arXiv subjects

Thomas M. Kremmer

Publications and source records attributed to Thomas M. Kremmer.

5 recordsLinked to original sources

arXiv2026

Recrystallisation phenomena in an ultrafine-grained Al-Mg-Si alloy revealed by correlative in situ EBSD and TEM heating

Ultrafine-grained (UFG) aluminium alloys are promising lightweight structural materials for space applications, where a high grain-boundary density can act as sinks for irradiation-induced defects. Their deployment, however, is contingent on thermal stability: aluminium components in low-Earth orbit can reach $\sim$200 $^\circ$C under solar irradiation, close to where severely deformed aluminium alloys recrystallise. Accurate, bulk-representative determination of recrystallisation onset is therefore essential, yet conventional in situ transmission electron microscopy (TEM) heating is compromised by thin-film effects, ambiguous grain-boundary contrast, and small sampling volumes. Here, a UFG AA6061 (Al-Mg-Si) alloy produced by high-pressure torsion was studied by a direct comparison of in situ TEM heating and in situ electron backscatter diffraction (EBSD) heating, complemented by differential scanning calorimetry (DSC), analytical scanning transmission electron microscopy (STEM-EDX) and microhardness. In situ EBSD sampled $\sim 10^{3}$ grains from bulk material and resolved the microstructural evolution into sequential recovery, recrystallisation and grain-growth regimes, placing the onset of instability at $\sim$198 $^\circ$C. Calorimetry, microhardness and nanoscale elemental mapping showed that grain refinement suppresses GP-zones formation and shifts precipitation to lower temperatures, with precipitation neither retarding recrystallisation nor restoring strength once the UFG structure is consumed. Revisiting the Brailsford-Bullough-Hayns sink-strength theory with a KAM-informed, temperature-dependent internal sink strength, we show that the grain-boundary sink strength collapses as recovery and recrystallisation proceed. We establish in situ EBSD heating as an in operando method for bulk-representative determination of microstructural instabilities in advanced metallic systems.

cond-mat.mtrl-sci↗

arXiv2026

Direct nanoscale observation of melting and solute redistribution in a hypoeutectic Al-Cu alloy with $\it{in\ situ}$ STEM

Melting and solidification of eutectic systems are classical topics in physical metallurgy, yet the mechanisms at nanoscale are less investigated, due to experimental limitations in spatiotemporal resolution. The advent of $\it{in\ situ}$ STEM heating with MEMS technology has recently enabled investigation of eutectic behavior as a function of temperature, time and electrical resistivity. Using this methodology, we investigate the evolution of a nanocrystalline hypoeutectic Al-Cu alloy. Melting initiated in the hotter central region and propagated outward, with grain boundaries acting as preferred sites for eutectic liquid formation via Cu enrichment. The Al$_2$Cu phase melted prior to complete matrix melting. Liquid-state Cu redistribution over a distance of 258 $μ$m - several orders of magnitude beyond solid-state diffusion limits - resulted in Al-rich rim accumulations and Cu enrichment at the outermost edge of the observed chip region. These observations are discussed in the context of classical predictions for melting of eutectic systems.

cond-mat.mtrl-sci↗

arXiv2026

Unlocking nanoscale microstructural detail in aluminium alloys through differential phase contrast segmentation in STEM

Differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM) maps projected electric fields through the phase sensitivity of segmented low-angle detectors. Although typically applied to atomic-resolution imaging at low beam currents, STEM-DPC is here demonstrated as a rapid micro- and nanoscale image-segmentation tool for materials characterization in advanced aluminium alloys. Decomposition of false-colour DPC micrographs in hue-saturation-value space enables simultaneous identification and quantification of nanoclusters, GP zones, intermediate precipitate phases, dislocation cores, and associated strain fields within a single field of view. The method is demonstrated across multiple alloy systems, including clustering and strain-field mapping in a deformed AlMgZn(Cu) crossover alloy, precipitate identification in a paint-baked automotive sheet alloy, phase-variant segmentation in overaged AA7075-T7, and nanopore and nanoparticle detection in an anodic coating on AA2024-T3. Coupling DPC with neural-network segmentation further enables automated grain-boundary delineation and quantification in nanocrystalline aluminium thin films. Combined with STEM-EDX, DPC-based segmentation enables correlative microstructural analysis, establishing DPC as a rapid complement to techniques such as SPED and 4D-STEM.

cond-mat.mtrl-sci↗

arXiv2022

MEMS-based in situ electron-microscopy investigation of rapid solidification and heat treatment on eutectic Al-Cu

The solidification behavior of a eutectic AlCu specimen is investigated via in situ scanning transmission electron microscope (STEM) experiments. Solidification conditions are varied by imposing various cooling conditions via a micro-electro-mechanical system (MEMS) based membrane. The methodology allows the use of material processed by a melting and casting route close to industrial metallurgically fabricated material for in situ STEM solidification studies. Different rapid solidification morphologies could be obtained solely on a single specimen by the demonstrated strategy. Additional post-solidification heat treatments are investigated in terms of observation of spheroidization of lamellas during annealing at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2020

A contamination-free electron-transparent metallic sample preparation method for MEMS experiments with in situ S/TEM

Microelectromechanical systems (MEMS) are currently supporting ground-breaking basic research in materials science and metallurgy as they allow in situ experiments on materials at the nanoscale within electron-microscopes in a wide variety of different conditions such as extreme materials dynamics under ultrafast heating and quenching rates as well as in complex electro-chemical environments. Electron-transparent sample preparation for MEMS e-chips remains a challenge for this technology as the existing methodologies can introduce contaminants, thus disrupting the experiments and the analysis of results. Herein we introduce a methodology for simple and fast electron-transparent sample preparation for MEMS e-chips without significant contamination. The quality of the samples as well as their performance during a MEMS e-chip experiment in situ within an electron-microscope are evaluated during a heat treatment of a crossover AlMgZn(Cu) alloy.

cond-mat.mtrl-sci↗