Welding Metallurgy of

Results from Pt model catalyst surfaces have demonstrated that surface defects, in particular surface concavities, can improve the oxygen reduction reaction (ORR) kinetics. It is, however, a challe...

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Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach

A Zr-based bulk metallic glass produced using selective laser melting (SLM) was compared to the same alloy fabricated using traditional suction-casting. Analysis of the fracture toughness through single edge notched beam bending experiments showed a significantly reduced damage tolerance for the laser-processed material (KQ ~ 138.0 ± 13.1 → 28.7 ± 3.7 MPa √m), even though X-ray diffraction and microhardness responses were identical. Using uniaxial quasistatic micro-pillar compression, it was found that as-cast samples more readily underwent shear transformations (evidenced through discrete load drops) below the nominal 0.2% yield stress, which was connected to the higher macroscopic toughness. Differential scanning calorimetry demonstrated that the increased barrier to shear transformation for the SLM material could not be explained by the relative relaxation states. Rather, it was attributed to the greater dissolved oxygen concentration in the laser-processed material, which is postulated to decrease atomic mobility in the structure and thereby increase the activation energy required to initiate shear transformations.

Relating fracture toughness to micro-pillar compression response for a laser powder bed additive manufactured bulk metallic glass

Characterization of the microstructure in friction stir welds of EN AW-2219 using coincident Doppler-broadening spectroscopy

Abstract Additive manufacturing of bulk metallic glasses (BMGs) has opened this material class to an exciting new range of potential applications, as bulk-scale, net-shaped amorphous components can be fabricated in a single step. However, there exists a critical need to understand the structural details of additive manufactured BMGs and how the glassy structure is linked to the mechanical properties. Here, we present a study of structure and property variations along the build height for a laser powder bed fusion (LPBF) processed Zr-based BMG with composition Zr 59.3 Cu 28.8 Nb 1.5 Al 10.4 commercially termed AMZ4, using hardness testing, calorimetry, positron annihilation spectroscopy, synchrotron X-ray diffraction, and transmission electron microscopy. A lower hardness, more rejuvenated glassy structure was found at the bottom of the build compared to the middle region of the build, with the structure and properties of the top region between the two. Such differences could not be attributed to variability in chemical composition or crystallisation; rather, the softer bottom region was found to have a larger medium range order cluster size, attributed to heat dissipation into the build plate during processing, which gave faster cooling rates and less reheating compared to the steady-state middle of the build. However, at the top of the build less reheating occurs compared to the middle, leading to a somewhat softer and less relaxed state. Graphical abstract 

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Advanced structural analysis of a laser additive manufactured Zr-based bulk metallic glass along the build height

The motion of a spin excitation across topologically nontrivial magnetic order exhibits a deflection that is analogous to the effect of the Lorentz force on an electrically charged particle in an orbital magnetic field. We used polarized inelastic neutron scattering to investigate the propagation of magnons (i.e., bosonic collective spin excitations) in a lattice of skyrmion tubes in manganese silicide. For wave vectors perpendicular to the skyrmion tubes, the magnon spectra are consistent with the formation of finely spaced emergent Landau levels that are characteristic of the fictitious magnetic field used to account for the nontrivial topological winding of the skyrmion lattice. This provides evidence of a topological magnon band structure in reciprocal space, which is borne out of the nontrivial real-space topology of a magnetic order. Description Magnons in a skyrmion lattice Electrons in two-dimensional solids placed in an external magnetic field fill the so-called Landau energy levels. In materials with a nontrivial spin texture, spin excitations (magnons) may have an analogous energy-level structure. However, showing this effect in an experiment is tricky. Weber et al. used inelastic neutron scattering and numerical simulations to demonstrate this effect in the skyrmion lattice phase of the material manganese monosilicide. —JS Inelastic neutron-scattering measurements indicate the formation of Landau levels for magnons in the skyrmion phase of MnSi.

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Topological magnon band structure of emergent Landau levels in a skyrmion lattice

We report on a newly developed scanning positron microbeam based on threefold moderation of positrons provided by the high intensity positron source NEPOMUC. For brightness enhancement a remoderation unit with a $100\,$nm thin Ni(100) foil and 9.6% efficiency is applied to reduce the area of the beam spot by a factor of 60. In this way, defect spectroscopy is enabled with a lateral resolution of $33\,\mu$m over a large scanning range of $19\times19\,$mm$^{2}$. Moreover, 2D defect imaging using Doppler broadening spectroscopy (DBS) is demonstrated to be performed within exceptional short measurement times of less than two minutes for an area of $1\times1$mm$^{2}$ ($100\times100\,\mu$m$^{2}$) with a resolution of $250\,\mu$m ($50\,\mu$m). We studied the defect structure in laser beam welds of the high-strength agehardened Al alloy (AlCu6Mn, EN AW-2219 T87) by applying (coincident) DBS with unprecedented spatial resolution. The visualization of the defect distribution revealed a sharp transition between the raw material and the welded zone as well as a very small heat affected zone. Vacancy-like defects and Cu rich precipitates are detected in the as-received material and, to a lesser extent, in the transition zone of the weld. Most notably, in the center of the weld vacancies without forming Cu-vacancy complexes, and the dissolution of the Cu atoms in the crystal lattice, i.e. formation of a supersaturated solution, could be clearly identified.

Defect Imaging and Detection of Precipitates Using a New Scanning Positron Microbeam

https://iopscience.iop.org/article/10.1088/1367-2630/aa915b/pdf

Lukas Beddrich

Papers

Topological magnon band structure of emergent Landau levels in a skyrmion lattice

Defect Imaging and Detection of Precipitates Using a New Scanning Positron Microbeam

Defect imaging and detection of precipitates using a new scanning positron microbeam