Untersuchung der Mikrostruktur von rührreibgeschweißten Stählen mittels in situ Experimenten mit hochenergetischer Röntgenstrahlung

Mit hochenergetische Röntgenstrahlung ist es möglich, tief in Werkstoffe einzudringen und Informationen über ihre Struktur zu gewinnen. Dank der hohen Intensität der Röntgenstrahlung am neuen PETRA III Speicherring und der fortschreitenden Entwicklung schneller Detektoren für hohe Photonenenergien ist es möglich, Prozesse in situ zu untersuchen. Um einen grundlegenden Einblick in die beim Rührreibschweißen ablaufenden Nicht-Gleichgewichtsprozesse zu bekommen, wurde am Helmholtz-Zentrum Geesthacht mit den Experten für Festphase-Fügeprozesse die portable Rührreibschweißanlage „FlexiStir“ entwickelt. In den in dieser Arbeit vorgestellten Diffraktionsexperimenten ist es erstmals mit praxisrelevanten Schweißparametern gelungen, die während des Schweißens ablaufenden Transformationen in der Mikrostruktur von Stählen zu untersuchen. Mit der Kombination der Ergebnisse der Rietveldanalyse und der protokollierten Schweißparameter konnte ein einzigartiger Einblick in den Rührreibschweißprozess gewonnen werden. Mit einer umfassenden zusätzlichen Charakterisierung der für die Rührreibschweißexperimente verwendeten Werkstoffe, die sowohl vor als auch nach dem Schweißen stattfand sowie der Durchführung ergänzender in situ Experimente im Dilatometer, die die Materialeigenschaften detailliert im für den Schweißvorgang relevanten Temperatur- und Zeitintervall zugänglich machten, konnten die Ergebnisse der in situ FSW-Experimente weitergehend interpretiert werden. Für den unlegierten Baustahl S355 konnte mit Hilfe der für die Umwandlung von Austenit zu Ferrit benötigten Zeit und dem anschließenden Vergleich mit den im Dilatometer gemessenen Umwandlungszeiten des gleichen Materials für unterschiedliche Schweißparameter die Abkühlraten des betrachteten Probenvolumens im Nahtzentrum berechnet werden. Mit den Ergebnissen aus den Messungen am Superduplexstahl 1.4410 konnte die Temperaturabhängigkeit des Ferritgehalts genutzt werden, um den Zusammenhang zwischen der Schweißleistung und der Maximaltemperatur in der Rührzone zu quantifizieren

Anisotropic strain variations during the confined growth of Au nanowires

The electrochemical growth of Au nanowires in a template of nano-porous anodic aluminum oxide was investigated in situ by means of grazing-incidence transmission small- and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray fluorescence (XRF) and 2-dimensional surface optical reflectance (2D-SOR). The XRF and the overall intensity of the GTWAXS patterns as a function of time were used to monitor the progress of the electrodeposition. Furthermore, we extracted powder diffraction patterns in the direction of growth and in the direction of confinement to follow the evolution of the direction-dependent strain. Quite rapidly after the beginning of the electrodeposition, the strain became tensile in the vertical direction and compressive in the horizontal direction, which showed that the lattice deformation of the nanostructures can be artificially varied by an appropriate choice of the deposition time. By alternating sequences of electrodeposition to sequences of rest, we observed fluctuations of the lattice parameter in the direction of growth, attributed to stress caused by electromigration.. Furthermore, the porous domain size calculated from the GTSAXS patterns was used to monitor how homogeneously the pores were filled.Comment: Short communication manuscript. Four figure

Effect of composition and thermal history on deformation behavior and cluster connections in model bulk metallic glasses

The compositional dependence and influence of relaxation state on the deformation behavior of a Pt-Pd-based bulk metallic glasses model system was investigated, where platinum is systematically replaced by topologically equivalent palladium atoms. The hardness and modulus increased with rising Pd content as well as by annealing below the glass transition temperature. Decreasing strain-rate sensitivity and increasing serration length are observed in nano indentation with increase in Pd content as well as thermal relaxation. Micro-pillar compression for alloys with different Pt/Pd ratios validated the greater tendency for shear localization and brittle behavior of the Pd-rich alloys. Based on total scattering experiments with synchrotron X-ray radiation, a correlation between the increase in stiffer 3-atom cluster connections and reduction in strain-rate sensitivity, as a measure of ductility, with Pd content and thermal history is suggested

In situ\textit{In situ} hydride breathing during the template-assisted electrodeposition of Pd nanowires

We investigated the structural evolution of electrochemically fabricated Pd nanowires $\textit{in situ}$ by means of grazing-incidence transmission small- and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray fluorescence (XRF) and 2-dimensional surface optical reflectance (2D-SOR). This shows how electrodeposition and the hydrogen evolution reaction (HER) compete and interact during Pd electrodepositon. During the bottom-up growth of the nanowires, we show that $\beta$-phase Pd hydride is formed. Suspending the electrodeposition then leads to a phase transition from $\beta$- to $\alpha$-phase Pd hydride. Additionally, we find that grain coalescence later hinders the incorporation of hydrogen in the Pd unit cell. GTSAXS and 2D-SOR provide complementary information on the volume fraction of the pores occupied by Pd, while XRF was used to monitor the amount of Pd electrodeposited.Comment: 17 pages, 11 figures, 4 appendice

Hierarchical supercrystalline nanocomposites through the self-assembly of organically-modified ceramic nanoparticles

Biomaterials often display outstanding combinations of mechanical properties thanks to their hierarchical structuring, which occurs through a dynamically and biologically controlled growth and self-assembly of their main constituents, typically mineral and protein. However, it is still challenging to obtain this ordered multiscale structural organization in synthetic 3D-nanocomposite materials. Herein, we report a new bottom-up approach for the synthesis of macroscale hierarchical nanocomposite materials in a single step. By controlling the content of organic phase during the self-assembly of monodisperse organically-modified nanoparticles (iron oxide with oleyl phosphate), either purely supercrystalline or hierarchically structured supercrystalline nanocomposite materials are obtained. Beyond a critical concentration of organic phase, a hierarchical material is consistently formed. In such a hierarchical material, individual organically-modified ceramic nanoparticles (Level 0) self-assemble into supercrystals in face-centered cubic superlattices (Level 1), which in turn form granules of up to hundreds of micrometers (Level 2). These micrometric granules are the constituents of the final mm-sized material. This approach demonstrates that the local concentration of organic phase and nano-building blocks during self-assembly controls the final material's microstructure, and thus enables the fine-tuning of inorganic-organic nanocomposites' mechanical behavior, paving the way towards the design of novel high-performance structural materials.The authors gratefully acknowledge the financial support from the German Research Foundation (DFG) via the SFB 986-M3, projects A1, A6, Z2, and Z3. We thank Dr. F. Beckmann (Helmholtz-Zentrum Geesthacht, Geesthacht, Germany) for scanning the sample with the technique SRµCT and for reconstructing the slices, and Dr. I. Greving (Helmholtz-Zentrum Geesthacht, Geesthacht, Germany) for her inputs on SRµCT. Dr. F. Brun (National Institute of Nuclear Physics, Trieste, Italy) is acknowledged for the discussion regarding quantitative analysis using Pore3d

Iron oxide-based nanostructured ceramics with tailored magnetic and mechanical properties: Development of mechanically robust, bulk superparamagnetic materials

Nanostructured iron-oxide based materials with tailored mechanical and magnetic behavior are produced in bulk form. By applying ultra-fast heating routines via spark plasma sintering (SPS) to supercrystalline pellets, materials with an enhanced combination of elastic modulus, hardness and saturation magnetization are achieved. Supercrystallinity-namely the arrangement of the constituent nanoparticles into periodic structures-is achieved through self-assembly of the organically-functionalized iron oxide nanoparticles. The optimization of the following SPS regime allows the control of organics' removal, necking, iron oxide phase transformations and nano-grain size retention, and thus the fine-tuning of both mechanical properties and magnetic response, up until the production of bulk mm-size superparamagnetic materials.Deusche Forschungsgemeinschaft (DFG

Strong macroscale supercrystalline structures by 3D printing combined with self-assembly of ceramic functionalized nanoparticles

To translate the exceptional properties of colloidal nanoparticles (NPs) to macroscale geometries, assembly techniques must bridge a 106-fold range of length. Moreover, for successfully attaining a final mechanically robust nanocomposite macroscale material, some of the intrinsic NPs’ properties have to be maintained while minimizing the density of strength-limiting defects. However, the assembly of nanoscale building blocks into macroscopic dimensions, and their effective macroscale properties, are inherently affected by the precision of the conditions required for assembly and emergent flaws including point defects, dislocations, grain boundaries, and cracks. Herein, a direct-write self-assembly technique is used to construct free-standing, millimeter-scale columns comprising spherical iron oxide NPs (15 nm diameter) surface functionalized with oleic acid (OA), which self-assemble into face-centered cubic (FCC) supercrystals in minutes during the direct-writing process. The subsequent crosslinking of OA molecules results in nanocomposites with a maximum strength of 110 MPa and elastic modulus up to 58 GPa. These mechanical properties are interpreted according to the flaw size distribution and are as high as newly engineered platelet-based nanocomposites. The findings indicate a broad potential to create mechanically robust, multifunctional 3D structures by combining additive manufacturing with colloidal assembly.Financial support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Projektnummer 192346071, SFB 986 -, the National Science Foundation CAREER Award (CMMI-1346638, to A.J.H.), and from the MIT-Skoltech Next Generation Program. A.T.L.T. was supported by a postgraduate fellowship from DSO National Laboratories, Singapore. XRM at the University of Bremen was funded within the CO 1043 12-1 (Call for Major Equipment, XRM)

Microstructural Investigations of Novel High Temperature Alloys Based on NiAl-(Cr,Mo)

Apart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae byadding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the first tendency to fiber formation was found at 9.6 at.% Cr. There is a compositional range, where both lamellae and fibers are present in the microstructure and a further decrease in Cr to1.8at.% Cr results in fully fibrous morphology. Alongside these morphology changes of the (Cr,Mo)$_{ss}$ reinforcing phase, its volume fraction was found to be from 41 to 11 vol.% confirming the trendpredicted by the CALPHAD approach. For mixed morphologies in-situ X-ray diraction experiments performed between room and liquidus temperature accompanied by EDX measurements reveal the formation of a gradient in composition for the solid solution. A new Mo-rich NiAl-9.6Cr-10.3Mo alloy clearly shows this effect in the as-cast state. Moreover, crystallographic orientation examination yields two different types of colonies in this composition. In the first colony type, the orientationrelationship between NiAl matrix and (Cr,Mo)$_{ss}$ reinforcing phase was (100)$_{NiAl}$||(100)$_{Cr,Mo}$ and $\langle 100 \rangle_{NiAl}$||$\langle 100 \rangle _{Cr,Mo}$. An orientation relationship described by a rotation of almost 60 about $\langle 111 \rangle$ was found in the second colony type. In both cases, no distinct crystallographic plane as phase boundary was observed

An in situ high-energy synchrotron x-ray diffraction study of directional solidification in binary TiAl alloys

Phase formation and microstructure selection during solidification in γ-TiAl alloysare highly relevant, both with respect to the microstructure and texture of castalloys and to directional solidification, which allows to obtain a unique combinationof properties. However, during cooling to room temperature, the solidifyingphases transform to low-temperature phases, which mask the priorsituation during solidification. A new inductive crucible-free zone melting furnaceis used in this work, which is specially designed for in situ investigations ofsolidification using synchrotron radiation. Herein, alloys with 45 and 48 at% Alare studied, and it can be shown that with varying withdrawal rate, a change fromα solidification to synchronous β þ α solidification occurs in Ti–48Al, as it ispredicted in the literature. Furthermore, it is observed that by increasing thewithdrawal rate, a preferred growth of the β phase occurs in alloys with45 at% Al, which is interpreted as a transition to dendritic solidification. Theobservations are compared with a microstructure selection map calculatedaccording to the nucleation and constitutional undercooling (NCU) model proposedin the literature

Strain-dependent fractional molecular diffusion in humid spider silk fibres

Spider silk is a material well known for its outstanding mechanical properties, combining elasticity and tensile strength. The molecular mobility within the silk's polymer structure on the nanometre length scale importantly contributes to these macroscopic properties. We have therefore investigated the ensemble-averaged single-particle self-dynamics of the prevailing hydrogen atoms in humid spider dragline silk fibres on picosecond time scales in situ as a function of an externally applied tensile strain. We find that the molecular diffusion in the amorphous fraction of the oriented fibres can be described by a generalized fractional diffusion coefficient Kα that is independent of the observation length scale in the probed range from approximately 0.3–3.5 nm. Kα increases towards a diffusion coefficient of the classical Fickian type with increasing tensile strain consistent with an increasing loss of memory or entropy in the polymer matrix