on the effect of grain size on the undercooling required to transform the material into B19’ and on the related heat of transformation.

Effect of Si addition on the oxidation resistance of Co–Re–Cr-alloys: Recent attainments in the development of novel alloys

Abstract The influence of silicon on the oxidation behaviour of Co—Re—Cr-alloys has been studied at 1 000°C and 1 100°C. Consideration was given to the synergetic effects between chromium and silicon with respect to the development of a protective Cr2O3 layer. The Si addition to the Co—Re-alloys produces a significant decrease in the evaporation rate of Re oxides. Moreover, the beneficial influence in the transient oxidation period results in a rapid formation of Cr2O3 scale. While the addition of 1 and 2 at.% Si to the ternary Co-17Re-23Cr alloy was insufficient to form a continuous Cr2O3 scale, the addition of 3 at.% silicon caused a change in the oxidation mode resulting in the formation of a nearly continuous Cr2O3 scale. On the oxide/alloy interface of the alloy Co-17Re-30Cr-2Si, a continuous and dense Cr2O3 scale was observed, which remained stable after 100 h exposure protecting the metallic substrate.

Quantitative Charakterisierung der Gefüge-Anisotropie einer stranggepressten TiAl-Legierung

Kurzfassung In dieser Arbeit werden quantitative mikrostrukturelle Untersuchungen am Duplex-Gefüge einer stranggepressten intermetallischen γ-Titanaluminidlegierung der 3. Generation (TNBV5) diskutiert. Die Mikrostruktur wurde in raster- und transmissionselektronenmikro-skopischen Untersuchungen von senkrecht und parallel zur Strangpressrichtung entnommenem Probenmaterial charakterisiert. Es erfolgten eine Bestimmung der Kornflächen lamellarer und globularer Körner, eine Bestimmung des Anteils lamellarer Kolonien sowie eine Analyse des Anteils der verzwillingten γ-Körner. Zudem wurde eine Zeiligkeit der lamellaren Kolonien in Strangpressrichtung nachgewiesen und der mittlere Grenzflächenabstand der lamellaren Bereiche bestimmt. In ersten Kriechversuchen an unterschiedlich orientierten Proben wurde der Einfluss der Gefüge-Anisotropie auf das Kriechverhalten dieser Legierung dokumentiert. Die hier ausführlich beschriebenen metallographischen Methoden ermöglichen eine quantitative Analyse des Einflusses mikrostruktureller Anisotropien auf die mechanischen Eigenschaften und das Kriechverhalten von Titan-aluminiden.

Influence of heat treatment and microstructure on the tensile pseudoelastic response of an Ni-rich NiTi shape memory alloy

Abstract The influence of microstructure on the stress–strain behavior of an Ni-rich NiTi shape memory alloy is examined. Specimens cut from a large-diameter bar of Ni50.7Ti49.3 shape memory alloy were analyzed in two states: (i) annealed and (ii) annealed and aged. The annealed state shows a fully austenitic structure with no precipitates and no distortions caused by residual stresses. The annealed and aged state has coherent Ni4Ti3 particles precipitated in the proximity of the austenite grain boundaries. The size of the precipitates increases moving away from the grain boundaries toward the grain interiors. The evolution of the two states in the stress–strain–temperature space has been analyzed using tensile specimens with special geometry. Due to the complex effects of the coherent precipitates, the specimens in the aged state exhibited lower stress plateaus in the tensile loading–unloading curves, which enabled the occurrence of transformation pseudoelasticity from room temperature to 333 K.