Breakthroughs in Optimization of Mechanical Properties of Nanostructured Metals and Alloys

While nanocrystalline metals can have strength and hardness values factors of 10 or more greater than their conventional grain size counterparts, ductility in tension has been disappointing, typically less than 2% elongation. This paper reviews the limitations to ductility in nanocrystalline materials and presents the results of recent breakthroughs wherein both high strength and good ductility are observed.     

Graphite Nucleation in Cast Iron Melts Based on Solidification Experiments and Microstructure Simulation

Microstructure strongly influences the mechanical properties of cast iron. By inoculating the melt with proper inoculants, foreign substrates are brought into the melt and eventually the graphite can crystallize on them. The elements and substrates that really play a role for nucleation are yet unknown. Until now there is very little knowledge about the fundamentals of nucleation, such as composition and morphology of nuclei. In this work we utilized EN-GJL-200 as a base material and examined several produced specimens. The specimens were cast with and without inoculants and quenched at different solidification states. Specimens were also examined with a high and low oxygen concentration, but the results showed that different oxygen contents have no influence on the nucleation in cast iron melts. Our research was focused on the microscopic examination and phase-field simulations. For studying the samples we applied different analytical methods, where SEM-EDS, -WDS were proved to be most effective. The simulations were conducted by using the software MICRESS, which is based on a multiphase-field model and has been coupled directly to the TCFE3 thermodynamic database from TCAB. On the basis of the experimental investigations a nucleation mechanism is proposed, which claims MnS precipitates as the preferred site for graphite nucleation. This theory is supported by the results of the phase-field simulations.     

On the Application of X‐ray Microtomography in the Field of Materials Science

The principle of the tomography technique and the different possible set‐ups, which can be used to obtain medium‐(10 μm) and high‐(1 μm) resolution, three‐dimensional, non‐destructive images, are shown in this paper. Illustrations are made of the applications of the technique in the field of materials science. Examples are given for medium‐resolution images of metallic foams and model metal matrix composites that are reinforced with spherical particles. High‐resolution examples are shown for aluminium alloys. For low‐absorbent materials we show that the phase contrast obtained using synchrotron radiation can provide a valuable solution. The quantitative use of these images, coupled with in‐situ tensile tests or used for the simple analysis of the initial microstructure of several structural materials, is also described.     

In Situ Characterization of Internal Damage in TRIP‐Steel/Mg‐PSZ Composites under Compressive Stress Using X‐Ray Computed Tomography

An experimental setup for in situ investigations under compressive stress using laboratory X‐ray computed tomography (XCT) was developed and successfully tested. Complete deformation curves can be taken. It could be shown that XCT scans are possible during brakes of the stopped in situ experiments. In this way the deformation behavior of defined sample regions can be investigated. This kind of experiments is well suited to investigate the deformation behavior of foams and other samples which are transferable for the X‐rays used. The compression of metal matrix composite foams lead to the cooperative collapse of connected cells. We observed deformation bands arising in regions of smaller cell wall thicknesses. The deformation was dependent on size, shape, and orientation of the cells under consideration. Obviously deformation bands start at bigger cells with a small cell wall thickness and some extension perpendicular to the deformation direction. The rising of this kind of deformation bands can be explained by the dramatic change of the stress distribution in the neighbor cells after the first brake of a cell wall.     

Monitoring Nanocrystal Self‐Assembly in Real Time Using In Situ Small‐Angle X‐Ray Scattering

Self‐assembled nanocrystal superlattices have attracted large scientific attention due to their potential technological applications. However, the nucleation and growth mechanisms of superlattice assemblies remain largely unresolved due to experimental difficulties to monitor intermediate states. Here, the self‐assembly of colloidal PbS nanocrystals is studied in real time by a combination of controlled solvent evaporation from the bulk solution and in situ small‐angle X‐ray scattering (SAXS) in transmission geometry. For the first time for the investigated system a hexagonal closed‐packed (hcp) superlattice formed in a solvent vapor saturated atmosphere is observed during slow solvent evaporation from a colloidal suspension. The highly ordered hcp superlattice is followed by a transition into the final body‐centered cubic superlattice upon complete drying. Additionally, X‐ray cross‐correlation analysis of Bragg reflections is applied to access information on precursor structures in the assembly process, which is not evident from conventional SAXS analysis. The detailed evolution of the crystal structure with time provides key results for understanding the assembly mechanism and the role of ligand–solvent interactions, which is important both for fundamental research and for fabrication of superlattices with desired properties.     

Variable‐Wavelength Quick Scanning Nanofocused X‐Ray Microscopy for In Situ Strain and Tilt Mapping

Compression of micropillars is followed in situ by a quick nanofocused X‐ray scanning microscopy technique combined with 3D reciprocal space mapping. Compared to other attempts using X‐ray nanobeams, it avoids any motion or vibration that would lead to a destruction of the sample. The technique consists of scanning both the energy of the incident nanofocused X‐ray beam and the in‐plane translations of the focusing optics along the X‐ray beam. Here, the approach by imaging the strain and lattice orientation of Si micropillars and their pedestals during in situ compression is demonstrated. Varying the energy of the incident beam instead of rocking the sample and mapping the focusing optics instead of moving the sample supplies a vibration‐free measurement of the reciprocal space maps without removal of the mechanical load. The maps of strain and lattice orientation are in good agreement with the ones recorded by ordinary rocking‐curve scans. Variable‐wavelength quick scanning X‐ray microscopy opens the route for in situ strain and tilt mapping toward more diverse and complex materials environments, especially where sample manipulation is difficult.     

Strain Profiling of a Ferritic‐Martensitic Stainless Steel Sheet — Comparing Synchrotron with Conventional X‐Ray Diffraction

To improve the fatigue resistance of stainless steel sheet, it is a common practice to induce compressive residual stress in the surface through shot‐peening or tumbling. Stress depth profiles obtained by tumbling of thin stainless steel tensile rods were analysed using laboratory and synchrotron X‐Ray Diffraction (XRD). Both the non‐destructive synchrotron and the laboratory XRD etch‐depth profile gave similar results: a residual stress profile decaying over a depth not exceeding 50 µm into the material.     

Ptychographic X‐Ray Imaging of Colloidal Crystals

Ptychographic coherent X‐ray imaging is applied to obtain a projection of the electron density of colloidal crystals, which are promising nanoscale materials for optoelectronic applications and important model systems. Using the incident X‐ray wavefield reconstructed by mixed states approach, a high resolution and high contrast image of the colloidal crystal structure is obtained by ptychography. The reconstructed colloidal crystal reveals domain structure with an average domain size of about 2 µm. Comparison of the domains formed by the basic close‐packed structures, allows us to conclude on the absence of pure hexagonal close‐packed domains and confirms the presence of random hexagonal close‐packed layers with predominantly face‐centered cubic structure within the analyzed part of the colloidal crystal film. The ptychography reconstruction shows that the final structure is complicated and may contain partial dislocations leading to a variation of the stacking sequence in the lateral direction. As such in this work, X‐ray ptychography is extended to high resolution imaging of crystalline samples.     

In‐Situ Processing of Al Alloy Composites

In‐situ processing of discontinuously reinforced Al alloy composites (DRACs) by gas‐bubbling method was investigated in this paper. Fine SiC and AlN particles were synthesized by bubbling C‐bearing and N‐bearing gases respectively. These results show that processing of DRACs through in‐situ gas‐bubbling route is technically feasible. Compared with conventional processing techniques, this method has the potential to produce DRACs with better microstructure and properties while with lower cost.